Principles of Guided Missiles and Nuclear Weapons, 1959, was created to introduce Navy personnel to these weapons. This document was never classified and does not contain any classified information. It is provided as a historical document demonstrating the technology and implied tactics midway through the US-Soviet Cold War.
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THE UNITED STATES NAVY
GUARDIAN OF OUR COUNTRY
The United States Navy is responsible for maintaining control of the sea and is a ready force on watch at home and overseas, capable of strong action to preserve the peace or of instant offensive action to win in war.
It is upon the maintenance of this control that our country's glorious future depends; the United States Navy exists to make it so.
WE SERVE WITH HONOR
Tradition, valor, and victory are the Navy's heritage from the past. To these may be added dedication, discipline, and vigilance as the watchwords of the present and the future.
At home or on distant stations we serve with pride, confident in the respect of our country, our shipmates, and our families.
Service to God and Country is our special privilege. We serve with honor.
THE FUTURE OF THE NAVY
The Navy will always employ new weapons, new techniques, and greater power to protect and defend the United States on the sea, under the sea, and in the air.
Now and in the future, control of the sea gives the United States her greatest advantage for the maintenance of peace and for victory in war.
Mobility, surprise, dispersal, and offensive power are the keynotes of the new Navy. The roots of the Navy lie in a strong belief in the future, in continued dedication to our tasks, and in reflection on our heritage from the past.
Never have our opportunities and our responsibilities been greater.
This is the second volume of a three-volume series of texts dealing with Naval weapons. The series is intended for use in the Naval Science curriculum of NROTC universities, and in other Navy training programs.
The first volume, Principles of Naval Ordnance and Gunnery, NavPers 10783, deals with conventional Naval weapons, and with the principles of fire control.
The present volume describes the principles of guided missiles and nuclear weapons, insofar as they can be discussed in an unclassified text. The treatment is necessarily of a general nature, with minimum reference to actual weapons in current use.
The third volume, a classified supplement to this text, will describe specific Navy missiles and nuclear weapons.
CHAPTER 1 INTRODUCTION TO GUIDED MISSILES
A GUIDED MISSILE is an unmanned vehicle that travels above the earth's surface; it carries an explosive war head or other useful payload; and it contains within itself some means for controlling its own trajectory or flight path. A glide bomb is propelled only by gravity. But it contains a device for controlling its flight path, and is therefore a guided missile.
The Navy's guided missiles, including Terrier, Talos, Sidewinder, Sparrow, Regulus, and Polaris, meet all the requirements of the above definition.
The Army's Honest John is a 3-ton rocket that is capable of carrying a nuclear warhead. But because it contains no guidance system, Honest John is not a guided missile. The Navy's homing torpedoes are self-propelled weapons with elaborate guidance systems. The homing torpedo can hunt for a target and, when it finds one, steer toward it on a collision course. But because it does not travel above the earth's surface, the homing torpedo is not a guided missile.
A MISSILE is any object that can be projected or thrown at a target. This definition includes stones and arrows as well as gun projectiles, bombs, torpedoes, and rockets. But in current military usage, the word MISSILE is gradually becoming synonymous with GUIDED MISSILE. It will be so used in this text; we will use the terms MISSILE and GUIDED MISSILE interchangeably.
1A2. Scope of the text
Part 1 of this book is a brief introduction to the basic principles that govern the design, construction, and use of guided missiles. Many of the principles we will discuss apply to all missiles; most of them apply to more than one. The treatment will necessarily be general. Security requirements prevent any detailed description of specific missiles in an unclassified text. This text will therefore contain very little information about specific missiles; they
will be described in some detail in a supplementary volume.
The reader will find some repetition in this text; this is intentional. The subject is complex; it deals with many different phases of science and technology. The beginning student of guided missiles faces a paradox. We might say that you can't thoroughly understand any part of a guided missile unless you understand all the other parts first. We will deal with this problem by first discussing the guided missile as a whole, with a brief consideration of its propulsion, control, guidance, and launching systems. Each of these subjects will then be treated at some length in one or more later chapters.
All guided missiles contain electronic devices; some of these devices are very complex. A sound understanding of the operating principles of missile guidance is impossible without some background in basic electricity and electronics. Appendix A of this text covers these subjects briefly. It may be used for a quick review. Students who have no background in electronics should use appendix A as an introduction to the subject; it should, if possible, be supplemented by further reading in basic texts on electricity and electronics.
1A3. Purposes and uses of guided missiles
As you well know, the primary mission of our Navy is control of the seas. We propose to keep the sea lanes open for our own and for friendly commerce; in time of war, we propose to deny use of the sea to our enemy. Historically, this mission has been accomplished by the use of warships armed with the most advanced weapons of their time. When John Paul Jones challenged the British control of the seas, his warships carried guns having an effective range of a few hundred yards. The Union Navy maintained a successful blockade of southern ports with the help of guns that could shoot a little more than a mile. The battleships of World War I carried rifled guns with an effective range in the order of 20 miles. When aircraft became more effective
weapons than guns, in both range and striking power, aircraft became the primary weapon of the Navy. The battle of the Coral Sea, in 1942, was the first major naval engagement in which surface ships did not exchange a single shot.
When a navy so controls the seas that it can safely approach the enemy coast, it can extend its striking power inland to the distance its weapons can reach. A battleship can bombard enemy installations more than 20 miles inland. Carrier-based aircraft extend the Navy's force for hundreds of miles over enemy territory. Thus, during the Korean War, the whole of North Korea was subject to attack by carrier-based aircraft of the U. S. Navy. The Navy's Regulus guided missile has a range comparable to that of carrier-based aircraft. And because it can be launched from submarines, Regulus can be used effectively even where we do not control the surface of the sea. The Polaris missile, also submarine-launched, will extend the Navy's striking power to 1,500 miles inland. And only a relatively small part of the earth's land surface lies more than 1,500 miles from the sea.
One of the strongest elements in our national defense is the Strategic Air Command, which can launch a devastating nuclear attack against any enemy on a few minutes notice. But SAC bases are large, and expensive to build and maintain. Their position is known to our possible enemies. At the outbreak of war, they would probably be the first objective in a surprise attack.
The intercontinental ballistic missile (ICBM) will carry a nuclear or thermonuclear warhead. It will reach its target, on another continent, within minutes after launching. It will approach the target at such a speed that any countermeasures may be very difficult. Its launching sites will be small, relatively cheap to build and maintain, and relatively easy to conceal. Because they can be widely dispersed, they will be difficult to attack even if their location is known. And the ICBM will not face the problem of returning safely to friendly territory after completing its mission, for guided missiles are expendable by design, while our strategic bombers and their crews are not. It is likely that SAC will first be supplemented, then replaced, by the ICBM.
The Navy's Polaris is an intermediate-range ballistic missile (IRBM). Polaris is another important element in our national
defense planning. It will be armed with a nuclear or thermonuclear warhead. While SAC bases and ICBM launching sites are fixed in position, and therefore subject to attack, Polaris will be launched from submerged submarines, whose location the enemy cannot know or predict. The nuclear-powered missile-launching submarine may well be the capital ship of the future.
Modern military aircraft can fly so high and so fast that conventional antiaircraft guns are ineffectual against them. As you know, a gun is not aimed directly at a moving target; it must be so aimed that both the projectile and the target will reach a predicted point at the same time. During the flight time of the projectile, a high-speed high-altitude aircraft will travel several miles. Any slight change of course during that time will take it beyond the lethal range of the projectile burst.
The surface-to-air guided missile is a more effective means of defense against enemy aircraft. The missile can intercept attacking aircraft at greater heights, and greater ranges, than any projectile. And the aircraft is unlikely to escape a missile by taking evasive action. The missile is faster and more maneuverable. If the attacking aircraft changes its course, the missile guidance system will change the course of the missile accordingly, up to the instant of interception.
Guided missiles are becoming increasingly important in aircraft armament. When two jet aircraft are approaching each other head-on, the range closes at a speed between half a mile and one mile per second. Under these conditions it is difficult even to see an enemy aircraft, and hitting it with conventional aircraft weapons would be largely a matter of luck. But the air-to-air missile can "lock on" the hostile aircraft while it is still miles away, and it can pursue and hit the target in spite of its evasive maneuvers.
In the future, the defense of a naval task force against air attack will be somewhat similar to that of an American city or industrial area. The enemy attack will be detected by long-range search radar while the attacking planes are hundreds of miles from the target. Ashore, the early warning radars are located at distant outposts in Canada. At sea, they will be aboard picket vessels at some distance from the main body of the task force. The
first line of defense will probably be interceptor aircraft, which will attack the enemy planes with air-to-air missiles. A second line of defense may consist of moderate range surface-to-air missiles, which will intercept the attacking planes at ranges from about 30 to more than 100 miles. A third line would consist of shorter range missiles, designed to intercept at ranges between about 5 and 20 or 30 miles. Against enemy aircraft that penetrate these three lines of defense, conventional antiaircraft guns will be a last resort.
Because the defense system outlined above is formidable, it is improbable that enemy aircraft will try to bomb our cities, or attack a task force with bombs or torpedoes. Enemy aircraft are more likely to attack with air-to-surface missiles, launched at a range of perhaps a hundred miles.
One question remains: how are we to defend ourselves against enemy intercontinental ballistic missiles, and air -to-ground missiles? Our present surface-to-air missiles such as Nike and Terrier were designed for defense against jet aircraft. But Nike and Terrier are not fast enough for reliable defense against enemy missiles, which will approach at several times the speed of sound. The answer is an anti-missile missile, which will be relatively small, capable of launching on very short notice, extremely fast, and extremely maneuverable. Such missiles are now being developed.
When the anti-missile missile becomes operational it will probably lead to further developments. Our aircraft carry air-to-air missiles for defense against enemy aircraft; an intercontinental ballistic missile might carry air-to-air missiles for defense against other missiles. These might be called anti-anti-missile missile missiles, though if we have the ingenuity to develop such weapons we may be able to think of a shorter name for them.
Such speculations about the future are not very instructive. But this prediction is safe: the effort to develop faster and better missiles, and the race between missiles and missile countermeasures, will continue as long as the threat of war exists, or until some new and unforeseen weapon makes guided missiles obsolete.
1A4. Introduction to missile types
To perform the various functions outlined above, missiles of many different types must be developed. A list, later in this chapter, will show the number of missile types now operational or in various stages of development. It can be assumed that other missiles, not yet announced, are being developed.
The Navy's Sidewinder is a relatively small air-to-air missile with a range of a few miles. A Sidewinder costs about as much as a good used car. It resembles an ordinary aircraft rocket; it differs, of course, in having a guidance system, and movable control surfaces by which the guidance system can control its flight path. At the other extreme, the ICBM has a range of thousands of miles, with size and weight in proportion; its proportional cost is even higher. The ICBM, like most missiles, has the familiar rocket shape. But the Air Force Snark and the Navy's Regulus I, among others, resemble conventional aircraft; they differ in having a guidance system rather than a pilot, and they are designed to dive into their targets rather than release a bomb load and return.
Guided missiles are classified in a number of different ways; perhaps most often by function, such as air-to-air, surface-to-air, or air-to-surface. A nonballistic missile is propelled during all or the major part of its flight time; the propulsion system of a ballistic missile operates for a relatively short time at the beginning of flight; thereafter, the missile follows a free ballistic trajectory like a bullet (except that this trajectory may be subject to correction, if necessary, by the guidance system). Some missiles are designed to travel beyond the earth's atmosphere, and re-enter as they near the target. Others depend on the presence of air for proper operation of the control surfaces, the propulsion system, or both.
Missiles may be further classified by type of propulsion system, such as turbo-jet, ramjet, or rocket; or by type of guidance, such as command, beam-riding, or homing.
1A5. Introduction to missile guidance
The missile guidance system keeps the missile on the course that will cause it to intercept the target. It does this in spite of
initial launching errors, in spite of wind or other forces acting on the missile, and in spite of any evasive actions that the target may take. The guidance system may be provided with certain information about the target before launching. During flight it may receive additional information, either by radio from the launching site or other control point, or from the target itself. On the basis of this information, the guidance system will calculate the course required to intercept the target, and it will order the missile control system to bring the missile onto that course.
From the paragraph above, it might be inferred that the guidance system is an intelligent mechanism that can think. This, of course, is untrue. The missile guidance system is based on a relatively simple electronic computer. But even the most complex computers, such as Univac and other "giant brains," cannot think. Thinking is a conscious process, confined to man and a few of the higher animals. No matter how complex it may be, a computer is simply a machine built so that when certain things happen, certain other things will result. The design of a computer is nothing more than an advanced exercise in the logic of cause and effect. A computer can take no action that isn't built into it by its designer (except, of course, the erratic action that might result from a bad connection or a faulty component).
But in the later chapters of this text you will find many statements such as this: "When Terrier detects an AM signal, it knows it is off the beam center; but it does not know, from the AM signal, which way to go to get back to beam center." We will make such statements without further apology. But it is essential that the students understand what we are doing. We are using a convention, because it saves time and space. Remember that a missile doesn't "know," or "see," or "think," or "decide."
Several distinct types of guidance are possible; a given missile may use one type, or a combination of two or more. Although it can not be called a guided missile, the air-steam torpedo has a simple guidance system. Before launching, its gyro is set for a predetermined course; the gyro holds the torpedo on that course throughout its run to the target. The torpedo is capable of steering itself, but it receives no information after the instant of
launching. This is PRESET guidance. The German V-1 is another example. Before launching, it was set to follow a given course, and to dive on its target after traveling a preset distance.
The German V-2 used a combination of preset and COMMAND guidance. Before launching, it was set to climb vertically for a certain distance, and then turn onto the desired course. Speed and position of the V-2 were determined by a radar at the launching site. This information was analyzed by a computer, which determined when the missile had reached a position and speed that would carry it, along a ballistic trajectory, to its target. At that instant, the missile propulsion system was shut down by radio command.
The Army's Nike surface-to-air missile is a more modern example of command guidance. Throughout the missile flight, radars at the launching site track both the missile and its target. A computer continuously calculates the course that the missile must follow to reach the point of intercept. Throughout its flight, Nike is steered along the desired course by radio commands from the ground.
Sidewinder has a HOMING guidance system. Sidewinder is sensitive to infrared (heat) radiation, and will steer itself toward any strong source of infrared. The exhaust of a jet aircraft is such a source, and Sidewinder can steer itself "right up the tailpipe" of an enemy jet.
Infrared is not the only basis for homing guidance. A missile can be designed to home on light, radio, or radar energy given off by, or reflected from, the target. (It could also, like a homing torpedo, be designed to home on a source of sound waves; but because a guided missile travels at from one to a dozen times the speed of sound, such a system would not be practical.)
Because its source of information is energy given off by the target itself, Sidewinder guidance is an example of PASSIVE homing. Other missiles carry a radar transmitter, "illuminate" the target with a radar beam, and home on the radar energy reflected from the target. This is an ACTIVE homing guidance system. A SEMI-ACTIVE system is also possible; the target is illuminated by a radar beam from the launching site or other control point, and the missile homes on energy reflected from the target.
The Navy's Terrier is similar to Nike in both function and performance; but its guidance system is entirely different. Terrier uses BEAM-RIDER guidance. A radar transmitter at the launching site keeps a narrow beam of radar energy continuously trained on the target. Terrier simply rides up the beam.
Intermediate-range (around 1,500 miles) and long-range (3,000 miles or more) missiles may use a NAVIGATIONAL guidance system. The missile determines its own position in relation to the target, calculates the course required to reach the target position, and steers itself along that course. A missile may be designed to navigate with the help of radio or radar beacons, just as a ship may navigate with the help of Loran. A missile may navigate by dead reckoning, through the
use of an INERTIAL guidance system. It may navigate by taking star fixes through a telescope, or by examining the ground with radar and comparing what it sees with a map. Or it may use a combination of two or more of these methods.
As previously stated, a missile may have more than one type of guidance system, and switch from one to another during its flight. For example, a long-range missile may climb to a preset height and turn onto a preset course shortly after launching, then navigate to the target vicinity, and finally home on the infrared or other energy given off by the target. Or a surface-to-air missile may ride a radar beam until it gets near the target, then switch over to active homing guidance.
B. History of Guided Missiles
The brief sketch that follows will enable the student to view the present-day guided missile in a historical perspective, and to consider the most recent developments in their relation to early experiments. It serves no other purpose; it is not necessary to memorize the dates listed here.
Guided missiles, as defined at the beginning of this chapter, were first used in World War II. But they could not have been built at that time without previous experiments in both propulsion systems and guidance. We will look briefly at early developments in both of those fields. Our latest missiles, of course, are based also on developments in many other fields, including mass production techniques, metallurgy, aerodynamics, radar, and electronic computers; but we cannot describe the evolution of those developments here.
1B2. Propulsion systems
Glide bombs and other gravity-powered missiles are obsolete. And although propeller-driven aircraft, under radio control, have been used as target drones, a propeller-driven guided missile would be too slow to be effective. All current missiles depend on some form of jet or rocket propulsion
In France, in 1909, Guillaume outlined the basic theory of turbo-jet propulsion. In 1927, the Italian Air Ministry built and tested a plane driven by a form of mechanical jet propulsion. The fuselage of this plane was shaped like a tube, with flaring ends. A conventional propeller was mounted in the throat of the tube, forming a "ducted propeller" installation. This craft had good maneuverability and good stability, but in other respects its performance was poor. In 1932 Campini, an Italian, designed and later flew the first plane powered by a thermal jet; it differed from modern jets in using a piston engine, rather than a turbine, as a compressor.
After Campini's successful flight, development of improved jet engines was undertaken in several countries. In England, in 1930, Frank Whittle patented a jet engine based on the principles used in modern jet aircraft. After combustion, the exhaust gases of the jet were used to spin a turbine; the turbine, in turn, drove the compressor. The first successful flight of a turbo-jet powered aircraft was made in England in May, 1941. In the U. S., development of jet engines was turned over to General Electric Company, because of its experience with turbine-driven superchargers. At present, nearly every manufacturer of aircraft engines is developing and building turbo-jet engines.
The pulse-jet engine uses the forward motion of the missile or aircraft, rather than a turbine, to compress the air and fuel vapor before combustion. The pulse-jet principle was patented by a German engineer in 1930, and further developed by Bleeker, an American, in 1933. The pulse-jet engine was much improved by the Germans during World War II, and was used to power their V-1 guided missile.
The ram-jet also depends on forward motion for compression, but it differs from the pulse-jet in having no moving parts. The basic idea of a ram-jet was patented by Rene Lorin, a French engineer, in 1913. This was followed by a Hungarian patent in 1928, and another French patent, by Leduc, in 1933. None of these patents resulted in a workable ram-jet engine. The basic ideas were sound; but successful development of a ram-jet engine had to wait for extensive data on the behavior of fluids at extremely high speeds. In June of 1945, the Applied Physics Laboratory of the Johns Hopkins University made the first successful ram-jet flight, in the course of developing a power plant for the Navy's Talos missile.
Turbo-jets, pulse-jets, and ram-jets all depend on the presence of air for the combustion of their fuel. Consequently, none of them can operate beyond the earth's atmosphere. Rockets, on the other hand, carry their own source of oxygen for combustion; and they operate even more efficiently in a vacuum than they do in air. Rocket-propelled vehicles are theoretically capable of flight to the moon and the planets.
The principle of rocket propulsion has been known for nearly 2,000 years. In the Far East, rockets were used in warfare as early as the 13th century. Several western armies used rocket projectiles in the early part of the 19th century, but not very effectively. They seem to have been of more value in frightening the enemy than in doing physical damage. The British used rockets in their attack on Washington in 1812; and in the Star Spangled Banner, Francis Scott Key referred to the "rocket's red glare" during the bombardment of Fort McHenry. (But some historians believe that the British were using rockets as signals, rather than weapons.) Military interest in rockets lapsed after the middle of the 19th century, because developments in gunnery made gun projectiles superior to rockets in range, and far superior in accuracy.
Among rocket engineers, Robert H. Goddard is known as the "Father of Rocketry." Goddard was born in Massachusetts in 1882. By the time he earned his Bachelor of Science degree in 1908, he was obsessed by thoughts of rockets and rocket propulsion. He believed, quite correctly, that rocket propulsion would be the most suitable means for sending measuring instruments to the top of the earth's atmosphere, and eventually to the moon. Up to that time no one had investigated the physics of rocket propulsion, and no one had worked out the necessary mathematics. Goddard decided to do both.
Before Goddard's experiments, rockets consisted of a quantity of propellant packed in a cylindrical tube. Goddard discovered that by forming the after end of the tube into a smooth, tapered nozzle, he could increase the ejection velocity of the combustion gases eight times, without increasing the weight of the fuel. According to Goddard's calculations this would, for a given weight of fuel, drive the rocket eight times as fast and sixty-four times as far.
Goddard was given a Navy commission in 1917, and assigned to the job of improving the Navy's signal rockets. This assignment enabled him to continue his development of rocket theory. After the war he summarized his theories and experience in a paper called A Method of Reaching Extreme Altitudes. This report was published by the Smithsonian Institution in 1920. It consisted almost entirely of equations, formulas, and tables, but it contained one statement of general interest. It proposed the idea of multi-stage or step rockets that is, one rocket carrying another-and said that by this means a rocket could be sent to the moon, where it could explode a charge of flash powder to make a light visible from the earth.
During the twenties and early thirties, Goddard continued his experiments with the help of a small salary (as professor of physics at Clark University) and grants from the Guggenheim and Carnegie Foundations. His list of accomplishments is impressive. We have mentioned his idea of multi-stage rockets, and his design of the tapered nozzle. He was the first to suggest that a liquid-fueled rocket could provide the sustained thrust necessary for sending a vehicle into space. He was the first to actually launch a successful
liquid-fueled rocket. (That was on 16 March, 1926; the rocket reached an altitude of 184 feet.) He proved, first by calculation and later by experiment, that rocket propulsion can be used in a vacuum. He was the first to fire a rocket that traveled faster than sound; he was the first to develop a gyroscopic steering mechanism for rockets; and he was the first to use vanes in the jet exhaust stream to stabilize the rocket during the first phase of its flight.
But Goddard was forced to end his experiments in 1935, for lack of funds. During World War II he again worked for the Navy, this time to develop rockets to aid the take-off of the Navy's flying boats. He died in 1945.
A group or rocket enthusiasts, inspired by Goddard's experiments, formed the American Rocket Society in 1930. During the thirties this group performed a number of important experiments with rocket motors, but their work was limited in scope by lack of money.
Hermann Oberth is a German counterpart of Goddard. Like Goddard, he worked on the physics and mathematics of rocket propulsion during the first World War. There is good evidence that he independently conceived the idea of multiple-stage liquid fuel rockets. He read Goddard's report shortly after it was published, and in 1923 published a book of his own, called The Rocket into Interplanetary Space. Goddard's principal interest was in scientific exploration of the upper atmosphere; but to Oberth, every improvement in rocketry was simply a step toward the eventual development of space ships. Oberth's book discussed the possibility of putting an artificial satellite into orbit around the earth. (Except for a science-fiction story published in 1870, that was the first time this idea had been expressed in print.) Oberth believed that passengers could travel to and from the satellite in smaller "landing rockets." In this way, the satellite could be transformed into a manned space station, which could ultimately serve as a launching point for space ships. Neither Goddard nor Oberth mentioned the possible use of rockets as military weapons.
The German "Society for Space Travel, Inc." was organized in 1927, with Oberth as president and Willy Ley as vice president. The society began at once to experiment with liquid-fueled rocket engines. The rockets carried two tanks-one of gasoline and one of
liquid oxygen. These two liquids had to be fed simultaneously, and in the right proportions, to the combustion chamber, where they were mixed and burned. Most of the attempted launchings ended in failure, for one of two reasons. First, liquid oxygen is extremely cold; it froze the valves, so that they refused to open or close at the proper time. (There is still no completely reliable solution for this problem.) Second, the combustion temperature was so high that the rocket burned up after a few seconds. In later experiments, the combustion chamber was surrounded by a cooling jacket filled with water. With this model, the society launched a number of rockets that burned for about thirty seconds, and reached an altitude of half a mile or more. The next step was to omit the water from the cooling jacket, and circulate the fuel through the jacket before burning it. When the society tried to launch such a rocket, using gasoline as fuel, it immediately exploded. Ley suggested using ethyl alcohol, slightly diluted with water, in place of gasoline. This system worked very well. The same system, and the same fuel combination, were later used in the German V-2 missiles, the American Viking rockets, and the rocket-propelled experimental planes X-1 and X-1A.
The Versailles peace treaty limited the German army to 100,000 men; it was forbidden to have aircraft or antiaircraft guns, or field artillery of more than 3-inch caliber. This may explain why the German army took an early interest in rocket development; the treaty of Versailles didn't mention rockets at all. In 1932, the army established a small research project under the direction of Captain (later General) Walter Dornberger, to develop liquid-fueled rockets for use as weapons. No one in Germany had any experience with rocket propulsion, except the members of the Society for Space Travel. Dornberger visited the society, and hired a very young member named Wernher von Braun.
The team of Dornberger and von Braun, with a small staff of assistants, began to test rocket motors on an artillery testing range near Berlin. In December of 1934, they succeeded in firing two rockets to a height of about 6,500 feet. This news eventually filtered up to the high command. In 1936, General von Fritsch went to the test range for a demonstration. The general was impressed. The result
was anew and much bigger research institute-the Peenemunde Project.
1B3. Guidance systems
The history of guidance systems is short. All of the significant developments are recent, principally because the state of electronics before the nineteen forties was relatively primitive.
The Americans developed a flying bomb called the Bug during the first World War; it was simply a pilotless aircraft, with a range of about 400 miles. The Bug was ready for production by the middle of 1918. But by that time it was apparent that the war would be over in a few months, and the Bug was never produced. Its accuracy would have been poor; it had no guidance system. But the Bug led to the suggestion that pilotless aircraft could be controlled by radio. Beginning in 1924, both the Army and Navy experimented with radio-controlled planes. Several moderately successful flights were made, with the pilotless plane controlled by radio from a parent plane that flew nearby. This project was dropped in 1932 for lack of money.
In 1935, an American high-school student named Walter Good built and flew a radio-controlled model airplane. This was the first time on record that a plane of any kind had been successfully launched, flown, and landed while under complete radio control from the ground. One of the problems that plagued the armed forces was stabilization-keeping the aircraft on an even keel so that it could respond properly to radio commands. Because a well-built model airplane is inherently stable, Good didn't have to worry about this problem. His contribution was to design and build a miniature radio receiver coupled to the control surfaces through a miniature servo system.
The Army and Navy resumed their experiments with radio command during the late thirties, and by 1940 both had developed radio-controlled planes for use as target drones. As we will note below, missiles with elementary preset and command guidance were used during World War II. But successful beam-riding, radar and infrared homing, hyperbolic, and inertial guidance systems are all postwar developments.
It may be worth mentioning at this point that many of the pioneers in the fields of missile guidance and propulsion are still (in 1959) actively at work on guided missile development. Dr. Walter Good is working at the Applied Physics Laboratory of Johns Hopkins University, where he helped to develop the guidance system for the Navy's Terrier. Dornberger is head of the missile department of Bell Aircraft in Buffalo, N. Y. Wernher von Braun is chief of the U. S. Army's Ballistic Missile Agency in Huntsville, Alabama, and Oberth is one of his assistants. And Willy Ley is probably the world's most popular author on the subjects of rockets, missiles, and space travel.
1B4. Guided missiles in World War II
During World War II, the Japanese developed and used two devices of interest in the history of guided missiles. One of these was an air-launched, radio-controlled, rocket-assisted glide bomb. Its performance was limited. It had to be launched from a plane at low altitude, within two and a half miles of the target. This made the launching planes highly vulnerable to antiaircraft fire, especially after we began to use the proximity fuze. The Japanese dropped this project before the end of the war.
The second Japanese missile was the baka bomb. This was a rocket-propelled glide bomb designed for use against shipping. It carried a human suicide pilot; for this reason we can't call it a true guided missile. The baka bomb had poor maneuverability, and because of this we were able to shoot down a great many of them with antiaircraft fire.
Of the guided missiles used during World War II, those made by the Germans were the most advanced, and the most effective. The V-1 and V-2 are familiar to nearly everyone. The V-1 was developed early in the war, and was successfully flight-tested at Peenemunde as early as the spring of 1942. By 1943, the Peenemunde center was working on 48 different antiaircraft missiles. Because of this dilution of effort, progress was slow. The work was later consolidated into 12 projects in an effort to get the missiles into production in time to influence the outcome of the war.
The V-1 was a robot bomb-a pulse-jet midwing monoplane with a conventional airframe and tail construction. It used gyro
stabilization and preset compass guidance. It was launched from a ramp with the help of boosters, and had to reach a speed of about 200 mph before its engine developed enough thrust to keep it airborne. V-1 missiles were launched against England in large numbers, and their 1-ton warheads did enough damage to have a serious effect on morale. But the V-1 missiles were slow, and after proximity fuzes were rushed to England to combat them, about 95% of them were brought down by antiaircraft fire.
The V-2 was a large missile, with a length of 46 ft 11 in., and a diameter of 5 ft 5 in. Its total weight at launching was over 14 tons, including a 1650-pound warhead. The V-2 was propelled by liquid-fuel rockets. It was launched vertically, and preset to tilt over to a 41- to 47-degree angle a short time after launching. When it reached a speed calculated to take it to the target, its propulsion system was shut down by radio command, and it then traveled a ballistic trajectory. Its accuracy was not high, and its maximum range was only about 200 miles. But it descended almost vertically on its target, at speeds of from 1800 to about 3300 mph. Active countermeasures against it were impossible; no V-2 missile was ever intercepted, or shot down by antiaircraft fire. If armed with a nuclear warhead, the V-2, within the limits imposed by its range, would be a formidable weapon even now.
Five other German missiles, which were in various stages of final testing when the war ended, are worth a brief mention:
Rheinbote was a surface-to-surface missile propelled by a three-stage rocket, with booster-assisted take-off. Its overall length was 37 ft; length of the third stage was 13 ft. The third stage carried 88 pounds of explosive; it reached a speed of over 3200 mph about 25 seconds after launching, and had a range of about 135 miles.
Wasserfall was a supersonic surface-to-air missile, propelled by a liquid-fuel rocket and guided by radio command. Length: 25 ft; weight: 4 tons; speed: 560 mph; range: 30 miles; war head: 200 pounds.
Schmetterling was a smaller version of Wasserfall, intended for use against low-altitude targets at ranges up to 10 miles. It carried a 55-pound warhead.
Enzian was another surface-to-air missile, designed for use against large bomber formations. Its length was about 12 ft, and wingspan about 14 ft. It carried about 1000 pounds of explosive. It was propelled by a liquid-fuel rocket, and was launched with four solid-fuel booster rockets.
The X-4 was an air-to-air missile designed for launching from fighter aircraft as shown in figure 1B1. It was propelled by liquid-fuel rocket, and stabilized by four fins placed symmetrically. Length: about 6-1/2 ft; span about 2-1/2 ft; range 1-1/2 miles; speed 560 mph at an altitude of 21,000 ft. The X-4 was guided by commands from the launching aircraft, through a pair of fine wires that unrolled from two coils mounted on the tips of the missile fins. The X-4 was successfully tested before the end of the war, but it was not used in combat.
In the United States, the Army Air Corps began the development of guided glide bombs in 1941. Azon was a vertical bomb controlled in azimuth only; it was put into production in 1943. Razon, a bomb controlled in both azimuth and range, was started in 1942 but not completed until the end of the war. The limits of control of Azon and Razon bombs are indicated in figure 1B2. A medium-angle glide bomb called the ROC, and a 12,000-pound bomb called Tarzon, both controlled in azimuth and range, were developed during the war but were not used in combat. The Tarzon project was dropped in 1946 but resumed in 1948, and Tarzon was used successfully during the Korean war.
In 1944, we carried out a glide-bomb mission against Cologne, Germany, and a majority of the bombs reached the target area. In this same year remote television-control equipment was developed and installed in bombing aircraft. These aircraft were used to control television-sighted, explosive-laden bombers unfit for further service. These radio-controlled bombers saw some service over Germany in the "Weary Willie" project.
Our first jet-propelled missile was a radio-controlled flying wing; a later version was a copy of the German V-1, with a few improvements.
By the end of the war, the Navy had a number of guided missile projects in various
Figure 1B1.-Launching of an X-4.
Figure 1B2.-Limits of control of the Azon and Razon.
stages of development. The Gargoyle was an air-launched, powered, radio-controlled glide bomb with a flare for visual tracking. Another Navy glide bomb, the Glomb, carried a television monitor through which the pilot of the launching aircraft could observe its approach to the target; it was guided by radio command. The Loon was a U. S. Navy version of the German V-1, intended for shore bombardment. The Gorgon IIC was propelled by a ram-jet engine, tracked by radar, and guided by radio command. In 1944, the Navy assigned development of the Bumblebee project to the Applied Physics Laboratory of the Johns Hopkins University. This project has produced Terrier, Talos, and Tartar, as well as the now discontinued Triton.
1B5. Missile developments after World War II
As we have shown, the principal guided missile developments during World War II were German. The United States lagged far behind. Japanese and British missile developments were insignificant, and as far as we know the Russians had none at all. In 1945, the Russians captured most of the production engineers and technicians of the V-2 project, as well as several tons of missile data and perhaps a few V-2 missiles. We were luckier than the Russians. The design staff of the Peenemunde project, including von Braun and his principal assistants, took pains to surrender to the Americans rather than to the
Russians. And we captured and shipped to the proving ground at White Sands, New Mexico, enough intact V-2' s and spare parts to make, eventually, about 70 complete missiles.
During the first few years after the war, both American and Russian missile effort was partially devoted to assimilating the German developments. Our own experiments with the captured V-2' s provided valuable training for launching crews, and valuable knowledge of missile engineering. Our "V-2 Program" ran from March 1946 to June 1951. One of its principal successes was a high-altitude record of 250 miles, achieved by a WAC-Corporal
missile boosted by a V-2. This record stood for many years. (At the moment of writing, the high-altitude record is about 80,000 miles; we expect this figure to look small by the time you read this.)
Postwar missile development has been rapid. Many missiles are now operational; many others have been abandoned at various stages of development, or rendered obsolete by more advanced weapons. We will not try to cover these developments here; a list of obsolete missiles would probably be longer than a list of those now current.
C. Classification of American Guided Missiles
Although missiles are popularly known by their names, such as Sidewinder or Terrier, every missile is assigned a designation consisting of letters and numerals. The first three letters indicate the intended use of the missile:
These three letters are followed by a hyphen and then a service letter: A for Air Force, N for Navy, and G for Army. After another hyphen comes the model number, followed by a lowercase letter indicating successive modifications. A one-letter prefix may be used to indicate the status of the missile: X means experimental, Y means service test, and Z
means obsolete. No prefix is used if the missile is operational. For example:
ASM-G-3c-An air-to-surface missile used by the Army; third model and third modification, operational.
XSAM-N-7-A surface-to-air missile used by the Navy, seventh model, experimental. (This designation was used for an early version of Terrier.)
All missiles in service, as well as most of those still under development, have been given popular names. Some of these names follow this pattern:
AAM-Winged creatures (except birds of prey and game birds). Example: Sparrow.
SAM-Mythological terms. Example: Talos.
SSM-Astronomical terms. Example: Polaris.
At the present time, most missiles appear to be exceptions to the above "rules." For example, Sidewinder and Bullpup are not winged creatures; Terrier is not a mythological term; Snark, Thor, Lacrosse, and Dart are not astronomical terms.
D. Current American Service Missiles
Because of the rapid developments in the guided missile field, the lists given below will be out of date before you can read them. Some of the missiles listed may have become obsolete. Others, now under development, will probably be announced.
1D2. Army missiles
Nike-Ajax SAM is the Army's first supersonic anti-aircraft guided missile. It is designed to intercept and destroy attacking enemy aircraft regardless of evasive action. Nike guided missile units are now deployed around vital industrial, highly populated, and strategic
areas of the United States. Nike-Ajax is about 20 ft long and 1 ft in diameter, with two sets of fins for guidance and steering. It is boosted to supersonic speed by a solid-propellant booster, and maintained by a liquid-fuel sustainer motor. The missile and booster together weigh more than a ton. There are 12 launchers in each Nike battery, which is operated by about 100 officers and men.
Corporal SSM may be equipped with either a nuclear or a conventional warhead. It can engage tactical targets at ranges of 75 miles or more. Corporal gives the Army field commander great firepower on the battlefield, and enables him to strike selected targets deep in enemy rear areas. Corporal follows a ballistic trajectory during most of its flight; weather and visibility conditions place no restriction on its use. The propulsion system uses a liquid-fuel rocket motor. The missile travels through space at several times the speed of sound. A Corporal battalion has 250 men. Each battalion has two batteries-a firing battery and a Headquarters Service battery. There are two operational launchers to a battalion. Corporal battalions are now deployed in Europe.
Sergeant SSM is a ballistic guided missile intended to replace Corporal, with improvements in power, range, and accuracy.
Redstone SSM is a supersonic ballistic missile with a range of several hundred miles, designed to extend and supplement the range and fire power of Army artillery.
Jupiter SSM is the Army's intermediate-range ballistic missile. Its range is in the order of 1,500 miles, and it is propelled by a liquid-fuel rocket.
Lacrosse SSM is used in close tactical support of ground troops. It is an all-weather missile, propelled by a solid-fuel rocket motor, and capable of carrying highly effective area-type warheads. It will supplement, and perhaps eventually replace, conventional artillery. The Lacrosse system includes the missile, a launcher mounted on a standard Army truck, and other ground equipment.
Dart SSM is a guided anti-tank missile, propelled by a solid-fuel rocket, and designed for use by front-line troops. It carries a warhead capable of defeating the heaviest known enemy armor, and delivers this warhead with pinpoint accuracy. The Dart can be launched
by a lightweight launcher from a variety of vehicles.
Nike-Hercules SAM is capable of carrying a nuclear warhead; it is designed for use against either single aircraft or whole formations of aircraft. The missile is 27 ft long, the booster 14-1/2 ft long. Both use solid propellant. The warhead is provided with a safety feature, so that it can detonate only at altitudes sufficiently high to prevent damage to friendly surrounding terrain.
Hawk SAM is designed to supplement the Nike missile system by destroying attacking aircraft at low altitudes. The launching facilities are sufficiently portable to be used by fast-moving combat troops. Hawk is propelled by a solid-fuel rocket. The missile is about 17 ft long, and about 14 in. in diameter.
Nike-Zeus SAM is an anti-missile missile equipped with a nuclear warhead, and designed to defend the United States against attack by enemy intercontinental ballistic missiles.
Talos SAM Defense Unit is a land-based version of the Navy's Talos, Shipboard Missile System.
1D3. Air Force missiles
Matador SSM is a tactical missile driven by a turbojet engine at a speed of 650 mph. It has a length of about 29 ft and a wing span of about 40 ft. It can carry a nuclear warhead, and may be guided by radio command or by a navigational system. Its range is more than 650 miles. Tactical missile groups armed with Matador are now deployed in Europe and on Formosa.
Falcon AAM comes in two versions, one with radar guidance and the other with infrared homing. Falcon is a supersonic missile, propelled by a solid-fuel rocket. It weighs about 100 pounds, and is about 6 ft long.
Genie AAM is a rocket-propelled air defense missile that may be armed with a nuclear warhead.
Snark SSM is actually a pilotless aircraft. It can carry a nuclear warhead at high subsonic speed. Snark has an inertial or other navigational guidance system. In tests it has been accurately placed on a target at a range of 5,000 miles.
Rascal ASM is a rocket-powered missile 32 ft long and 4 ft in diameter. It is designed for launching from B-47 Stratojet bombers at high altitude and high speed, at
such a distance from the target that the bombers and crews are not exposed to local defenses.
Bomarc SAM is a long-range air defense missile that can destroy attacking aircraft at ranges of more than 100 miles, and altitudes above 60,000 ft. The missile is about 47 ft long, has a wing span of about 18ft, and weighs about 15,000 pounds. It is launched vertically by solid-fuel boosters, and sustained in flight by twin ram-jet engines.
Thor SSM is the Air Force's intermediate range ballistic missile (IRBM). It is propelled by a liquid-fuel rocket at a speed of Mach 10; range is over 1,500 miles. Thor is provided with an inertial guidance system.
Atlas SSM is an intercontinental ballistic missile with a range of more than 5,000 miles. It is launched by rocket engines that develop many tons of thrust, and millions of horsepower, within a few seconds. Atlas reaches a top speed of about Mach 15-more than 10,000 miles an hour. It will descend on its target at that speed, from a height of about 800 miles.
Titan SSM is also an intercontinental ballistic missile. In general it is similar to Atlas, except that Titan has a second-stage motor. It is likely that one of these two missiles will be discontinued, and all the effort now going into both developments will be concentrated on only one of them.
Minuteman SSM is a solid-fuel intercontinental ballistic missile that will eventually replace both Atlas and Titan.
1D4. Navy missiles
Sidewinder AAM (fig. 1D1) is probably the
simplest and cheapest of all guided missiles. It is about 9 ft long, and weighs about 155 pounds. It has only about 24 moving parts, and no more electronic parts than a table radio. It attains a speed of Mach 2 relative to the launcher, and a range of several miles; it is designed to destroy high-performance aircraft from sea level to altitudes above 50,000 ft. It has an infrared homing system. Sidewinder was named after a desert rattlesnake. (The Sidewinder snake, like all of the pit vipers, has infrared receptors on its head that enable it to detect the presence of prey by its body heat.) Sidewinder is now the primary airborne missile used by squadrons in
Figure 1D1.-Sidewinder missile and pilot with pressure suit.
the Sixth Fleet in the Mediterranean, and the Seventh Fleet in the Western Pacific.
Sparrow I AAM is 12 ft long and weighs 300 pounds; it reaches a speed of Mach 2.5 relative to the launcher, within a few seconds after launching. It is provided with beam-rider guidance, and is propelled by a solid-fuel
rocket. Navy planes can carry two to four of the missiles, and can fire them singly or in salvos.
Sparrow II AAM was developed as an experimental missile, and not intended to become operational. It has, however, been adopted for operational use by the Royal Canadian Air Force.
Sparrow III AAM is very similar to Sparrow I, but with a much more sophisticated guidance system. It is slightly heavier than Sparrow I, a little faster, and has a longer range. It will first supplement, and then replace Sparrow I in the Fleet.
Petrel ASM is nearly obsolete; although a few of these missiles may still be found in the Fleet, they are no longer in production. Petrel
is a subsonic missile with radar homing, powered by a turbojet engine. Its payload is not a warhead, but a homing torpedo.
Bullpup ASM is 11 ft long and weighs about 540 pounds. It is relatively inexpensive, simple in design, and extremely accurate. Bullpup is a tactical missile with a conventional warhead, designed for use by carrier-based aircraft against small targets such as pillboxes, tanks, and truck convoys, in support of ground troops. It is powered by a solid-fuel rocket, and has a range of 15,000 ft at a speed of Mach 2.
Corvus ASM is a supersonic missile, propelled by a solid-fuel rocket, and designed for use by carrier-based aircraft. No further details have been released.
Figure 1D2.-Terrier missiles and launchers.
Terrier SAM (fig. 1D2) is a supersonic beam-riding antiaircraft missile with a range of more than 10 miles. It is launched by a solid-fuel booster rocket, and propelled by a solid-fuel sustainer rocket. Terrier is about 15 ft long without its booster, and weighs 1-1/2 tons. Terrier batteries have been installed on the guided missile cruisers Boston and Canberra, and on the guided missile destroyer Gyatt. Terriers will be used by the aircraft carriers Kitty Hawk and Constellation, cruisers Topeka, Providence, and Springfield, the nuclear cruiser Long Beach, and frigates Farragut, Luce, MacDonough, Coontz, King, Mahan, and Dewey.
Tartar SAM is similar in function to Terrier; except it is propelled by a dual-thrust rocket, and is launched without a booster. The Tartar system will be installed aboard the guided missile destroyers 2 through 14, and aboard the cruisers Chicago, Albany, and Fall River.
Talos SAM (fig. 1D3) is designed to bring down attacking enemy aircraft and missiles at ranges of 65 miles or more. It is 20 ft long, and weighs 1-1/2 tons. It is launched with solid-fuel boosters, and sustained in flight by a ram-jet; it reaches a speed in excess of Mach 2 within about 10 seconds of launching. It can be armed with a nuclear warhead. During the first part of its flight, Talos is a beam rider. As it approaches its target, it switches over to homing guidance. Talos systems are now installed on the cruisers Galveston, Little Rock, and Oklahoma City. It is scheduled for installation on the cruisers Albany, Fall River, and Chicago, and on the nuclear powered cruiser Long Beach.
Regulus I SSM (fig. 1D4) is intended primarily for use against enemy shore installations, but it can also be used against ships. Regulus I is about 30 ft long, and resembles a conventional swept-wing fighter aircraft. It is powered by a turbojet engine, and flies at the speed of Mach 1 for a range of about 500 miles. It can be armed with a nuclear warhead. Launching equipment for this missile can be installed in a short time on several types of ships, at relatively low cost and with little modification of the ship itself. Among the ships that can now launch Regulus I are the cruisers Macon, Helena, Toledo, and Los Angeles, submarines Tunny and Barbero, and carriers Randolph, Hancock, Forrestal,
Figure 1D3.-Talos missiles on launcher at White Sands Proving Ground.
Saratoga, Lake Champlain, Franklin D. Roosevelt, Lexington, Bennington, Bon Homme Richard, and Shangri-La. Two different versions of Regulus I have been developed. The tactical version is nonrecoverable. The test version is provided with retractable landing gear and parachute braking. It can be launched and recovered repeatedly-a factor that drastically reduces the cost of evaluating and testing the missile system.
Regulus II SSM (fig. 1D5) is 57 ft long, and has a 20-ft wingspan. It is propelled by a turbojet engine with afterburner at a speed of Mach 2. Its range is more than 1,000 miles, and its altitude capability more than 60,000 ft. Like Regulus I it can carry a nuclear warhead, and is made in both a tactical and a recoverable test version. Regulus II may be guided by either a command system or an inertial navigation system. Unlike ballistic missiles, which are capable of only one path of approach to a target, Regulus can be guided to its target
Figure 1D4.-Regulus I on fantail of USS Helena (CA-75).
in any of a number of ways. For example, it may approach at 60,000 feet, and then descend to 500 feet when it approaches within 50 miles of the target. It may power-dive vertically onto the target from 60,000 feet. Or it may approach the target at low altitude, and then climb to high altitude before diving. It can change target in mid-flight. It can be launched at the first indication of enemy attack, and then called back in the event of a false alarm.
However, due to budgetary limitations the Regulus II program has been cancelled. The
remaining Regulus II missiles will be used in other missile programs as high speed targets.
Polaris SSM-USM (fig. 1D6) is the Navy's intermediate-range ballistic missile, with a range of about 1,500 miles. It is designed for launching either from surface ships or from submerged submarines. It is propelled by solid fuel. There are, at present, plans to build nine submarines capable of launching Polaris. Each submarine will carry 10 or more missiles.
Figure 1D5.-Regulus II missiles (test version).
CHAPTER 2 FACTORS AFFECTING MISSILE FLIGHT
A guided missile, by definition, flies above the surface of the earth. Aerodynamic long-range missiles, as well as all missiles of short and medium range, are subject throughout their flight to the forces imposed by the earth's atmosphere. Ballistic missiles, though they follow a trajectory that takes them into space, must climb through the atmosphere after launching, and must descend through it before striking the target. All missiles are subject to gravitational and inertial forces. This chapter will briefly discuss the principal forces that act on a guided missile during its flight. It will show how the missile trajectory may be controlled by designing the missile airframe and control surfaces to utilize or overcome the forces acting on them.
An understanding of missile aerodynamics requires a familiarity with several of the
basic laws of physics. These laws will be briefly summarized. A detailed study of air in motion, and the mathematical analysis of the various forces present, are beyond the scope of this text. The discussion will be general and qualitative, and no mathematical development will be attempted.
In general, missile aerodynamics are the same for both subsonic and supersonic flight. The basic requirement is common to all craft intended to fly: in order to fly successfully, the craft must be aerodynamically sound. But the high speeds and high altitudes attained by current guided missiles give rise to new problems not encountered by most conventional aircraft. An example is the shock wave that is produced when a flying object attains the speed of sound. Problems of oxygen supply for air breathing missiles arise at high altitudes, and problems of skin heating by friction with the air arise at high speeds.
B. Physics of Flight
2B1. Forces acting on a missile in flight
Gravity, friction, air resistance, and other factors produce forces that act on all parts of a missile moving through the air. One such force is that which the missile exerts on the air as it moves through it. In opposition to this is the force that the air delivers to the missile. The force of gravity constantly attracts the missile toward the earth, and the missile must exert a corresponding upward force to remain in flight.
Figure 2B1 illustrates the forces acting on a body in level flight through the air, at a uniform speed. Note that the force tending to produce motion (toward the left) exactly balances that resisting the motion. The force of gravity is exactly opposed by the lifting force. In accordance with Newton's first law (discussed below), a moving body on which all forces are balanced will continue to move in the same directions and at the same speed.
Figure 2B1.-Forces acting on a body moving through air.
Figure 2B2 illustrates the effect of unbalanced forces acting on a body. The length of the arrows is proportional to the respective magnitude of the forces, and the arrowheads point in the direction in which these forces are applied. The illustration shows that forces
Figure 2B2.-Unequal forces acting on a body.
A and B are equal and opposite, and that C and D are equal and opposite. But force F is opposite to and greater than force E. As a result, the body shown will accelerate in the direction of force F. This figure is an example of vector representation of the forces acting on a body. Any number of forces may be shown by vector representation. They can be resolved, or simplified, into resultant force that is the net effect of all the forces applied.
2B2. Relativity of motion
To an observer standing on the ground and watching the flight of a missile through the air, it appears that the missile is moving and the air standing still. It would seem that the opposing force exerted by the air is entirely the result of the missile motion through it. But if it were possible for an observer to ride the missile itself, it would appear that the missile is standing still, and that the air is moving past the missile at high speed.
This illustrates the basic concept of relativity of motion. The forces that the air exerts on the missile are the same, regardless of which is considered to be in motion. The force exerted by the air on an object does not depend on the absolute velocity of either but only on the relative velocities between them. This principle can be put to good use in the study of missile aerodynamics, and in the design of missile airframes and control surfaces. In a wind tunnel, the missile or model remains stationary, while air moves past it at high speed. The measured forces are the same as those that would result if the missile, or model, were moving at the same relative speed through a stationary mass of air.
2B3. Newton's laws of motion
Newton's first law states: "A body in a state of rest remains at rest, and a body in motion remains in uniform motion, unless acted upon by some outside force." This means that if an object is in motion, it will continue in the same direction and at the same speed until some unbalanced force is applied. And, whenever there are unbalanced forces acting on an object, that object must change its state of motion. For example, if you were to push against a book lying on a table, you would have to supply sufficient force to overcome friction in order to set the book in motion. If you would eliminate all of the restraining forces acting on the book once it is in motion, it would continue to move uniformly until acted upon by some outside force. It is these restraining forces with which we are mainly concerned in the study of aerodynamics.
Newton's second law states: "The rate of change in momentum of an object is proportional to the force acting on the object, and in the direction of the force." The momentum of an object may be defined as the force that object would exert to resist any change of its motion.
Newton's third law states: "To every action there is an equal and opposite reaction." This law means that when a force is applied to any object, there must be a reaction opposite to and equal to the applied force. If an object is in motion, and we try to change either the direction or rate of that motion, the object will exert an equal and opposite force. That force is directly proportional to the mass of the object, and to the change in its velocity. This can be stated as:
Force = Mass times Acceleration,
F = ma
Thus any object in motion is capable of exerting a force. Whenever a force is applied through a distance, it does work. We can express this as:
Work = Force times Distance
W = Fd
Figure 2B3.-Forces acting on a flat surface in an airstream.
Any mass that is in motion is capable of applying a force over a distance, and therefore of doing work. Whenever the motion of a mass is changed, there is, in accordance with Newton's second law, a change in momentum.
2B4. Lift and drag
Figure 2B3 represents a flat surface moving through an airstream. In accordance with the principle of relativity, the forces acting on the surface are the same, regardless of whether we think of the surface as moving to the left, or of the airstream as moving to the right. One of the forces acting on the surface is that produced by friction with the air. This force acts in a direction parallel to the surface, as indicated by the small white arrow at the lower right. As the air strikes the surface, it will be deflected downward. Because the air has mass, this change in its motion will result in a force applied to the surface. This force acts at a right angle to the surface, as indicated by the long black arrow in figure 2B3. The resultant of the frictional and deflection forces, indicating the net effect of the two, is represented by the long white arrow. We can resolve this resultant force into its horizontal and vertical components. The horizontal component, operating in a direction opposite to the motion of the surface, is drag. The vertical force, operating upward, is lift. The angle that the moving surface makes with the air stream is the angle of attack. This angle affects both the frictional and the deflection force, and therefore affects both lift and drag.
Bernoulli's theorem states that the total energy in any system remains constant. Air flowing past the fuselage or over the wing of a guided missile forms a system to which this theorem can be applied. The energy in a given air mass is the product of its pressure and its velocity. If the energy is to remain constant, it follows that a decrease in velocity will produce an increase in pressure, and that an increase in velocity will produce a decrease in pressure.
Figure 2B4 represents the flow of air over a wing section. Note that the air that passes over the wing must travel a greater distance than air passing under it. Since the two parts of the airstream reach the trailing edge of the wing at the same time, the air that flows over the wing must move faster than the air that flows under. In accordance with Bernoulli's theorem, this results in a lower pressure on the top than on the bottom of the wing. This pressure differential tends to force the wing upward. and gives it lift.
Figure 2B4.-Airflow over a wing section.
Figure 2B4 represents the general shape of a section of the wing of a conventional aircraft. In such an aircraft, the major part of the necessary lift is provided by the Bernoulli effect. As we will explain later, a wing of this shape is not suitable for use on missiles flying at or above the speed of sound. None of the Navy missiles listed in chapter 1 depends on a wing of this shape for lift. All of them get the necessary lift entirely from the angle of attack, as illustrated in figure 2B3.
Boundary layer refers to a condition that occurs as a result of friction between an airfoil surface and the air moving past it. The air tends to cling to the surface. This is a serious problem in missile design. Since lift depends on the flow of air past the surface, reduction in flow produces a reduction in lift. The boundary layer effect has been overcome to some extent by using highly polished surfaces, as free as possible from any irregularities.
C. Aerodynamic Forces
A discussion of the problems of aerodynamic forces involves the use of several flight terms that require explanation. The following definitions are intended to be as simple and basic as possible. They are not necessarily the definitions an aeronautical engineer would use.
AIRFOIL. An airfoil is any structure around which air flows in a manner that is useful in controlling flight. The airfoils of a guided missile are its wings or fins, its tail surfaces, and its fuselage.
DRAG is the resistance of an object to the flow of air around it. It is due in part to the boundary layer, and in part to the piling up of air in front of the object. One of the problems of missile design is to reduce drag while maintaining the required lift and stability.
STREAMLINES are lines representing the path of air particles as they flow past an object, as shown in figure 2B4.
WING SPAN is the measured distance from the tip of one wing to the tip of the other.
ATTITUDE. This term refers to the orientation of a missile with respect to a selected reference.
STABILITY. A stable body is one that returns to its initial position after it has been disturbed by some outside force. If outside forces disturb a stable missile from its normal flight attitude, the missile tends to return to its original attitude when the outside forces are removed. If a body, when disturbed from its original position, assumes a new position and neither returns to its origin nor moves any farther from it, the body is said to be neutrally stable. If the attitude of a neutrally
stable missile is changed by an outside force or by a change in its controls, the missile remains in the new position until other forces influence it.
A third type of stability is negative stability, or instability. In this case a body displaced from its original position tends to move even farther away. For example, if an unstable aircraft is put into a climb, it tends to climb more and more steeply until it stalls.
AXIS. A missile in flight can be considered to move about three axes, as shown in figure 2C 1. In normal level flight, the vertical line is the yawing axis; the longitudinal line through the missile center is the rolling axis; and the horizontal line through the center of gravity at right angles to the rolling axis is the pitching axis. Whenever there is a displacement of a missile about any of these three axes, the missile may do any one of the following:
1. It may oscillate about the axis.
2. It may increase its displacement and get out of control.
3. It may return to its original position readily, without oscillation.
The last possibility, which indicates a stable missile, is the one desired. We will show later how this problem of stability is met in missile design.
2C2. Effects of aerodynamic forces
CENTER OF PRESSURE. On every point of a moving airfoil, a small force is present. This force is different in both magnitude and direction from that acting on any other point on the airfoil. It is possible to add mathematically all of these small forces. Their sum is the resultant force. The resultant has
Figure 2C1.-Flight attitude of a guided missile.
magnitude, direction, and location. The point at which the resultant force can be considered as applied to the wing is called the center of pressure.
In actual flight, a change in the angle of attack will change the airspeed. But if for test purposes we maintain a constant velocity of the airstream while changing the angle of attack, the results on a nonsymmetrical wing will be as shown in figure 2C2. The sketches show a wing section at various angles of attack, and the effect of these different angles on the resultant force and the position of the center of pressure.
The burble point referred to in the lower sketch is the point at which airflow over the upper surface becomes rough, causing an uneven distribution of pressure. The burble point is generally reached when the angle of attack is increased to about 18° or 20°. At small angles of attack, the resultant is comparatively small. Its direction is upward and back from the vertical, and its center of pressure is well back from the leading edge. Note that the center of pressure changes with the angle
of attack, and the resultant has an upward and backward direction. At a positive angle of attack of about 3° or 4°, the resultant has its most nearly vertical direction. Either increasing or decreasing the angle causes the direction of the resultant to move farther from the vertical.
Figure 2C2.-Effect of angle of attack on center of pressure.
As we have shown, the resultant force on a wing can be resolved into forces perpendicular and parallel to the relative wind; these components are lift and drag. The lift force depends on the contour of the wing, the angle of attack, air density, area of the wing, and the square of the air speed. If a missile is to continue in level flight, its total lift must equal its weight. As the angle of attack increases, the lift increases until it reaches a maximum value. At the angle of maximum lift, the air no longer flows evenly over the wing, but tends to break away from it. This breaking away (the burble point) occurs at the stalling angle. If the angle of attack is increased further, both lifting force and airspeed decrease rapidly.
Drag is the resistance of air to motion through it. The drag component of the resultant force on a wing is the component parallel to the direction of motion. This force resists the forward motion of the missile. If the missile is to fly, drag must be overcome by thrust-the force tending to push the missile forward. Drag depends on the missile area, the air density, and the square of the velocity. Air resists the motion of all parts of the missile, including the wings, fuselage, tail airfoils, and other surfaces. The resistance to those parts that contribute lift to the missile is called induced drag. The resistance to all parts that do not contribute lift is parasitic drag.
From Newton's laws, we know two things: First, if all the forces applied to a missile are in balance, then if the missile is stationary it will remain so; if it is moving, it will continue to move in the same direction at the same speed until an outside force is applied to it. Second, if an unbalanced force-one not counteracted by an equal and opposite force-is applied to the missile, it will accelerate in the direction of the unbalanced force.
At the instant of launching, missile speed is zero, and there is no drag. (We will, for the moment, disregard air-launched missiles.) The force of thrust developed by the propulsion system will be unbalanced, and as a result the missile will accelerate in the direction of thrust. (A solid-fuel rocket develops full thrust almost instantly. When a long-range liquid-fuel rocket is launched, it may be physically held down until its engines have developed sufficient thrust.) When thrust weight ratio reaches its maximum value,
acceleration of the missile is at a maximum. But, during the launching phase, missile speed quickly increases. Because drag is proportional to the square of the speed, drag increases very rapidly. The force of thrust is thus opposed by a steadily increasing force of drag. The missile will continue to increase in speed, but its acceleration (rate of increase of speed) will steadily decline. This decline will continue until thrust and drag are exactly in balance; the missile will then fly at a uniform speed as long as its thrust remains constant.
If the propulsive thrust is decreased for any reason (such as a command from the guidance system, or incipient fuel exhaustion) the force of drag will exceed the thrust. The missile will slow down until the two are again in balance. When the missile fuel is exhausted, or the propulsion system is shut down by the guidance system, there is no more thrust. The force of drag will then be unbalanced, and will cause a negative acceleration, resulting in a decrease in speed. But, as the speed decreases, drag will also decrease. Thus the rate of decrease in speed also decreases.
As we have shown, a missile will maintain a uniform forward motion when thrust and drag are equal. The power required to maintain uniform forward motion is equal to the product of the drag and the speed. If drag is expressed in pounds, and speed in feet per second, the product is power in foot-pounds per second. By definition, one horsepower is 550 foot-pounds per second. The horsepower expended by a missile in uniform forward motion is then
hp = DV/550
where D is the drag in pounds, and V the speed in feet per second.
2C3. Problems of missile control
A missile must be so designed and constructed that it will fly a specified course without continual changes in direction. The degree of stability of a missile has a direct effect on the behavior of its controls, and for this reason a high degree of stability must be maintained. As the speed of a missile increases, its stability is changed by shifts in the center of pressure. A pressure shift causes changes in the airflow acting on the missile surfaces. Even
in pure supersonic flow, variations in speed will cause shifts in center of pressure.
Figure 2C3 represents a missile in level flight; it is longitudinally stable about its lateral axis through the center of gravity. Airflow over the wing is deflected downward toward the elevator. This angle of deflection is called the downwash angle. When lift decreases as a result of reduced speed, this downwash angle decreases, and produces pressure changes. There will be certain speeds at which unstable conditions are set up as a result of such pressure shifts. When an unstable condition occurs, the control system must quickly compensate by moving the control surfaces or changing the missile speed; otherwise the missile may get out of control.
As we will show in the next section of this chapter, unstable conditions are most serious at transonic speeds. Most missiles have dive control and roll recovery devices to overcome unstable conditions. For example, the horizontal tail surfaces may be placed high on the fin, to minimize the effects of downwash.
Unstable airflow over the wings of a missile may cause the ailerons to oscillate, creating a condition known as "buzz." A similar condition called "snaking" may exist about the yaw axis as a result of rudder oscillation. The troubles may be partially compensated for by nonreversible control systems, or by variable - incidence control surfaces.
STABILITY ABOUT THE VERTICAL AXIS is usually provided for by vertical fins. If a missile begins to yaw to the right, air pressure on the left side of the vertical fins is increased. This increased pressure resists the yaw, and tends to force the tail in the opposite direction. In some missiles the vertical fin may be divided, and have a movable part, called the rudder, that is used for directional control. In addition to the rudder,
there may be trim tabs that can be set for a particular direction of flight relative to the prevailing wind. The vertical sides of the fuselage also act as stabilizing surfaces. the same action takes place here as on the fin, but with a lesser correcting force.
Another means for obtaining yaw stability is by sweepback of wings. Sweepback is the angle between the leading edge of a wing and a line at right angles to the longitudinal axis of the missile. If a missile yaws to the right, the leading edge of the left sweptback wing becomes more perpendicular to the relative wind, while the right wing becomes less so. This puts more drag on the left wing, and less on the right. The unbalanced drag at the two sides of the missile tends to force it back to its original attitude.
STABILITY ABOUT THE LONGITUDINAL AXIS may be provided by dihedral--an upward angle of the wings. As the missile starts to roll, the lift force is no longer vertical, but moves toward the side to which the missile is rolling. As a result, the missile begins to sideslip. This increases the angle of attack of the lower wing, and decreases that of the upper. Lift on the lower wing will therefore increase, while lift on the upper wing decreases. This unbalanced lift tends to roll the missile back to its original attitude.
STABILITY ABOUT THE LATERAL AXIS is accomplished by horizontal surfaces at the tail of the missile. The stationary part of these surfaces is the stabilizer; the movable part is the elevator. Pitch stability results from the change in forces on the stabilizer when the missile changes its angle of attack. For example, if the missile nose begins to pitch downward, the force of the airstream against the upper surface of the stabilizer will increase. This will tend to push the tail downward, and thus return the missile to its original attitude.
D. Aerodynamics of Supersonic Missile Flight
SHOCK WAVE. As a missile moves through the air, the air tends to be compressed, and to pile up in front of the missile. Because compressed air can flow at speeds up to the speed of sound, it can flow smoothly around a low-speed missile. But as the missile approaches the speed of sound, the air can no longer get out of the way fast enough. The missile surfaces split the airstream, producing shock waves. A shock wave is a sharp boundary between two masses of air at different pressures.
Shock waves can seriously alter the forces acting on a missile, requiring radical changes in trim. The missile tail surfaces may be seriously buffeted and wing drag rises. Any deflection of the control surfaces in an attempt to overcome these conditions may cause new shock waves, which interact with those already present. For this reason there may be certain speeds at which the controls become entirely useless.
MACH NUMBER is the ratio of flight speed to the speed of sound. It was named in honor of Ernst Mach (pronounced mock), an Austrian scientist who first pointed out its importance in 1887. If a missile travels at twice the speed of sound, it has a flight speed of Mach 2.0. If its speed is half that of sound, it has a flight speed of Mach 0.5. The speed of sound varies with both pressure and temperature. It decreases from an average of about 760 mph at sea level to about 675 mph at 30,000 feet.
REYNOLDS NUMBER. During the development of a new missile design, scale models of the proposed missile are tested in wind tunnels. But the performance of the model does not necessarily indicate the performance of the actual missile, even when all known variables are scaled down. In some cases the effect of a given variable on the model may be opposite to its effect on the full-size missile. Reynolds number is a mathematical ratio involving relative wind speeds, air viscosity and density, relative sizes of the model and missile, and other factors. The use of this ratio makes it possible to predict missile behavior under actual flight conditions from the behavior of the model in the wind tunnel.
HEAT BARRIER is not a barrier in a physical sense, but its effect tends to limit the maximum speed of a missile through the atmosphere. Heat results not only from friction, but from the fact that at high speeds the air is compressed by a ram effect. The temperature rise caused by the ram effect is proportional to the square of the Mach number. The average temperature at sea level is considered to be 59°F; temperature decreases steadily with altitude to about 46,000 feet, above which it is assumed to be constant. At sea level, ram temperature is about 88°F at Mach 1-29° higher than the standard temperature. At Mach 2, ram temperature at sea level is about 260°F, and at Mach 4 about 1000°F. Missiles capable of flying at these speeds must be capable of withstanding these temperatures. This problem is particularly serious with ballistic missiles intended to plunge down into the atmosphere at speeds in the order of Mach 12. A significant part of the development effort for long-range ballistic missiles has been devoted to development of nose cones capable of withstanding extreme temperatures.
SPEED CLASSIFICATIONS. Missile speeds may be divided into four categories: subsonic, transonic, supersonic, and hypersonic. A missile is moving at subsonic speed when the relative velocity of air at all points on its surface is less than the speed of sound. In the transonic range of speeds, air moves over some parts of the missile at less than the speed of sound, and over other parts at more than the speed of sound. Under these conditions, shock waves are present; the airflow is turbulent, and the missile may be severely buffeted. A high-speed missile should be made to accelerate through the transonic zone in the least possible time, to prevent these disturbances. A missile is moving at supersonic speed when the relative speed of the air at all points of its surface is greater than the speed of sound. In supersonic flow, little turbulence is present.
When any object moves through the air, the molecules of air require a finite time to adjust themselves to its presence, and to readjust themselves after it has passed. This period of adjustment and readjustment is called the relaxation time. If the time required for a
missile to pass a given point is equal to or less than the relaxation time, the missile is moving at hypersonic speed. Relaxation time is longer at high altitudes, and the beginning of the hypersonic speed zone is correspondingly lower. Under most conditions, this zone begins somewhere between Mach 5 and Mach 10.
MACH ANGLE. This term is illustrated by analogy in figure 2D1, which represents a boat in four different conditions of motion over the surface of a lake. Assume that any wave
or ripple formed on this lake will move at a speed of 10 mph. And, just for the purposes of this illustration, assume that Mach number means the ratio of boat speed to wave speed.
In the sketch at the upper left of figure 2D1, the boat is at rest. If the wind makes the boat bob up and down, the boat will generate a series of ripples that spread out in concentric circles at the rate of 10 mph. In the upper right sketch, the boat is moving at 5 mph (representing a speed of Mach 0.5). After the boat generates a ripple, it will move with respect to that
Figure 2D1.-Mach angle analogy.
ripple before it generates the next one. The waves still spread out at 10 mph, but they are no longer concentric.
A the lower left, figure 2D1, the boat is traveling at the speed of wave propagation, representing a speed of Mach 1.0. Because the boat is moving at the same rate the waves spread out, all the waves are tangent to each other at the bow of the boat. At the lower right, the boat is moving at Mach 2.0-twice the wave speed-and it leaves the ripples behind. The wave pattern now becomes a wedge on the surface of the water. In the air, with three dimensional flow, the pattern would be a cone. The semi-vertex angle is the MACH ANGLE. The greater the speed above Mach 1, the smaller the angle. The bow wave of the boat is closely analogous to the conical shock wave
that spreads from the nose of a supersonic missile. And both have the same cause: the fact that an object is moving through a fluid faster than the fluid itself can flow.
NORMAL SHOCK WAVE. This term refers to a shock wave at a right angle to the direction of motion, that appears on any surface over which air is moving at the speed of sound. Figure 2D2 represents the flow of air over a section of a missile wing at four different speeds. In the upper sketch the missile is moving at subsonic speed. Air flows past every point on the wing at less than the speed of sound. The second wing section from the top is that of a missile in the transonic zone. The missile itself is moving at less than the speed of sound. But the air, in order to travel over the curved surface of the wing, must
Figure 2D2.-Effect of missile speed on airflow pattern.
increase its relative speed. At a certain point on the curved surface it reaches the speed of sound, and a normal shock wave forms at that point.
The third wing section from the top (fig. 2D2) is moving at exactly the speed of sound. The normal shock wave now forms at the leading edge of the wing. Airflow over the whole wing surface is turbulent; lift and control are decreased, or lost altogether.
OBLIQUE SHOCK WAVE. The lowest of the four wing sections shown in figure 2D2 is moving at supersonic speed. The shock wave still forms at the leading edge of the wing; but now, because the missile is moving faster than the air can flow, the Mach angle is less than 90°, and an oblique shock wave spreads out from the leading edge of the wing. As we have said, a shock wave is a sharp boundary between two masses of air at different pressure. Air behind the oblique shock wave has a lower relative speed than that in front, and therefore has a higher pressure.
2D2. Control of supersonic missiles
Aerodynamic control is the connecting link between the guidance system and the missile flight path. Effective control of the flight path requires smooth and exact operation of the missile control surfaces. The control surfaces must have the best possible design configuration for the intended speed of the missile. They must be moved with enough force to produce the necessary change of direction. Methods must be found for balancing the various controls, and for changing them to meet the variations of lift and drag at different Mach speeds.
EXTERNAL CONTROL SURFACES. The simplest control surfaces are fixed fins. Fixed fins abaft the center of gravity provide "weathercock stability," in the same way that the feathers on an arrow give it a stable flight. Fixed fins are used on most current missiles. They are usually called vertical stabilizers or horizontal stabilizers, depending on their position and function.
All guided missiles are also provided with movable control surfaces, since stationary fins cannot provide the precise control needed to keep the missile on a desired course. Movable control surfaces can be divided into two
types; primary and secondary. The primary controls include ailerons, elevators, and rudders. An aileron is attached to the trailing edge of the wing or main lifting surface, as shown at the upper right in figure 2D3.
The two ailerons move differentially. The three lower diagrams in figure 2D3 show how they control the roll of the missile about its longitudinal axis. With both ailerons in neutral position, both wings have the same lift. But when an aileron moves up, it decreases lift; when it moves down, lift increases. Thus, the wing with the raised aileron will move down and the other will move up, as shown in the diagram.
Figure 2D3 also shows the effect of the elevators and rudder. The elevators are the movable parts of the horizontal stabilizers; they control the movement of the missile about its axis of pitch. Both elevator surfaces move together. If they move down, they will deflect air downward from the tail. The tail will react by moving upward, and the nose of the missile will pitch downward.
The rudder consists of one or two movable surfaces on the trailing edge of the vertical stabilizer. If the rudder is in two parts, both move together. The rudder controls the movement of the missile about its axis of yaw. For example if the rudder moves to the right, the tail moves to the left, and the missile yaws to the right.
Secondary control surfaces include tabs, spoilers, and slots. A tab is a small independently movable surface on the trailing edge of a larger control surface. Tabs control the missile indirectly. For example, consider a tab on an elevator. If the tab moves upward, the deflected air will exert a downward force on the elevator. The elevator will then move down. Note that it is still the elevator, not the tab, that directly controls the missile.
A spoiler can be any of several devices-for example a hinged flap on the upper surface of the wing. Suppose that a gust of air causes the left wing to lose lift. The spoiler on the right wing can be raised to "spoil" the smooth flow of air over it, and thus decrease its lift to equal that of the other wing.
A slot is basically a high-lift device located at the leading edge of the wing. At a normal angle of attack, it has no effect. At high angles of attack the slot can be opened to allow air to spill through and thus prevent a stall.
Figure 2D3.-Effect of control surfaces on missile flight.
DUAL PURPOSE CONTROL. In some types of missiles, two control functions are combined in a single set of control surfaces. Elevons (also called ailevators) combine the function of elevators and ailerons. Such surfaces might be mounted on the trailing edges of delta wings. If operated together, they serve as elevators; if operated differentially, they serve as ailerons. If the missile tail surfaces were inclined upward, to form a V with the missile axis, controls on the trailing edges of these surfaces could be used as ruddervators. By suitable combinations of movements, they could control the missile in both pitch and yaw.
CONTROL AT STARTING SPEEDS. Surface-launched missiles start out with zero
velocity, and accelerate to flying speeds. For a short time after launching, airspeed over the control surfaces is slow, and these surfaces are unable to stabilize the missile or control its course. With small, booster-launched missiles, this problem is not serious. Terrier, for example, builds up enough speed for aerodynamic stability in a fraction of a second. But heavy intercontinental ballistic missiles rise slowly from their launching pads, and may require auxiliary control devices for a number of seconds after launching. Two types of auxiliary control have been used.
EXHAUST VANES are surfaces mounted directly in the exhaust path of a jet or rocket engine. When the exhaust vanes are moved, they deflect the direction of exhaust, and thus
produce a lateral component of thrust that can be used to keep the missile pointed in the desired direction. This is possible because the exhaust velocity is very high, even when the missile has just begun to move. Because of the tremendous heat in the exhaust, the life of exhaust vanes is short. The German V-2 used exhaust vanes made of carbon. The melting point of carbon is far above the exhaust temperature. But because carbon burns, the vanes were eroded rapidly. By the time the V-2 reached a speed at which the vanes were no longer needed, they were burned away completely.
JET CONTROL is similar to exhaust vane control in that both deflect the exhaust to produce a lateral component of thrust. One method of jet control consists in mounting the engine itself in gimbals, and turning the whole engine to deflect the exhaust stream. This system requires that the engine be fed by flexible fuel lines; and the control system that turns the engine must be very powerful. Another method of jet control consists in mounting several auxiliary jets at various points on the missile surface. By turning on one or more of the auxiliary jets, it is possible for the guidance and control systems to change the missile course as required. The use of auxiliary jets makes it possible to eliminate the outside control surfaces entirely. This is the steering method most likely to be used for control of missiles after they leave the atmosphere (and, eventually, for the control of space ships).
2D3. Effects of missile configuration
The configuration of a guided missile is the principal factor controlling the drag and lift forces that act on it. And these two forces largely determine the overall efficiency of the missile.
DRAG REDUCTION. It is essential that supersonic missiles be designed for minimum drag. A low drag configuration makes it possible to use a smaller power plant, with a lower rate of fuel consumption. The resulting saving in bulk and weight can be used to extend the range of the missile, add to its war head payload, reduce its over-all size, or any combination of these three.
The effects of thickness distribution, Reynolds number, surface imperfection, and Mach number all influence missile drag. Wing drag is influenced by thickness ratio, sweepback, aspect ratio, and section of airfoil. Total drag of the missile is made up of fuselage drag, wing and fin drag, and another factor not present in subsonic flight: mutual interference between the drags of the individual parts. For example, the drag of a wing may be strongly affected, for better or worse, by the shape of the body on which it is mounted.
LIFT EFFECTIVENESS. A steady lift force, equal to the weight of the missile, must be maintained to keep the missile in level flight. Additional lift must be available for maneuvering. A missile must be so designed that the necessary lift is provided with minimum drag. And, for satisfactory control response, lift must vary smoothly with the angle of attack.
The conditions of flight associated with subsonic airflow are well known. Airflow phenomena at supersonic speeds are orderly, and can be analyzed mathematically. But in the transonic speed range, major design problems arise. A great deal remains to be learned about airflow in this range.
Airflow over an ideal wing would be subsonic until the missile reaches a velocity of Mach 1, and it would then immediately become supersonic. In other words, an ideal wing, if it were possible to make one, would eliminate the transonic range. Actually, the transonic range begins when the flow over any part of the missile becomes supersonic, and continues until the flow over all parts of the missile becomes supersonic. The free-stream Mach number at which transonic flow begins on any given missile is called the critical Mach number for that missile.
Every missile is designed for a cruising speed either below the transonic region or above it; no missile is intended to cruise within this region. For supersonic missiles,
the effects of the transonic zone can be minimized in two ways. First, the range of speeds included within the transonic zone can be narrowed by suitable design of the missile. Figure 2D4 illustrates some of the devices that have been used for this purpose. Second, by maintaining maximum powerplant thrust until after supersonic velocity is reached, the missile can pass through the transonic region in minimum time. Supersonic missiles are often launched with the help of boosters. A booster may be considered as an auxiliary powerplant. It consumes fuel at a rapid rate, and develops a high thrust. After the missile has passed through the transonic region its booster falls away. The missile is then propelled at supersonic speed by its own powerplant, which has a lower rate of fuel consumption and a smaller thrust than the booster.
Figure 2D5 represents three common airfoil plan forms for guided missiles. The optimum arrangement of airfoils on a missile is governed by many factors, such as speed, rate of acceleration during the launching phase, range, and whether or not the missile is to be recovered. The sketches in figure 2D6 show some of the more common arrangements of missile airfoils. In some missile designs, arrangements shown in the figure as "tail units" may be used at the mid-section or even at the nose of the missile; in others, some of the "wing arrangements" shown in the figure may be used as tail units.
Figure 2D6.-Common arrangements of airfoils.
E. Guided Missile Trajectories
2E1. Trajectory curves
Missile trajectories include many types of curves. The exact nature of the curve is determined by the type of guidance and the nature of the control system used. For some missiles, the desired trajectory is chosen before the missile is designed, and the missile is closely
limited to that trajectory. Other missiles, such as Regulus, may offer a choice of trajectories.
HYPERBOLIC SYSTEM. A missile using a hyperbolic guidance system will first climb to the desired altitude, then follow an arc of a hyperbola before diving on its target. If the control stations are ideally located with respect
to the target, the hyperbolic course is a close approach to a straight line. This system is described in a later chapter.
PURSUIT CURVE. Some homing missiles, and some beam riders, follow a pursuit curve. At any given instant, the course of the missile is directly toward the target. If missile and target are approaching head-on, or if the missile is engaged in a tail chase, the pursuit curve may be a straight line unless the target changes course. But a missile that pursues a crossing target must follow a curved trajectory. As the missile approaches a crossing target, the target bearing rate increases, and the curvature of the missile course increases correspondingly. In some cases the extreme curvature of the pursuit course may be too sharp for the missile to follow.
LEAD ANGLE COURSE. Some homing missiles follow a modified pursuit course. The deflection of the missile control surfaces is made proportional to the target bearing rate. The missile flies not toward the target, but toward a point in front of it. The missile thus develops a lead angle, and the curvature of its course is decreased.
A further refinement is possible if a computer, either in the missile or at a control
station, can use known information about the missile and target to calculate a point of intercept which missile and target will reach at the same instant. Because the missile is guided directly toward the point of intercept, its trajectory is a straight line. If the target changes course during the missile flight, a new point of intercept will be calculated, and the missile course will be turned toward the new point of intercept.
BEAM-RIDER TRAJECTORY. As we will explain in a later chapter, a beam-rider missile may follow either a pursuit curve or a lead-angle course, depending on the type of system used.
FLAT TRAJECTORY. An intermediate-range or long-range air-breathing missile is usually made to climb as quickly as possible to the altitude at which its propulsion plant operates most efficiently-somewhere between 30,000 and 90,000 feet. After reaching this altitude the missile flies a flat trajectory to the target area. Regulus, for example, can be made to climb steeply to a desired altitude, level off, fly a flat trajectory to the target area, then dive straight down.
Figure 2E1 illustrates a probable trajectory for along-range ballistic missile. The missile is launched vertically, so that it can get through
Figure 2E1.-High-angle rocket trajectory.
the densest part of the atmosphere as soon as possible. At a certain altitude which may be controlled by either preset or command guidance, the missile turns to a more gradual climb. After burnout, or shutdown, of the propulsion system by radio command, the missile "coasts" along a ballistic trajectory to the target.
2E2. Factors affecting missile trajectory
The principle factor affecting a missile's trajectory is, of course, the design of the missile and its guidance system. This article will deal only with other external forces affecting the trajectory. Such factors include wind, gravity, magnetic forces, and the coriolis effect. In the use of any long-range missile, all of these must be taken into account.
All missiles fly through the atmosphere, either during their entire flight or at the beginning and end of it. They are therefore liable to be pushed off the desired course by the force of the wind. The magnitude and direction of the prevailing winds at various points on the earth are well known. But the prevailing winds are much modified by a number of factors such as local topography, thermal updrafts due to local heating of the earth's surface, the distribution of high-pressure and low-pressure air masses, and storms and their associated turbulence. All of these factors can be predicted to some extent, but the reliability of the prediction decreases with both time and distance. For that reason, air-breathing missiles must be provided with means for correcting any deviation in course that might result from unpredicted winds. A ballistic missile may be subject to correction as it rises through the atmosphere. But it descends on its target at such a speed that the effect of wind is unlikely to produce a serious error.
A long-range missile using a navigational guidance system may use the direction of the center of the earth as a reference. It does so by using a pendulum, plum bob, or some similar device, to measure the direction of the gravitational force. But the measuring device is acted on by two forces: gravity, which tends to pull it toward the center of the earth; and
centrifugal force, caused by the earth's rotation, acting at a right angle to the earth's axis. The direction indicated by the measuring device is that of apparent gravity-the resultant of the two forces. The motion of the missile itself will create additional forces that tend to disturb the gravity-measuring device. Any missile guidance system that uses a gravitational reference must compensate for these disturbing forces.
Some missile may use the strength or the direction of the earth's magnetic field as a reference for navigation. But both strength and direction of the field vary from point to point on the earth. In general, these variations have been measured and plotted. But at any given point on the earth, the magnetic field is subject to annual, monthly, and even daily variations. It is subject to non-periodic variations in "magnetic storms" that result from bursts of ions or electrons radiated from the sun. Most of these variations are predictable with reasonable accuracy, and can be taken into account in the missile guidance system.
The CORIOLIS FORCE must also be compensated for. It is caused by the earth's rotation, and tends to deflect a missile to the right in the northern hemisphere, and to the left in the southern hemisphere. As the earth turns on its axis, its surface moves toward the east at a rate determined by latitude. At the equator, the earth's surface is moving to the east with a speed of more than 1,000 mph; at the poles, its speed is zero.
Assume that a missile is launched directly northward in the northern hemisphere. At the instant of launching, it will be moving to the east at the same rate as the surface from which it is launched. But as it moves northward, it flies over points whose eastward velocity is less than its own. As a result it will be deflected eastward, or toward the right. Now imagine a missile fired southward in the northern hemisphere. It will fly over points whose eastward velocity is greater than its own. It will therefore be deflected westward (still to the right) with respect to the surface.
The amount of deviation produced by the coriolis force depends on the latitude, length, and direction of the missile flight. Since it can be accurately predicted, suitable corrections can be made by the missile guidance system.
CHAPTER 3 GUIDED MISSILE COMPONENTS
This chapter is intended to provide an overall view of a guided missile and its various components. Most of the material in this chapter is covered in more detail elsewhere in this text. Airframes and control surfaces, for example, were treated in chapter 2. Propulsion systems, control systems, and guidance systems are each given one or more separate chapters. Their principal features will be briefly summarized here. War heads which are not covered elsewhere in this text, will be treated in some detail.
The principal components of a guided missile are:
WAR HEAD. The war head maybe designed to inflict any of several possible kinds of damage on the enemy. The war head is the reason that the missile exists; the other components
are intended merely to insure that the war head will reach its destination.
AIRFRAME. The airframe is the physical structure that carries the war head to the enemy, and contains the propulsion, guidance, and control systems
PROPULSION SYSTEM. This system provides the energy required to move the missile from the launcher to the target.
CONTROL SYSTEM. The control system has two functions. It keeps the missile in stable flight, and it translates the commands of the guidance system into motion of the control surfaces, or into some other means for modifying the missile trajectory.
GUIDANCE SYSTEM. This system determines whether or not the missile is on the course required to reach the target. If the missile is off course, the guidance system sends appropriate commands to the control system.
The term AIRFRAME has the same meaning for guided missiles as it has for the conventional airplane. It serves as a vehicle to carry all the other parts of the missile, and it provides the aerodynamic characteristics required for successful flight. Research on airframes is a major part of our missile development effort. In the event of war or national emergency, the manufacture of missile airframes will probably assume the proportions of a major industry.
3B2. Body configuration
The Navy missiles described and illustrated in chapter 1 show the range of body configuration found in operational missiles. In general, the design of the airframe is determined by performance requirements. For example, Regulus 1 is strikingly similar in appearance to a jet fighter of similar performance, in the high subsonic speed range. Its wings, like those of carrier aircraft, may
be folded. Before launching, the wings are extended and locked in place. The test version of Regulus has retractable landing gear. (Regulus differs from a conventional aircraft in that its wings depend only on angle of attack, rather than the Bernoulli effect, for lift).
Regulus II has some resemblance to a conventional aircraft, but its form has been greatly modified for supersonic speed. It is long and narrow as compared to Regulus I. Its nose is long and sharp, and its wings are severely swept back. Both versions of Regulus carry control surfaces similar in location and function to those of conventional aircraft.
Terrier, Sidewinder, and Talos are typical of a second class of airframe. In general, the airframe is a long, slender cylinder without wings. Its tail fins provide weathercock stability. A second set of fins, mounted near the center of the missile or forward of the center, provide additional stability. Control of the missile is accomplished by pivoting one or more pairs of fins, rather than by the use of conventional ailerons, rudders, and elevators. Note that, as in Terrier, the forward
fins may be mounted at 45° angles to the horizontal and vertical.
The nose shape is determined by other requirements. The Terrier nose is long and slender, because that shape has been found highly efficient at supersonic speeds. Sidewinder, although it travels at a comparable speed, has a hemispherical nose. This is necessary because of the infrared seeking device located immediately behind the nose surface; a surface of any other shape would transmit a distorted indication of the target position. The nose of Talos is relatively blunt; it contains the air intake for the ramjet engine.
Talos is, in effect, a double-walled tube. Its central part is taken up by the ram-jet engine. All of its other components-war head, fuel tanks, guidance and control systems-are crowded within the space between the inner and outer walls. (In one model, one of these components is carried inside the central diffuser in the nose.) The central part of Terrier is taken up by a chamber filled with solid rocket propellant. The war head, fuze, and the major part of the guidance and control systems, are located forward of the fuel chamber. But a part of the guidance and control system is located in the double-walled cylinder that surrounds the exhaust duct at the after end of the missile. Electric cables and
pneumatic lines to maintain communication between the two parts of the guidance system pass through covered channels along the outside of the missile.
Polaris, pictured in chapter 1, represents still a third type of missile body configuration. Note that there are no external control surfaces; any necessary changes in trajectory are accomplished by jet deflection. Note also that the nose is bluntly rounded. Because Polaris is an intermediate-range ballistic missile, its trajectory takes it far beyond the earth's atmosphere. It descends on its target at a steep angle, and at tremendous speed. As it re-enters the atmosphere, friction with the air generates a great deal of heat. The Polaris nose cone will probably become white-hot on re-entry. Its shape and construction have been determined by the requirements of this problem. The shape of the nose cone is designed to cause a minimum increase in temperature, and to distribute the temperature build-up uniformly over the nose cone, rather than allowing it to concentrate in a small area. The materials for the outer surface have been especially developed to resist high temperature. Suitable insulation must be provided between the outer skin of the nose cone and the internal components, to prevent damage to or premature detonation of the war head.
C. Propulsion Systems
Because chapter 4 is devoted to propulsion systems, they will be covered very briefly here. The powerplants of guided missiles have been referred to as "reaction engines." But strictly speaking, any engine designed to propel a vehicle is a reaction engine. All of them operate in accordance with Newton's third law, which states that for every action there is an equal and opposite reaction. For example, the force that the tires of a car apply to the road is opposed by an equal and opposite force and it is this reaction that drives the car forward. A propellant-driven aircraft operates by increasing the momentum of the air; the resulting reaction is applied to the propeller and its shaft, and, through a thrust bearing, to the airframe. As we have pointed out
earlier, speed requirements make it impossible to use propeller-driven missiles. Because the speed of the propeller tip exceeds the speed of the airframe, the propeller tip enters the transonic zone while the aircraft speed is considerably below the speed of sound. In the transonic zone, the thrust developed by the propeller drops off rapidly, and a further increase in aircraft speed becomes impracticable. Therefore, all current guided missiles depend on some form of jet propulsion.
3C2. Types of jet propulsion systems
Popular terminology makes a distinction between jets and rockets: a jet takes in air from the atmosphere, and propels itself forward by increasing the momentum of the air; a rocket needs no air supply, since it carries
its own source of oxygen. But this is a rather arbitrary distinction. Both types of engine operate by expelling a stream of gas at high speed from a nozzle at the after end of the vehicle. For our purposes, a rocket can be considered as a type of jet engine.
The pulse-jet, which propelled the German V-1, was used by the Navy to propel an early missile that is now obsolete. No current missiles are driven by pulse-jets. Talos, as you know, is propelled by a ram-jet. The Navy uses turbo-jets for its Regulus I and II, and for the now obsolete Petrel. Terrier, Sidewinder, and Polaris are propelled by solid-fuel rockets. Current developments appear to indicate that, until the advent of atomic propulsion, most if not all of our future missiles will be powered by solid-fuel rockets.
The liquid-fuel rocket is used in the Air Force ICBM's, and in that application it has certain advantages. Liquid fuel provides more energy than an equivalent weight of solid fuel, and can maintain a high thrust for a relatively long time. And a liquid-fuel propulsion system can be shut down by radio command at any desired instant, whereas a solid-fuel system can not. But the liquid-fuel rocket must have a rather complex system of fuel and oxidizer lines and pumps, and it requires relatively elaborate equipment at the launching site. At present, a large missile propelled by a liquid-fuel rocket requires a lengthy "count-down" before firing. The last two factors make the liquid-fuel rocket impracticable for ship-launched missiles.
D. War Heads
The war head is the reason-for-being of any service guided missile; it may contain any of a large number of destructive agents. This versatility must not be confused with interchangeability, because the design of the missile and war head must be thoroughly integrated.
The guided missile fuze may be defined as that device which causes the war head to detonate in such a position that maximum damage will be inflicted on an average target. A fuze may be any of several types, such as impact, time, or proximity.
Guided missiles are precision-built weapons; they are expensive in manpower, materials, and money. The most accurate control and guidance systems will be of little value if the war head cannot produce enough lethal effect at the right time to destroy or at least cripple the target. The war head problem must be solved for each type of missile, to permit final crystalization of any integrated missile plan.
The ultimate aims and desires of weapon-makers have always been to strengthen the "arm" of the user. The rock in the hand of primitive man added strength and distance to the blows that he could deliver; the bow and arrow great 1 y multiplied man's effective striking distance; and the rifle and cannon have progressively increased the strength and
range of his striking power. Guided missiles are a new means for lengthening the arm of the user. But the striking effect depends on the nature of the war head and on how accurately it can be delivered to the intended target. The war head of a guided missile is its payload, and justification for employment of the missile lies in its ability to deliver its payload to the target.
A guided missile may carry one or more war heads, and one or more fuzes. Missile war heads intended for use against ships or land targets present similar design problems to those of older weapons. Surface-to-air missiles present a somewhat different problem. The effective radius of damage from a high-explosive war head in the air depends on the type, shape, and size of the charge, and on the nature of the target itself.
The designer of the payload for any type of military weapon is faced with a number of variables, some of which are unpredictable. For example, in the design of an anti-aircraft missile, he must consider the following factors among others:
1. Altitude affects the lethal radius of a fragmentation war head; the fragments maintain a lethal velocity through a greater distance at high altitudes.
2. The relative velocity of target and missile has a direct bearing on the optimum angle of ejection of fragments. The designer must
determine the angle at which the greatest mass of fragments should be ejected. The timing sequence of fuze operation, as well as the guidance system of the missile, must function with great precision if the target is to be destroyed. The fuze must be both sensitive and fast to ensure success against high-speed aircraft or supersonic missiles.
3. The armor of the target, if any, influences the design specifications for fragment size and velocity. Fragments that are effective against conventional aircraft of today may be too light or too slow to penetrate the protective covering of the airplanes or missiles of the future.
The multiplicity of types of ground targets has led to the development of numerous types of lethal devices-from hand grenades to hydrogen bombs. A 500-pound bomb may be armor-piercing, semi-armor-piercing, or general-purpose. It may be equipped with an impact fuze, a time fuze, a proximity fuze, or a combination of these types.
The type of target is the most influential factor in war head design. A fragmentation-type war head might be effective against conventional aircraft, or against missiles of moderate speed. But a missile intended for use against a whole fleet of attacking bombers, or against a high-speed ballistic missile, would require an entirely different type of war head. Because of the wide variety in types of surface targets, it is necessary either to have missiles that can use several types of war heads interchangeably, or to develop a whole family of missiles for this application.
3D2. Types of war heads
The types of war heads that might be used with guided missiles include: external blast, fragmentation, shaped-charge, explosive-pellet, chemical, biological, and nuclear. (Other types of war heads will be discussed in a classified supplement to this text.)
BLAST-EFFECT WAR HEAD. The blast effect war head consists of a quantity of high-explosive material in a metallic case. The force of the explosion sets up a pressure wave in the air or other surrounding medium; the pressure wave causes damage to the target. This type of war head is most effective against underwater targets, because water is incompressible, and relatively dense. Torpedo warheads
are of this type. Blast-effect war heads have been used successfully against small ground targets. They are considerably less effective against aerial targets because the density of the air, and therefore the severity of the shock wave, decreases with altitude.
FRAGMENTATION WAR HEAD. The fragmentation war head uses the force of a high-explosive charge to break up the war head casing into a number of fragments, and to propel them with enough velocity to destroy or damage the target. The size and velocity of the fragments, and the pattern in which they are dispersed, can be controlled by variation in the design and construction of the war head. The velocity of the fragments depends on the type and amount of explosive used, and on the ratio of explosive-to-fragment weight. The average size of fragments depends on the shape, size, and brittleness of the war head casing, and on the quantity and type of explosive. Greater uniformity in fragment size can be achieved by scoring or otherwise weakening the casing in a regular pattern, as shown in figure 3D1.
The damage produced by a fragmentation war head depends on the amount of metal available to form fragments, and on the amount of .explosive available for breaking the casing and propelling the fragments. Aerial targets are more susceptible to damage by fragments if the war head explodes a short distance away, rather than in contact with the target. Against a partially protected surface target, a fragmentation war head is most effective when exploded in the air above the target, rather than on the ground. Figure 3D2 shows this effect. Fragments from the air burst strike the partially protected target and the entrenched personnel.
Figure 3D1.-Basic construction of a fragmentation war head.
Figure 3D2.-Effect of fragmentation war head on surface targets.
SHAPED-CHARGE WAR HEAD. A shaped charge consists of a casing and a quantity of high explosive. The explosive is so shaped that the force of the blast it produces is largely concentrated in a single direction. As a result, a shaped charge has high penetrating power. It is widely used against armored surface targets. For example, the antitank bazooka rocket used during World War II and in Korea uses a shaped charge.
Figure 3D3 shows how the shape of the charge affects the penetrating ability of the
blast. All three of the charges shown in the figure have the same weight of explosive. The flat charge at the left produces an explosive force rather evenly distributed over a given area of the target; this charge produces little penetration. The shallow-cone shape of the middle charge produces a greater concentration of the explosive force, and penetration of the target is deeper. The deep-cone shape at the right has concentrated the explosive force so as to penetrate the target armor. Metallic fragments of the armor can now reach the interior of the target, and do additional damage.
EXPLOSIVE-PELLET WAR HEAD. An explosive-pellet war head consists of a group of separately fuzed explosive pellets housed in a casing. The casing contains an additional quantity of explosive to eject the pellets from the main war head casing. The pellets themselves do not explode until they contact or penetrate the target. If the target is an aircraft or missile, maximum destruction can be accomplished when the pellets are detonated after penetrating the outer skin of the target. Each pellet contributes both blast effect and high-velocity metallic fragments when detonation occurs. (Because of high cost due to manufacturing difficulties, war heads of this type are rarely used.)
CHEMICAL WAR HEADS. A chemical war head is designed either to eject poisonous
Figure 3D3.-Effects of various charge shapes.
substances and thus produce personnel casualties, or to destroy combustible targets by the use of incendiary materials. Although the use of poisonous gases in warfare appears to be obsolete, the possibility of their use remains as a threat. It is likely that quantities of poisonous war gases are included in the arsenal of every major power. Missile war heads can, of course, be designed to carry any type of poisonous gas.
Incendiary war heads contain chemicals that burn violently at high temperature, cover a large area when released, and are difficult to extinguish. They are used principally against surface targets, but may also be effective against aerial targets that contain combustible materials. Incendiary materials suitable for use in war heads include magnesium, jellied oil or gasoline, and phosphorous.
BIOLOGICAL WAR HEADS. A biological war head contains bacteria or other living organisms capable of causing sickness or death. The biological agent can be specifically chosen for use against personnel, livestock, or crops. Antipersonnel agents might be chosen to cause either temporary disability or death, depending on the objectives of the attacker. An explosive charge placed in a biological war head would ensure ejection and initial dispersion of the biological agents. Special attention must be given to the design and construction of biological war heads, in order that the bacteria or other agent will remain alive, and be carried to the target under the most favorable conditions.
NUCLEAR WAR HEADS. A number of current guided missiles have nuclear capability. Nuclear war heads are suitable for use against large surface targets, against fleets of bombers, and against intercontinental ballistic missiles. Nuclear weapons are discussed in detail in the second part of this text.
A fuze is a device that initiates the explosion of the main charge in an explosive weapon. In a guided missile the fuze may or may not be a physical part of the war head. In any case, it is essential to proper war head operation. A large variety of fuze types is available. The fuze type for a given application depends on characteristics of the target, the missile, and the war head. To insure the highest probability of lethal damage to the target, fuze design must be based on the location, vulnerability, speed, and physical structure of the target.
IMPACT FUZE. Impact fuzes are actuated by the inertial force that occurs when a missile strikes a target. Figure 3D4 is a schematic representation of an impact fuze. As shown in the left-hand diagram, a charge of sensitive explosive is contained in the forward end of the fuze; a movable plunger is mounted in the after end, where it is held in place by a spring or other suitable device.
During the flight of the missile, the plunger remains in the after end of the fuze. When the missile strikes the target, it decelerates suddenly, and the inertia of the plunger carries it
Figure 3D4.-Impact fuze before and after impact.
forward. As shown in the right-hand diagram figure 3D4, the plunger strikes the shock. sensitive explosive and detonates it. The fuze charge in turn detonates the main bursting charge of the war head. A time delay element is sometimes used in conjunction with an impact fuze, so that the war head can penetrate the target before detonation.
An impact fuze may be used in conjunction with a fuze of another type, such as a proximity fuze. If the proximity fuze fails to operate as the missile approaches the target, the impact fuze will still function on contact.
TIME-DELAY FUZE. Time delay fuzes are used in some types of gun projectiles. This fuze is designed to detonate the war head when a predetermined time has elapsed after firing or launching. One type of time delay element consists of a burning powder train; another uses a clock-like mechanism. In either type, the time interval can not be changed after launching. For that reason, time-delay fuzes are unlikely to be used in guided missile war heads.
PROXIMITY FUZES. Proximity fuzes are often called VT (variable time) fuzes. They are actuated by some characteristic feature of the target or the target area. Several types of proximity fuze are possible; for example, photo-electric, acoustic, pressure, radio, or electrostatic. Each of these types could be preset to function when the intensity of the target characteristic to which it is sensitive reaches a certain magnitude.
Proximity fuzes are designed so that the war head burst pattern will occur at the most effective time and location relative to the target. Designing the fuze to produce an optimum burst pattern is not easy, since the most desirable pattern depends largely on the relative speed of missile and target. If targets with widely varying speeds are to be attacked, it might be possible to adjust the fuze sensitivity for the speed of the individual target, as predicted by a computer. Proximity fuzes activate the war head detonating system after integrating two factors: the distance to the target, and the rate at which the range is closing.
Since a proximity fuze operates on the basis of information received from the target, it is subject to jamming by false information. This is one of the important problems in proximity fuze design. The fuzes are designed for the
Figure 3D5.-Influence of fuze on point of warhead detonation.
maximum resistance to countermeasures consistent with other requirements. If the fuze is made inoperative by jamming, the missile can not damage the target unless it scores a direct hit. A more serious possibility is that jamming, instead of making the fuze inoperable, might cause premature detonation of the war head before the missile approached the target within lethal range.
Although any of the effects listed above-photoelectric, acoustic, pressure, radio, or electrostatic-can be used as the basis for proximity fuze action, and although all of them have been used at least experimentally, it has been found in practice that the radio proximity fuze is more effective than any of the others. The radio proximity fuze is the only type used by the Navy. This fuze transmits high-frequency radio waves, which are reflected from the target as the missile approaches it. Because of the relative motion of missile and target, the reflected signal, as received at the missile, is of a higher frequency than the transmitted signal. The two signals, when mixed, will generate a doppler
frequency, the amplitude of which is a function of target distance. When this amplitude reaches a predetermined level, the fuze functions and detonates the war head.
FUZE POSITION IN WAR HEAD. In general, fuzes may be classified as NOSE FUZES, located in the nose of the war head, or BASE FUZES, located at its after end. As previously stated, the fuze or combination of fuzes to be used, and their location in the war head, depend on the mission at hand and the effect desired.
WAR HEAD DETONATION POINTS. Figure 3D5 shows the effect of fuze type on the point at which the war head detonates. The war head of the missile in the upper drawing is provided with an impact fuze, and the war head detonates at the instant the missile strikes the target. The missile in the middle drawing has an impact fuze provided with a time-delay element. The time delay allows the war head to penetrate the target before detonation. In the lower drawing, the missile has a proximity fuze, which has been actuated by some characteristic of the target, causing war head detonation at a predetermined distance from the target.
E. Telemetering Systems
A missile-telemetering system consists of measuring devices and radio transmitting equipment, the purpose of which is to measure the performance of various missile components throughout the missile flight, and to transmit this information to recording instruments on the ground.
During the early stages in the design and development of a missile, electronic analog computers are used to predict the performance of proposed missile components. In later stages of development, missile components and systems must be tested in actual flight. The test versions of Regulus are designed for intact recovery after test flights, and recording equipment to indicate the second-by-second performance of missile components can be carried within the missile itself. But most missiles are non-recoverable. Telemetering equipment is therefore essential to an evaluation of component and system performance, and to indicate the cause of component failure. Telemetering data thus provides the basis for improved designs. Although telemetering is
most useful during the development stages before a missile becomes operational, it should be remembered that missile designs are constantly being improved. Telemetering equipment will play a useful part in practice firing of some operational missiles. It seems likely that, even when launched against an enemy in time of war, the very large missiles will be provided with some form of telemetering equipment.
TELEMETRY is a word of Greek origin meaning measurement from a distance. Telemetry includes (1) conversion of quantities to be studied into electric signals, (2) transmission of these signals over a radio link, (3) reception of the signals, and (4) presentation of the quantities in the form of indications and permanent recordings. The recordings are retained f or detailed analysis. This telemetering permits the measurement and study of missile component performance from a remote point. Photographic or other recording made in the missile itself (as in test flights of Regulus) are not considered telemetering, because
there is no great distance between the measuring and the recording instruments.
A simple telemetering system might measure only "yes-no" information, such as whether or not the fuze is armed at any given instant. This type of system tells an observer when an event has taken place. A usual method is to change an audio modulation frequency each time an event takes place. The frequency change gives evidence that the transmitter was working both before and after each successive event. In such a transmitter no rigid demands are made on the stability of the audio frequency, or upon its waveform.
An instrument panel observed through a television camera constitutes a form of telemetering with a large number of channels, in which quantitative information is made immediately available for observation and recording. Radiosondes suspended from weather balloons provide weather information by sampling the readings of various meteorological instruments in sequence. Resistance values are caused to vary with humidity, temperature, etc. This variation in turn causes modulation of the transmitter carrier frequency.
Telemetering in one form or another has been used in radio-controlled and other airplanes for a number of years. Guided missiles present their own peculiar problems, caused by limited space, high launching acceleration, high speed, and the varied and numerous measurements required.
A great deal of information is needed during the various stages of a missile test or development program, on such subjects as launching performance, flight data, and operation of the control and guidance systems. Data measured by the telemetering systems of guided missiles include (1) changes of attitude in roll, pitch, and yaw; (2) flight data such as air speed and altitude; (3) missile acceleration during launching or maneuvering; (4) ambient conditions of temperature, humidity, and pressure; (5) structural information such as vibration and strain; (6) control functions, such as operation of the control receiver, autopilot operation, servo operation, displacements of control surfaces, and operation of the homing or other target-seeking equipment; (7) propulsion information, including fuel flow and thrust; (8) ordnance functions such as fuze arming time; (9) upper-air research data; (10) the performance of the electric,
hydraulic, and pneumatic systems; and (11) information on the performance of the telemetering equipment itself, including reference voltages for calibration and time marks, to permit synchronizing recordings as received by several different receivers located along the flight path.
Many of these measurements are interrelated. Some of them require a high order of time resolution, especially as the speed of the missile increases. For others, a few samplings per second are adequate. A telemetering system must be capable of transmitting large amounts of varied data each second. With so much information to be handled, a multi-channel system is plainly indicated, because a single commutated channel would not give sufficient time resolution.
The nature of the telemetering installation is determined by the requirements of the particular test, and the exact functions to be telemetered for each flight must be carefully chosen. For example, a small number of functions may be studied with great precision, rather than a larger number of functions on a time-sharing basis. Such a selection might also result in a saving in the time required for missile instrumentation. Because most missiles are fired only once, the telemetering must be reliable or a sizable expenditure of time and money will be wasted. It follows, therefore, that accuracy, stability, and simplicity are imperative. Because of these requirements, telemetering personnel check their calibration work just prior to launching. Launching subjects the missile-borne telemetering equipment to severe conditions of acceleration, vibration, and sometimes condensation. For example, when a missile is launched at high altitude from a parent plane, its parts may be cold. When the missile reaches a lower altitude, the condensation that takes place may impair operation of the telemetering equipment.
The missile may roll, or it may be thrown into a climb or dive, and through all these gyrations the telemetering equipment must continue to function. A directional antenna may cause the signal to be lost entirely, along with valuable information, at a critical time; such an antenna requires a number of data-receiving stations to ensure continuous reception. The pattern of the antenna on the missile should be such that reception is not impaired
by changes in missile attitude. The antenna should be designed for minimum aerodynamic drag; with supersonic missiles, this requirement is particularly important. Nose probes or an insulated section of the missile nose are sometimes used as radiating surfaces. In some missiles, a part of the airframe itself is excited by a feedline and serves as a transmitting antenna.
As in any system of measurement, the telemetering system should neither impair the operation of the equipment it monitors, nor exert an undue influence on the quantities it measures. In small missiles, the distribution
of telemetering instruments must be so arranged that the center of gravity remains undisturbed. Telemetering equipment must be appropriately packaged to fit the particular requirements of the missile airframe-generally by utilizing a number of small components properly located with respect to sources of heat, vibration, etc.
Data transmitted from the missile may be observed on instruments as the flight progresses, and simultaneously recorded on film or magnetic tape. Suitable decoding and computing equipment are used to facilitate the work of data reduction and analysis.
CHAPTER 4 MISSILE PROPULSION SYSTEMS
Until the start of World War II, the reciprocating engine-propeller combination was considered satisfactory for the propulsion of aircraft. We have already explained the speed limitations of propeller-driven craft. As the speed of the propeller approaches the speed of sound, shock waves form and limit the development of thrust. This condition requires the use of extremely large engines to produce any further increase in speed. Research in the design of propellers may make it possible to overcome some of their limitations. But, at present some form of jet propulsion is required for high subsonic and supersonic speeds.
Guided missiles must travel at high speeds to lessen the probability of interception and destruction by enemy countermeasures. Although a few high- subsonic missiles, such as Regulus I and Snark, are still operational for use against surface targets, the increasing efficiency of countermeasures tends to make all subsonic missiles obsolete. Missiles intended for use against high-speed enemy missiles and manned aircraft must be capable of high speeds. All air-to-air and surface-to-air missiles now operational fly at supersonic velocities. Naturally, all guided missiles now operational depend on some form of jet engine for propulsion.
As we have explained in earlier chapters, jet propulsion is a means of locomotion brought about by the momentum of matter expelled from the after end of the propelled vehicle. This momentum is gained by the combustion of either a solid or a liquid fuel. Compared to reciprocating engines, jet propulsion systems are simple in construction. The basic components of a jet engine are a combustion chamber and an exhaust nozzle. Some systems require accessory components such as pumps, injectors, turbines, and ignition systems.
4A2. Classification of jet systems
Jet propulsion systems used in guided missiles may be divided into two types: ducted propulsion systems and rockets.
Missiles using a ducted propulsion system fly through the air: they are incapable of operating in a vacuum. The missile takes in a quantity of air at its forward end, increases its momentum by heating it, and produces thrust by permitting the heated air and fuel combustion products to expand through an exhaust nozzle. This process may be broken down into the following steps: air is taken in and compressed; liquid fuel is injected into the compressed air; the mixture is burned; and the resulting hot gases are expelled through a nozzle. The air may be compressed in any of several ways. In a turbo-jet engine, air is compressed by a rotary compressor, which in turn is operated by a turbine located in the path of the exhaust gases and mounted on the same shaft as the compressor. (A turbo-prop engine, now used in some manned aircraft but not in guided missiles, makes use of a propeller mounted at the forward end of the compressor turbine shaft.) In a pure duct system, such as the ram-jet, air is compressed by the forward motion of the missile through it. The now obsolete pulse-jet also depends on for- ward motion; it differs from the ram-jet in that combustion is intermittent, rather than continuous.
Rockets do not depend on air intake for their operation, and are therefore capable of traveling beyond the atmosphere. A rocket engine carries with it all the materials required for its operation. These materials usually consist of a fuel and an oxidizer. The oxidizer is a substance capable of releasing all the oxygen required for burning the fuel.
B. Principles of Jet Propulsion
4B1. Basic laws and formulas
In this article we will show, with the help of some elementary mathematics, how a
jet-propulsion system develops the thrust required to propel a guided missile. All jet-propulsion systems are based on the principles expressed in Newton's second and third
laws of motion. Newton's second law says that when a body is acted on by an unbalanced force, the body will accelerate in the direction of the applied force. The acceleration produced is directly proportional to the magnitude of the force, and inversely proportional to the mass of the body. This relation can be expressed as a formula:
F = Ma
or, force equals mass times acceleration. In this formula, force is expressed in pounds, acceleration in feet per second per second, and mass in SLUGS. The weight of any given mass varies, depending on the force of gravity. Gravity varies with the distance from the earth's center. The relation between weight and mass can be expressed in the formula:
M = W/g
in which M is the mass in slugs, W the weight in pounds, and g the acceleration due to gravity in feet per second per second (approximately 32.2 ft/sec2 at sea level).
Acceleration is the rate of change of velocity. This is expressed in the formula
a= (v2 - v1)/t
where v1 is the initial velocity of a mass, v2 its final velocity, and t the time during which this change of velocity occurs. If we substitute the above value of acceleration in the original formula, F = Ma, we get
F= (Mv2 - Mv1)/t
Since Mv is momentum, the above formula shows that the thrust produced by a jet engine is equal to the rate of change of momentum of its working fluid. We can write the above formula as follows:
where m represents M/t, and is called the mass rate of flow of the working fluid in slugs per second.
In the original equation, F = Ma, we can substitute the equivalent weight for mass, and get
F = Wa/g
When we apply this formula to a jet propulsion system, F is the unbalanced force that accelerates the working fluid through the exhaust nozzle, and a is the acceleration of the fluid in feet per second per second. In accordance with Newton's third law of motion, the forward thrust developed by the jet propulsion system is equal and opposite to the unbalanced force applied to its working fluid.
Now, let W equal the total weight of working fluid that flows through a missile propulsion system during the time the system is producing thrust, and let t equal the total time during which the system develops thrust. Then W/t is the weight rate of flow of working fluid, in pounds per second. Letting w = W/t, we can now write a formula for the thrust developed by a jet propulsion system:
T = w/g (v2 - v1)
where T is the thrust in pounds; w is the weight rate of flow of the working fluid, in pounds per second; v1 is the initial (intake) velocity of the working fluid; v2 is the final (exhaust) velocity of the fluid; and g is the acceleration due to gravity. This equation gives the thrust applied to expel the working fluid from the exhaust nozzle of the engine. And, in accordance with Newton's third law of motion, the same equation also expresses the forward thrust developed by the propulsion system to propel the missile.
Figure 4B1 represents a jet engine which is taking in air at its forward end at a speed of 1,000 feet per second. The burning fuel within the engine heats the air and increases its speed to 2,000 feet per second. If we assume that the working fluid flows through the engine at the rate of 64.4 pounds per second, application of the thrust formula shows that this engine is developing a thrust of 2,000 pounds.
Note that the thrust developed by an engine is always expressed in POUNDS OF FORCE, not in terms of work or horsepower. A jet engine that is fired in a test stand does not
Figure 4B1.-Practical example of Newton's second law of motion.
move. It therefore does no work, and consequently develops no horsepower, although it may exert its maximum thrust. For a missile in actual flight, it is possible to calculate the horsepower developed by the propulsion system from the formula:
Horsepower = VT/375
where V is the missile velocity in miles per hour, T is thrust in pounds, and 375 is a constant having the dimensions of mile-pounds per hour. For example, assume that a missile traveling at 3750 mph has 56,000 pounds of thrust. The above equation shows that the engine is developing 560,000 horsepower:
(3750 x 56,000) / 375 = 560,000 hp
Although it is possible to calculate the horsepower developed by the propulsion system of a missile in flight, the student should remember that jet-propulsion engines are always rated in terms of pounds of thrust, rather than in horsepower.
A rocket engine takes in no air from the atmosphere; its working fluid consists of the combustion gases resulting from burning fuel. Since the rocket carries its own supply of oxygen as well as its fuel supply, the initial velocity of the working fluid, relative to the missile, is zero. Thus, the formula for the thrust developed by a rocket engine reduces to
T = W/g ve
where w is the rate of fuel and oxidizer consumption in pounds per second, and ve is the exhaust velocity of the gases. But the above formula expresses only the thrust due to momentum of the working fluid. If the pressure of the working fluid, after it leaves the exhaust nozzle, is greater than the pressure outside the missile, the actual thrust is less than that given by the above formula. It is obvious that if the gases that have left the missile are at a higher pressure than the surrounding atmosphere or space, these gases are capable of doing work. That work, which might have been used to propel the missile, will be wasted. A more accurate formula for rocket thrust is
T = W/g Ve + (Pa - Pe) Ae
in which Pa is the pressure of the surrounding atmosphere (or space), Pe is the pressure of the exhaust jet, and Ae, is the cross-sectional area of the exhaust jet. If the exhaust nozzle can be so designed that it decreases the pressure of the exhaust jet to that of the surrounding space, the pressure term in the above equation becomes zero. This condition represents the maximum thrust available for any given propellant and chamber pressure. Although this condition cannot be fully attained
in actual practice, well-designed nozzles make it possible to approach it closely.
It is a common misconception that jet engines operate by pushing against the surrounding air. Ducted jets depend on air as a working fluid, but they do not need air for the exhaust to push against. Rockets require no air. Air acts only to impede the motion of a rocket, first by drag, and second by hindering the high-speed ejection of the exhaust gases. Thus rockets operate more efficiently in a total vacuum than they do in the atmosphere.
The fuels and oxidizers used to power a jet engine are called propellants. The chemical reaction between fuel and oxidizer in the combustion chamber of a jet engine produces large quantities of high-pressure high-temperature gases. When these gases are channeled through an exhaust nozzle, a large part of the heat energy they contain is converted into kinetic energy to propel the missile. When you read of an engine that can travel faster than a gun projectile, operate in a vacuum, deliver a great deal more energy than a reciprocating engine, and do so with few or no moving parts, you may get the idea that some very complex chemical mixture is used as the propellant. This is not so. Jet-propulsion engines can operate on such fuels as kerosene, gasoline, alcohol, gunpowder, and coal dust.
With regard to their physical state, propellants may be either solids, liquids, gases, or various combinations of these. However, gases are rarely used as missile propellants, for two reasons. First, liquids or solids have a higher density than most gases, even when the latter are highly compressed; thus a larger quantity of solid or liquid propellant can be carried in a given space. Second, a greater energy transformation results when a substance goes from solid or liquid to gas than results when a gas is merely accelerated to a higher velocity.
Several means have been worked out for rating, or comparing, various propellants. For example, they may be compared on the basis of TOTAL IMPULSE. Total impulse is the product of the thrust in pounds and the burning time in seconds. Solid propellants are often rated on the basis of SPECIFIC IMPULSE, or the amount of impulse per pound.
The specific impulse of a propellant, in pound-seconds per pound, is equal to the total impulse divided by the weight of the propellant. Liquid propellants are often compared on the basis of SPECIFIC THRUST. This value is similar to specific impulse, but is derived in a slightly different way. Specific thrust is defined as the thrust that would be produced by a given liquid propellant if it were consumed at the rate of one pound per second. It is equal to the total thrust, in pounds, divided by the weight rate of flow in pounds per second, and is usually expressed in SECONDS.
SPECIFIC PROPELLANT CONSUMPTION is the reciprocal of specific thrust; it is the rate of propellant flow, in pounds per second, required to produce one pound of thrust. MIXTURE RATIO designates the relative quantities of oxidizer and fuel used in a given propellant combination. It is equal to the weight of oxidizer flow divided by the rate of fuel flow. SOLID PROPELLANTS are of two types. One of these consists of a fuel, such as a hydrocarbon, mixed with a chemical capable of releasing large quantities of oxygen, such as a chlorate or a nitrate. A second type consists of a compound that releases large quantities of gases and heat when it decomposes, such as nitrocellulose. Of course it is possible to combine the two types in a single propellant mixture. The ingredients of a solid propellant are mixed so as to produce a solid of the desired chemical and physical characteristics. The finished product is called a GRAIN or STICK. A CHARGE may be made up of one or more grains.
An ideal solid propellant would have all of the following characteristics: high specific impulse; manufactured from easily obtainable substances; safe and easy to handle; easily stored; stable to shock and temperature changes; ignites and burns uniformly; maintains constant burning surface; non-hygroscopic (will not absorb water vapor); smokeless; and flashless. It is doubtful if a single propellant having all of these qualities will ever be developed. Some of these characteristics are obtained at the expense of others, depending on the performance desired.
While solid propellants are stored within the combustion chamber of the propulsion system, liquid propellants are stored in tanks, and INJECTED into the combustion chamber. In general, liquid propellants provide a longer
burning time than solid propellants. They have a further advantage in that combustion can be stopped and started at will by controlling the propellant flow.
When oxygen or an oxygen-rich chemical is used as an oxidizer, the best liquid fuels appear to be those rich in both carbon and hydrogen. Examples are ethyl alcohol and aniline.
In addition to the fuel and oxidizer, a liquid propellant may also contain a catalytic agent to increase the speed of the reaction. Inert additives, which do not take part in the chemical reaction, are sometimes combined with liquid fuels. An example is water, which is often added when alcohol is used as a fuel. Although such additives add no energy to the system, they contribute to a higher thrust by increasing the rate of mass flow through the system.
An ideal liquid propellant would have all of the following characteristics; easily manufactured from available raw materials; high heat of combustion per unit weight of mixture, to give a high chamber temperature; low molecular weight of the reaction products; low freezing point; high specific gravity; low toxicity and corrosiveness; low vapor pressure; and stability in storage. As with solid propellants, it is unlikely that all of these characteristics will ever be combined in a single fuel.
4B3. Components of jet-propulsion systems
The principal parts of any jet-propulsion system are the combustion chamber and the exhaust nozzle. Liquid-fuel systems require additional parts, such as injectors, pumps, and ignition systems.
The COMBUSTION CHAMBER is the enclosure within which the fuel is burned, and energy changed from potential to kinetic. The chamber is usually a cylinder, although it may sometimes be a sphere. Its length and diameter must be such as to produce a chamber volume most suitable for complete and stable combustion. The chamber length and the nozzle EXIT DIAMETER are determined by the propellants to be used. Both must be designed to produce the optimum gas velocity and pressure at the nozzle exit.
The INJECTOR is similar in function to the carburetor in a reciprocating engine. It vaporizes and mixes the fuel and oxidizer in the proper proportions for efficient burning.
Figure 4B2 shows schematic sketches of three types of injector. In the multiple-hole impingement type, oxidizer and fuel are injected through an arrangement of separate holes in such a way that the jet-like streams intersect each other at some predetermined point, where the fuel and oxidizer mix and break up into vapor-like droplets. A spray injector has oxidizer and fuel holes arranged in circles, so as to produce conical or cylindrical spray patterns that intersect within the chamber. The nonimpinging injector, shown in the lower sketch in figure 4B2, is one in which the oxidizer and fuel do not impinge at any specific point, but are mixed by the turbulence within the chamber.
Unless the fuel and oxidizer form a combination that ignites spontaneously, a separate IGNITION SYSTEM must be provided to initiate the reaction. The igniter must be located within the combustion chamber at a point where it will receive a satisfactory starting mixture that ignites readily. If either fuel or oxidizer accumulates excessively in the chamber before ignition begins, an uncontrolled explosion may result. In some systems, ignition is brought about by a SPARK PLUG similar to those used in reciprocating engines. A POWDER-CHARGE ignition system is often used for solid-fuel rockets. It consists of a powder squib which can be ignited electrically from a safe distance; it burns for a short time, with a flame hot enough to ignite the main propellant charge. A catalytic ignition system uses a solid or liquid catalytic agent that brings about chemical decomposition of the propellant.
An EXHAUST NOZZLE is a nonuniform chamber through which the gases generated in the combustion chamber flow to the outside. Its most important areas are the cross sections at the mouth, throat, and exit. These areas are identified in figure 4B3a. The function of the nozzle is to increase the velocity of the gases. Under conditions of steady flow, the weight of gas that passes any cross section in unit time is constant. (This is in accordance with Bernoulli's theorem). Thus, in subsonic flow, the velocity of the gases will increase at any point where the cross section
Figure 4B2.-Types of injectors.
Figure 4B3.-a. Location of nozzle components;
b. subsonic flow through a convergent nozzle;
c. supersonic flow through a divergent nozzle;
d. convergent-divergent, or DeLaval nozzle.
is constricted, provided the weight ratio of flow remains constant. Where the cross section becomes wider, the gas velocity will decrease. This relation of cross section to velocity always holds for subsonic flow.
Figure 4B3b shows a simple convergent nozzle. The speed of gases entering such a nozzle will increase. The greater the convergence, the greater the increase in speed, up to the local speed of sound. (When we refer to the local speed of sound at some point in the propulsion system, we mean the speed of sound that corresponds to the temperature and pressure at that specific point.) But, in a simple convergent nozzle, no further increase in speed will take place beyond Mach 1. The behavior of gases at supersonic speed differs considerably from that of gases at subsonic speed. For example, if gases at supersonic speed enter the convergent nozzle shown in figure 4B3b, they will SLOW DOWN.
Figure 4B3c shows a simple divergent nozzle. When gases enter such a nozzle at subsonic speeds, they slow down. But, when gases at supersonic speed enter a divergent nozzle, their speed is further increased. The decrease in speed in subsonic flow is in accordance with Bernoulli's theorem. Since the cross section increases and the weight-rate of flow remains constant, the velocity must decrease with a proportionate increase in pressure. But gases in supersonic flow are in a state of compression. When gases in supersonic flow enter a divergent nozzle, they
expand. A part of the potential energy contained in the compressed gas is converted into kinetic energy, and increases the velocity of flow.
To obtain a supersonic exhaust velocity, most rocket motors use a nozzle of the type shown in figure 4B3d. The exhaust nozzle first converges to bring the subsonic flow up to the local speed of sound. Then at a certain point the nozzle diverges, allowing the gases to expand and produce supersonic flow. A convergent-divergent nozzle of this type is often called a DeLaval nozzle.
A simple convergent nozzle is the most efficient type for certain combinations of fuel and missile speed. Such nozzles are often used on subsonic turbo-jets. It has been found that, with a nozzle of this type, if the internal pressure of the combustion chamber is more than about 1.7 times the external pressure, an excess pressure remains in the gases after they leave the nozzle. This excess pressure represents wasted energy. The performance of combustion systems using this type of nozzle is therefore limited.
The convergent-divergent nozzle, if properly designed, can be used to control the expansion of gases after they pass through the throat, and thus obtain higher velocity and increased thrust. The area of the throat section is determined by the weight rate of flow. The area at the exit of the divergent cone is determined by the desired ratio of expansion of the gases between throat and exit.
C. Air Jet Engines
Any jet-propelled system that obtains oxygen from the surrounding atmosphere to support the combustion of its fuel is an air-jet engine. Pulse-jets, ram-jets, turbo-jets, and turboprops are all of this type, although the latter are not used in guided missiles. Obviously, the operation of these engines is limited by the amount of oxygen available, and they can operate only at altitudes where the oxygen content of the air is adequate. The upper limit of operation depends on the type and design of the particular engine.
Pulse-jet engines are so called because of the intermittent or pulsating combustion process. This type of engine first drew international attention when it was used to propel the German V-1 "buzz bomb" during World War II. Although pulse-jet engines were used by the U. S. Navy to propel an early missile, they are now considered obsolete, and we will give them only brief treatment here.
Figure 4C1 illustrates the fundamental construction of the pulse-jet, as well as the stages of its combustion cycle. The principal
Figure 4C1.-Stages of pulse jet engine.
parts of a pulse-jet are the diffuser, grill assembly (containing air valves, air injectors, and fuel injectors), the combustion chamber, and the tail pipe. The DIFFUSER is a duct of varying cross section at the forward end of the engine, between the air intake and the grill. Between these two points the diameter increases; as a result the velocity of air entering the diffuser decreases, and its pressure increases.
The GRILL ASSEMBLY carries the fuel injectors, injectors for starting air, and the air-intake "flapper" valves. The latter are spring loaded, and are normally closed, so as to completely block off the diffuser from the combustion chamber. As the engine moves through the air, ram-air pressure builds up in the diffuser. When this pressure exceeds that of the combustion chamber and the spring, the valves open and air enters the combustion chamber. Fuel is then injected, and the air-fuel mixture is ignited by a spark plug. The
burning fuel produces a rapid increase in pressure within the chamber, and the flapper valves close.
As the pressure in the combustion chamber rises, it exceeds the pressure in the fuel system, and automatically shuts off the flow of fuel. The flaming gases rush down the tailpipe and exhaust to the atmosphere. The momentum given to the working fluid thus provides thrust to propel the engine. Because of the speed with which the combustion gases rush down the tailpipe, they overexpand and produce a partial vacuum within the combustion chamber (middle sketch in figure 4C1.) The ram pressure in the diffuser then exceeds the pressure in the chamber; the flapper valves open, and a fresh supply of air enters the chamber. Because of the decrease in pressure, the pressurized fuel system is able to inject a fresh supply of fuel. As a result of the partial vacuum, a portion of the hot exhaust gas is drawn back into the chamber; the temperature of this gas is high enough to ignite the air-fuel mixture, and a new cycle begins. Note that the spark plug ignition is required only to start the engine; after starting, its combustion cycle is self-sustaining.
The frequency of the combustion cycle is the resonant frequency of the combustion chamber and tailpipe. A formula for resonant frequency of a closed pipe is:
Frequency = (Velocity of sound) / (4 x length)
The frequency of various pulse-jet engines that have been used in the past ranges from about 50 to over 200 cycles per second.
At the instant of launching, there is no ram pressure in the diffuser of a pulse-jet. For that reason, most pulse-jets are incapable of developing enough static thrust to take off under their own power. They are therefore launched with the help of compressed air injected into the chamber along with the fuel, or from a catapult, or with booster rockets, or by a combination of these means. The speed of a pulse-jet is limited to the low subsonic range by the fact that at higher speeds the ram pressure developed in the diffuser exceeds the chamber pressure at all times throughout the combustion cycle; the flapper valves therefore cannot close, and the cycle can not maintain itself.
A turbo-jet engine is an air-dependent thermal jet-propulsion device. It derives its name from the fact that its compressor is driven by a turbine wheel, which is itself driven by the exhaust gases. Turbo-jets may be divided into two types, depending on the type of compressor. These are CENTRIFUGAL-FLOW TURBO-JETS (fig. 4C2) and AXIAL-FLOW TURBO-JETS (fig. 4C3). Both types are the same in operating principles. The major components of both are an accessory section, compressor section, combustion section, and exhaust section.
Figure 4C2.-Centrifugal-flow turbo-jet.
The ACCESSORY SECTION serves as a mounting pad for accessories, including the generator, hydraulic pump, starter, and tachometer, for various engine components, such as units of the fuel and oil systems, and for the front engine balancing support.
The primary function of the COMPRESSOR SECTION is to receive and compress large masses of air, and to distribute this air to the combustion chambers. The centrifugal compressor consists of a STATOR, often referred to as a DIFFUSER VANE ASSEMBLE, and a rotor or impeller. The rotor consists of a series of blades which extend radially from the axis of rotation. As the rotor revolves, air is drawn in, whirled around by the blades, and ejected by centrifugal force at high velocity.
The stator consists of diffuser vanes that compress the air and direct it into the various firing chambers. Air leaves the impeller wheel at high velocity. As it passes through the diffuser vanes it enters a larger space; its velocity therefore decreases, and its pressure increases.
The axial compressor is similar to a propeller. The rotor consists of a series of blades set at an angle, extending radially from the central axis. As the rotor of the axial compressor turns, the blades impart energy of motion in both a tangential and axial direction to the ram air entering through the front of the engine. The stator does not rotate. Its blades are set at an angle so as to turn the air thrown off the trailing edge of the first-stage rotor blades, and redirect it into the path of the second-stage rotor blades. One rotor and one stator comprise a single-stage compressor. A number of rotors and stators assembled alternately make up a multistage compressor, as in figure 4C3.
In a multistage compressor, air from the first row of compressor blades is accelerated and forced into a smaller space. The added
Figure 4C3.-Axial flow turbo-jet.
velocity gives the air greater impact force. This compresses the air into a smaller space, causing its density to increase. The increase in density results in a corresponding increase in static pressure. This cycle of events is repeated in each successive stage of the compressor. Therefore, by increasing the number of stages, the final pressure can be increased to almost any desired value.
The COMBUSTION SECTION includes combustion chambers, spark plugs, a nozzle diaphragm, and a turbine wheel and shaft. The combustion chambers, or BURNERS, in both types of turbo-jet engines, have the same function and produce the same results. But they differ in size and number, depending on the type of engine: In any case, each combustion chamber has the following parts: outer combustion chamber, inner liner, inner liner dome, flame cross over tube, and fuel-injector nozzle.
The outer combustion chamber retains the air so that a high-pressure supply is available to the inner liner at all times. This air also serves as a cooler jacket. The inner liner houses the area in which fuel and air are mixed and burned. Many round holes in the inner liner allow the air to enter and mix with the fuel and high-temperature combustion gases. The forward end of the inner liner is allowed to slide over the dome to accommodate expansion and contraction. The after end of the burners are convergent to increase the velocity of the gases just before they pass through the nozzle diaphragm. The flame cross over tube connects one chamber to the next, allowing ignition to occur in all chambers after the two chambers containing spark plugs have fired.
The EXHAUST SECTION consists primarily of a nozzle and an inner cone. This assembly straightens out the turbulent flow of the exhaust gases caused by rotation of the turbine wheel, and conveys these gases to the nozzle outlet in a more perfect and concentrated gas-flow pattern.
The exhaust-nozzle diaphragm is composed of a large number of curved blades standing perpendicular to the flow of combustion gases and arranged in a circle in front of the turbine wheel. By acting as both a restrictor and a director, this diaphragm increases the gas
velocity. Its primary function is to change the direction of the gases so that they strike the turbine-wheel vanes at, or nearly at, a 90° angle. The impact of the high-velocity gases against the buckets of the turbine wheel causes the wheel to rotate. The turbine-wheel shaft is coupled to the compressor-rotor assembly shaft. Thus, part of the energy of the exhaust gases is transformed and transmitted through the shaft to operate the compressor and the engine-driven accessories.
The operation of a turbo-jet may be summarized as follows: The rotor unit of the compressor is brought up to maximum allowable speed by the starter unit, which is geared to the compressor shaft for starting. Air is drawn in from the outside, compressed, and directed to the combustion chambers. Fuel is injected through the fuel manifold under pressure, and mixes with the air in the combustion chambers. Ignition occurs first in the chambers containing the spark plugs, and then in the other chambers an instant later by way of the flame cross over tubes. High-pressure combustion gases and coolant air pass through the exhaust nozzle diaphragm and strike the turbine blades at the most effective angle. Part of the energy of the exhaust stream is absorbed by the turbine, resulting in a high rotational speed. The remainder is thrust. The turbine wheel transmits energy through the coupled turbine and compressor-rotor shafts to operate the compressor. Once started, combustion is continuous.
The afterburner is an important part of jet fighter aircraft; afterburners have limited application in guided missile propulsion. They were developed to give additional thrust when needed for short periods of time, as in launching or during a steep programmed climb.
The additional thrust is obtained by burning additional fuel in the tailpipe section. That portion of the air which served only as a coolant for the main combustion chambers is sufficient to support combustion of the additional fuel. The added thrust is large, but the over-all efficiency of the turbo-jet decreases because the specific fuel consumption is greatly increased. During a missile launching, an afterburner could provide approximately 30% increase in thrust; when the missile reaches a speed of 600 mph, an afterburner can increase its thrust by from 70% to 120%.
A ram-jet engine derives its name from the ram action that makes its operation possible. (This engine is sometimes referred to as the athodyd, meaning aerothermodynamic duct).
Ram-jet operation is limited to altitudes below about 90,000 feet because atmospheric oxygen is necessary for combustion. The velocity that can be attained by a ram-jet engine is theoretically unlimited. The faster a ram-jet travels the more efficiently it operates, and the more thrust it develops. But its upper speed is limited, in practice, to about Mach 5.0, because of frictional heating of the missile skin. The major disadvantage of a ram-jet is that the higher the speed at which it is designed to operate, the higher the speed to which it must be boosted before automatic operation can begin.
Basically, a ram-jet consists of a cylindrical tube open at both ends, with a fuel-injection system inside. The engine is extremely simple in design, and it has no moving parts. Even though all ram-jets contain the same basic parts, the structure of these parts must be modified to produce satisfactory operation in the various speed ranges. The principal parts of a ram-jet engine are a diffuser section, and a combustion chamber that contains fuel injectors, spark plugs, flame holder, and an exhaust nozzle.
The DIFFUSER SECTION serves the same purpose in the ram-jet as it does in the pulse-jet. It decreases the velocity and increases the pressure of the incoming air. Since there is no wall or closed grill in the front section of a ram-jet, the pressure increase of the ram air must be great enough to prevent the escape of the combustion gases out the front of the engine. Diffusers must be especially designed for a specific entrance velocity, or predetermined missile speed. In other words, the desired pressure barrier is developed only when air is entering the diffuser at the speed for which that particular diffuser was designed.
The COMBUSTION CHAMBER is of course the area in which burning occurs and high-pressure gases are generated. The fuel injectors are connected to a continuous-flow fuel supply system, adequately pressurized to permit fuel to flow against the high pressures that exist in the forward section of the
combustion chamber. Combustion is started by a spark plug; once started, it is continuous and self-supporting. The flame holder prevents the flame front from being swept too far toward the rear of the engine, thus stabilizing and restricting the actual burning to a limited area. The flame holder also insures that the combustion-chamber temperature will remain high enough to support combustion.
The EXHAUST NOZZLE performs the same function as in any jet-propulsion engine.
A SUBSONIC RAM-JET engine cannot develop static thrust; therefore, it cannot take off under its own power. If fired at rest, high-pressure combustion gases would escape out the front as well as the rear. For satisfactory operation, the engine must be boosted to a suitable subsonic speed so that the ram air entering the diffuser section develops a pressure barrier high enough to confine the escape of combustion gases to the rear only. Figure 4C4 is a diagram of a subsonic ram-jet engine. Note the simple tubular construction, and the openings at front and rear.
As ram air passes through the diffuser section (fig. 4C4), the velocity of the air decreases while the pressure increases. This is brought about by the increase in cross section of the diffuser, in accordance with Bernoulli's theorem for incompressible flow. Fuel is sprayed into the combustion chamber through the fuel injectors. The atomized fuel mixes with the incoming air, and the mixture is ignited by the spark plug. As previously stated, burning is continuous after initial ignition, and no further spark plug action is needed.
The gases that result from the combustion process expand in all directions, as shown by the arrows in the central part of the combustion chamber (fig. 4C4). The gases, as they expand in the forward direction, are stopped by the barrier of high-pressure air and the internal sloping sides of the diffuser section, as indicated in the diagram by the short, wide black arrows. The only avenue of escape remaining for the combustion gases is through the exhaust nozzle, and here another important energy conversion occurs; The pressure energy of the combustion gases is converted to velocity. The gases enter the exhaust nozzle at less than the local speed of sound. But, while they pass through the convergent nozzle, the pressure energy of the gases decreases
Figure 4C4.-Structure and combustion processes of a subsonic ram-jet.
and the velocity increases up to the local speed of sound at the exhaust nozzle exit.
Thrust is developed in the ram-jet as a result of the unbalance of forces acting in the forward and rearward directions. The bombardment of combustion gases against the sloping sides of the diffuser and the ram-air barrier exert a force in the forward direction. This forward force is not balanced by the combustion gases that escape through the exhaust nozzle. The unbalanced force constitutes the thrust that propels the missile.
In order to operate, a LOW-SUPERSONIC RAM-JET must be boosted to a supersonic speed, approximately equal to its operating speed, before ignition. When the forward speed of the ram-jet becomes supersonic, a normal shock wave forms at the entrance to the diffuser section. The location of this shock wave is shown in figure 4C5. On the upstream side of the normal shock wave, the free-stream air is moving at a low supersonic velocity. As the supersonic air passes through the shock wave, its velocity drops abruptly to a subsonic value, with a corresponding increase
Figure 4C5.-Structure and combustion processes of a low supersonic ram-jet.
in pressure. Thus the shock wave produces a sudden increase in air pressure at the diffuser entrance. As the compressed subsonic air flows through the diverging diffuser section, an additional increase in pressure and decrease in velocity occurs.
The combustion process is essentially the same as that in a subsonic ram-jet. Fuel is mixed with the highly compressed air, the mixture is ignited initially by a spark plug, and burning is continuous thereafter. The potential energy possessed by the combustion gases is converted into kinetic energy by the exhaust nozzle.
The convergent-divergent nozzle shown in figure 4C5 allows the gases to exceed the local speed of sound. Therefore, with proper design modifications, the ram-jet engine can travel efficiently at supersonic speed.
Now, assume that we want to design a ramjet that will travel at HIGHER SUPERSONIC SPEEDS, say Mach 2.0. At speeds of around Mach 2.0, shock waves formed at the diffuser inlet are oblique, rather than normal. Air velocity in front of an oblique shock wave is high supersonic. When supersonic free-stream air passes through an oblique shock wave, an increase in pressure and a decrease in velocity occur, but the velocity is still supersonic. For example, air with a free-stream velocity of 1,500 mph may pass through an oblique shock wave and still have a velocity of 900 mph. Also, when supersonic air flows through
divergent-type diffuser actions, as shown in figures 4C4 and 4C5, the velocity of that air increases and the pressure decreases. Therefore, the diffuser design for high-supersonic ram-jets must be modified so that in progressing from diffuser inlet to combustion-chamber entrance, the obliqueness of the shock wave successively decreases until a normal shock wave followed by subsonic flow is produced.
This energy transformation is achieved by using a diffuser of the type shown in figure 4C6. The diffuser center body decreases the obliqueness of the shock waves, allowing supersonic air to flow inside the diffuser inlet.
As supersonic flow passes through the divergent section of the diffuser, the velocity is steadily decreased and the pressure correspondingly increased. But, at some predetermined point in the diffuser, air velocity approaches the sonic value and a normal shock wave forms. As previously stated, when low-supersonic air flows through a normal shock wave, an abrupt decrease in velocity and increase in pressure results. The subsonic air produced by the normal shock wave flows through the divergent section of the diffuser, where it undergoes an additional velocity decrease and pressure increase. Here again the diffuser has achieved a pressure barrier at the entrance to the combustion chamber. The combustion process is the same as that described for the subsonic ram-jet. The exhaust nozzle shown in the diagram is of the
Figure 4C6.-Structure and combustion processes of a high supersonic ram-jet.
convergent-divergent type designed to produce supersonic flow at the exit.
A ram-jet is designed to operate best at some given speed and altitude. The pressure recovery process in a diffuser designed for
oblique shock waves is more efficient than that in diffusers designed for subsonic flow or single normal shock waves. For that reason the ram-jet engine operates best at high supersonic speeds.
D. Rocket Motors
Unlike a jet engine, a rocket carries within itself all the mass and energy required for its operation. It is independent of the surrounding medium. In a rocket, the chemical reaction takes place at a very rapid rate. This results in higher temperatures, higher operating pressures, and higher thrust development than in jet engines. Because of the high pressures developed in rocket motors, the convergent-divergent nozzle is used so that more of the energy can be extracted from the gases after they have passed the throat section. The basic principles involved in the action of other jet-propulsion units also apply to rockets.
Depending on the physical state of the propellant used, rockets are designated as either SOLID or LIQUID type.
A solid rocket has a short burning time, simple design, heavy construction, and non-intermittent operation. It is therefore primarily used for booster units, and as a power-plant for relatively short-duration, high-speed missiles. But recent research seems to indicate that solid fuels will have increasing future applications in long-range missiles. The Navy's Polaris (IRBM) is propelled by a solid-fuel rocket.
The liquid rocket unit has a longer burning time, relatively complicated design, and intermittent operation possibilities. This system has been widely used as a powerplant for high-altitude, long-range missiles.
4D2. Liquid-fuel rockets
The major components of a liquid-rocket system are the propellant, propellant-feed system, combustion chamber, igniter, and exhaust nozzle. The propellant-feed system is the only one which has not been explained, in principle, in the preceding sections of this chapter. Feed systems may be of the pressure-feed type or the pump-feed type.
Pressure-feed systems may be subdivided into stored-pressure and generated-pressure systems. In the stored-pressure system, air or some other gas is stored under pressure in the missile before launching. It is injected, in controlled amounts, into the propellant storage tanks, causing a pressurized flow toward the combustion chamber. In a generated-pressure system, substances are carried within the missile to generate the high-pressure gas as it is needed. An example of such a substance is hydrogen peroxide, which, when passed through a catalyst, decomposes to form a high-pressure vapor. This vapor is then injected into the propellant storage tanks.
Many other devices such as valves, regulators, delivery tubes, and injectors, are necessary for the successful operation of either system.
Figure 4D1 shows the general relationship of the various major parts of a typical stored-pressure feed system. In the system shown, air is stored under a pressure of 200 psi. The hand-arming valve is opened manually, just before launching. This allows the system to be pressurized up to the motor-start valve. The air-pressure regulator decreases the pressure to the desired value required for operation of the system components-in this case 100 psi.
The motor-start valve is electrically operated. It is opened from a safe distance after all personnel have cleared the immediate launching area. Air at 100 psi enters and pressurizes the fuel and oxidizer tanks. These tanks must be made of material that is not affected by the respective propellants. In addition, they must be strong enough to withstand the added air pressure. At the same time the propellant tanks are pressurized, air also enters the hydraulic accumulator and pressurizes the hydraulic fluid. The hydraulic fluid displaces the piston in the propellant valve actuating cylinder, which in turn opens the propellant valves. Fuel and oxidizer,
Figure 4D1.-Stored-pressure feed system of a liquid rocket.
under pressure, now flow through the respective mixtures orifices, which regulate the flow so that the correct mixture ratio is maintained. These orifices are simply restrictions in the line, and are flow-checked prior to installation. In some cases the injectors perform this operation, and orifices are not necessary. The propellants are atomized by the injectors. Note that the oxidizer first circulates between the walls of the combustion chamber before passing through the cut-off valve. This action is called REGENERATIVE COOLING.
Pump-feed systems are used with power-plants designed to burn large volumes of propellants, and with plants requiring a high weight rate of flow. A pump-feed system consists of a fuel pump and an oxidizer pump, both driven by a turbine wheel. Power for driving the turbine wheel may be provided by a gas generated by chemicals carried within the missile for that purpose (turbine-pump system), or the turbine wheel may receive its
power from the exhaust gases of a rocket motor (turbo-pump system).
Because of the intense heat developed in liquid-rocket combustion chambers, it is important that the inner walls of the chamber, throat, and exit be cooled. Uncooled operation over a prolonged period reduces physical strength and may even melt parts of the motor.
The regenerative cooling method shown in figure 4D1 is often used. Before injection into the chamber, the fuel or oxidizer is circulated from front to rear between the walls of the combustion chamber. The heat absorbed by the fuel or oxidizer cools the chamber and adds to the energy originally contained in the propellant.
A film-cooling procedure consists of low-velocity injection of a portion of the fuel, oxidizer, or some nonreactive liquid into the chamber at critical points. The fluid forms a protective film on the inner walls, and absorbs heat from the walls as it evaporates.
Liquid propellants may be classified either as monopropellants or bipropellants. Mono-propellants are those in which the fuel and oxidizer are combined as a single substance. This may be a physical combination-for example hydrogen peroxide mixed with ethyl alcohol, or a chemical combination such as nitromethane. Monopropellants are stable at ordinary temperatures, but when activated by an ignition system they decompose and liberate hot combustion gases. Monopropellants are seldom used in guided missiles because of the possibility of detonation being propagated from the combustion chamber, through the fuel lines, to the storage tanks.
The fuel and oxidizer of a bipropellant are kept apart until they are injected into the combustion chamber. Most liquid propellants for guided missiles are of this type.
Aniline, hydrazine hydrate, and ethyl alcohol are among the more commonly used liquid rocket fuels. Analine is an oily clear liquid with a specific gravity of 1.022. It has a boiling point of about 363° F and a freezing point of about 21° F. On contact with red fuming nitric acid, it ignites spontaneously. A fuel and oxidizer combination that reacts in this manner is said to be HYPERGOLIC. This combination was successfully used in the WAC Corporal missile.
Hydrazine hydrate is a colorless liquid, slightly heavier than water. It is explosive when its concentration is above 25%. Hydrazine hydrate gives a hypergolic reaction with hydrogen peroxide.
Ethyl alcohol is a clear liquid, lighter than water. It is stable to shock and temperature changes. It is readily available because of its wide commercial market in the chemical and liquor industries.
Liquid oxygen, and various forms of nitric acid, are among the most commonly used oxidizers in liquid rockets. Liquid oxygen is made by liquefying air and boiling off the nitrogen and other gases. This bluish liquid has a boiling point of about minus 297° F, and a freezing point of minus 363° F. Because of its low boiling point, its rate of evaporation is very high. For this reason, storage and shipment to launching areas presents serious problems, and results in appreciable loss. When poured on metal at ordinary temperature, liquid oxygen acts like water dropped on a red-hot stove. Evaporation loss in the German
V-2 missile was about 4.4 pounds per minute between the time of fueling and launching.
The extremely low temperature of liquid oxygen causes water vapor from the surrounding atmosphere to collect and freeze on pipes and valves. This is a serious problem, which has yet to be fully solved. Liquid oxygen is noncorrosive and nontoxic, but will cause severe damage if it comes into contact with skin.
Nitric acid is used in several different forms as an oxidizer for liquid rockets. The most commonly used and the most powerful of these is RED FUMING NITRIC ACID (RFNA), which consists of nitric acid in which nitrogen dioxide is dissolved. It varies in color from orange to brick red, and gets its name from the reddish color of the nitric oxide fumes it gives off. RFNA is highly corrosive, and stainless steel must be used for storage tanks and delivery pipes. Its high vapor pressure presents storage and transfer problems. The fumes are extremely poisonous, and severe burns result from bodily contact with the liquid. This oxidizer has been successfully used with analine, giving up approximately 63.5% of its oxygen content for combustion.
Hydrogen peroxide is a colorless liquid which, in concentrations of from 70% to 90%, may be used as a monopropellant in guided missiles. When in contact with a suitable catalyst it decomposes, forming steam and gaseous oxygen. When 90% hydrogen peroxide decomposes, about 42% of the total weight of the decomposition products is gaseous oxygen. Therefore, it is also used as an oxidizer with such fuels as alcohol and hydrazine hydrate. A third use for hydrogen peroxide is as a pressurizing agent. The gaseous products of decomposition may be jetted against a turbine wheel which drives fuel and oxidizer pumps connected to the turbine shaft.
4D3. Solid-fuel rockets
A solid rocket unit consists of the propellant, combustion chamber, igniter, and exhaust nozzle. A typical solid rocket motor is shown in figure 4D2.
The combustion chamber of a solid rocket serves two purposes. First, it acts as a storage place for the propellant. Second, it serves as a chamber in which burning takes place. Depending the grain configuration used
Figure 4D2.-Components of solid rocket motor.
this chamber may also contain a device for holding the grain in the desired position, a trap to prevent flying particles of propellant from clogging the throat section, and resonance rods to absorb vibrations set up in the chamber.
The igniter consists of a small charge of black powder, or some other material that can be easily ignited by either a spark discharge or a hot wire. As it burns, the igniter produces a temperature high enough to ignite the main propellant charge.
The exhaust nozzle serves the same purpose as in any other jet-propulsion system. It must be of heavy construction, because of the high temperatures of the exhaust jet.
Operation of a solid rocket is simple. The propellant charge is ignited electrically from a safe distance. The igniter squib assembly is blown out, and the rocket burns continuously until the propellant supply is exhausted.
Cooling is not usually a serious problem in a solid rocket, because the burning time is relatively short. One method of preventing excessive heat from reaching the chamber walls is to use a hollow-restricted charge. With a charge of this type, burning takes place only on the inner surface, and the outer walls of the grain tend to act as an insulator. This becomes less and less effective as the grain is burned thinner and thinner.
Solid propellant charges are classified as restricted burning or unrestricted burning. A RESTRICTED-BURNING charge has some of its exposed surfaces covered with an INHIBITOR. This makes it possible to control the burning rate by confining the burning area
to the desired surface or surfaces. The use of inhibitors lengthen the burning time of the charge, and helps to control the combustion-chamber pressure. A burning cigarette can be considered as a model of an inhibited rocket grain, with the paper representing the inhibitor.
UNRESTRICTED-BURNING charges are permitted to burn on all surfaces at once. The unrestricted grain delivers a relatively large thrust for a short time, and the restricted grain yields a smaller thrust for a longer time.
The burning characteristics of a solid propellant depend on its chemical composition, initial temp era t u r e, combustion-chamber temperature and pressure, gas velocity adjacent to the burning surface, and size and shape of the grain. One propellant grain may burn in such a way that the burning area remains constant, producing constant thrust. This type of burning is known as NEUTRAL BURNING. Another grain may increase its burning area as burning progresses. In this case, PROGRESSIVE BURNING is taking place. Still another grain may show a constantly decreasing burning area as burning progresses. This is called DIGRESSIVE BURNING.
The BURNING RATE of a solid propellant is the rate at which the grain is consumed; it is a measure of linear distance burned, in inches per second, in a direction perpendicular to a burning surface.
As stated earlier, thrust depends on mass rate of flow and the change in velocity of the working fluid. For large thrust, a large burning area is necessary in order to yield a
large mass flow. A smaller burning area produces less mass flow and less thrust. Therefore, by varying the geometrical shape and arrangement of the charge, the thrust developed by a given amount of propellant in a given combustion chamber can be greatly influenced.
A restricted-burning charge is usually a solid cylinder which completely fills the combustion chamber and burns only on the end. The thrust is proportional to the cross-section area of the charge, and burning time is proportional to length. An unrestricted burning charge is usually hollow, and burns on both the outside and inside surfaces. Thrust is again proportional to the burning area. Since the inside area increases while the outside area decreases during burning, it is possible to maintain a nearly constant burning area. The burning time of hollow grains depends on the WEB THICKNESS-the distance between the inside and outside surfaces.
One limitation of solid propellants is sensitivity to temperature. The initial temperature of a grain noticeably affects its performance. A given grain will produce more thrust on a hot day than it will on a cold day. A grain designed to produce 1,000 pounds of thrust at 80° F may deliver only 600 pounds of thrust at 30° F. The initial temperature also affects the burning rate. Because of these characteristics, solid propellants must be stored in areas of controlled temperature until they are used. The percentage change in thrust per degree Fahrenheit temperature change is referred to as the TEMPERATURE SENSITIVITY of the propellant.
Temperature also affects the physical state of solid-propellant grains. At extremely low temperatures, some grains become brittle and are subject to cracking. Cracks increase the burning area and burning rate and therefore increase the combustion-chamber pressure. If this pressure exceeds that for which the chamber was designed, the chamber may explode. A propellant exposed to high temperature before firing may lose its shape, and become soft and weak. This, too, results in unsatisfactory performance. The temperature range for most solid propellants is from about 25° F to 120° F.
PRESSURE LIMITS play an important part in solid propellant performance. Below a certain chamber pressure, combustion becomes
highly unstable. Some propellants will not sustain combustion at atmospheric pressure. Ordinarily, chamber pressure for solid propellants must be relatively high. For a given propellant composition and burning area, the chamber pressure is determined by the area of the exhaust nozzle throat. If the throat area is too large, for example, proper chamber pressure cannot be maintained.
Decomposition and hygroscopic tendencies are other weaknesses of solid propellants, but both can be minimized by the use of certain additives.
Some of the more common propellants are discussed below. The chemical formulas of some of them are given, to show the carbon and/or hydrogen content, and the oxygen content of the oxidizers.
One of the first solid propellants used was BLACK POWDER. Its approximate composition is:
Potassium nitrate (KNO3)
Both charcoal and sulphur react readily with oxygen. Potassium nitrate, as shown by its formula, contains large quantities of oxygen. The three ingredients are thoroughly mixed, using some substance such as glue or oil as a BINDER.
When heat is applied to black powder, the potassium nitrate gives up oxygen. The oxygen reacts with the sulphur and carbon, producing intense heat and large volumes of carbon dioxide and sulphur dioxide. These two gases make up the major part of the exhaust jet. The heat produced by the reaction gives high velocity to the exhaust gases. Black powder has a specific impulse of about 65 lb-sec/lb. One of its drawbacks is that it is quite sensitive to storage temperatures, and tends to crack. Its exhaust velocity ranges from 1,500 to 2,500 feet per second. It is now used primarily for signal rockets, and as an igniter for other solid-propellant grains.
BALLISTITE is a double-base propellant; it contains two propellant bases, NITROCELLULOSE and NITROGLYCERINE. It also contains small amounts of additives, each performing a specific function. A STABILIZER absorbs the gaseous products of slow decomposition, and reduces the tendency to absorb moisture during storage. A PLASTICIZER serves as a binding agent. An OPACIFIER is
added to absorb the heat of reaction and prevent rapid thermal decomposition of the unburned part of the grain. A FLASH DEPRESSOR cools the exhaust gases before they escape to the atmosphere, thus preventing a burning-tail effect. A typical ballistite composition is
Nitrocellulose C24H40O20 (NO3)
1.45% (flash depresser)
Ballistite has a specific impulse of about 210 lb-sec/lb. Its exhaust is relatively smokeless. Storage temperatures between 40° F and 120° F are necessary to prevent rapid decomposition. The ingredients of ballistite are subject to detonation, and are toxic when they come in contact with the skin. The manufacturing process is difficult and dangerous.
GALCIT consists of about 25% asphalt-oil mixture, which serves as both fuel and binder, and 75% potassium perchlorate (KCI04), which serves as an oxidizer. In its finished form, Galcit resembles stiff paving tar. Recommended temperature limits for firing are 40° F to 100° F. The specific impulse of galcit is about 186 lb-sec/lb. It is quite stable to temperature; storage temperature limits are minus 9° F to 120° F. Galcit is relatively easy to manufacture. It is nonhygroscopic-that is, it does not absorb moisture. Its major disadvantage is that its exhaust develops dense clouds of white smoke.
NDRC PROPELLANTS were developed through research sponsored by the National
Defense Research Committee. A typical composition consists of about equal parts of ammonium picrate and sodium nitrate (46.5% each), and 7% resin binder (usually urea formaldehyde). This propellant has good thermal stability. It is hygroscopic, and must therefore be stored in sealed containers. Heavy smoke develops in the exhaust gases.
4D4. Nuclear-powered rockets
Serious consideration is being given to the use of nuclear power for missile propulsion. A nuclear powerplant would greatly increase both the speed and range of missiles. Present propulsion systems would become obsolete as major powerplants. But they may still serve as boosters for takeoff and initial acceleration, to prevent radioactive contamination of the launching area.
One of the main advantages in the use of nuclear power is that it provides an almost inexhaustible source of heat. In a missile propelled by nuclear energy, the fuel supply would remain practically constant throughout the flight. Enough fuel to start the reaction would be enough for sustained operation. But other material, such as water, will be required in the missile to absorb the heat developed by the powerplant, and be accelerated to produce thrust.
The major problems confronting the engineers are protecting the launching personnel from radiation damage, and developing a nuclear powerplant small enough to be carried in a guided missile. Many years of extensive technical development may be needed before nuclear energy can be harnessed for use as a missile powerplant. But the outlook is promising.