Introduction to torpedo fire control




The Nature of the Torpedo as a Weapon

In the last section of these instruction sheets you learned how surface ships employ various weapons against submarines. You saw that a good deal of intricate equipment is used to detect, locate and attack these underwater enemies. You may ask why antisubmarine warfare is so important -certainly a sub's deck guns, if it had any, would be of no value against more heavily armed surface ships. The answer, of course, is that a submarine's primary weapon is the torpedo-a self-propelled missile designed to travel underwater and carry an explosive charge to an enemy vessel. Using torpedoes, subs and small surface ships have a "Sunday punch" to throw at large surface ships.

Torpedoes are very powerful and effective weapons, and their use is not restricted to submarines. In addition to being fired from submerged subs, they may be dropped from planes or launched from torpedo (PT) boats and destroyers.

How the navy uses torpedoes.  Destroyer, aircraft, submarines, PT Boats

There are two general classifications of torpedoes-homing and non-homing (or aimed). The homing torpedo, which seeks out its own target, was discussed briefly in the preceding section. The aimed torpedo, which is set to follow a fixed course after firing, was for many years the only type in use. At present, the homing torpedo is rapidly coming to the fore, but the non-homing type will remain important for some time, and you should know something about aimed torpedoes and how they are used.

On the following sheets we will limit our discussion to the non-homing torpedo and the fire control problem it presents when fired from destroyers.




Comparing the Surface and Torpedo Fire Control Problems

The non-homing torpedo is a weapon which can be used effectively only against targets on the ocean's surface. In this section of instruction sheets we are considering only its use by destroyers-the Navy's primary submarine hunter. Our discussion, then, is concerned with the problem of controlling the firing of a missile by one surface ship against another surface ship. Thus the torpedo fire control problem is very closely related to the surface (or main-battery) fire control problem. In order to solve both problems it is necessary to determine the target's position by measuring its bearing and range only-no target elevation or depth is involved as in the antiaircraft and antisubmarine problems. When the problem has been solved the torpedo is launched and follows a fixed, pre-determined course calculated to intercept the target at its future position, just as main-battery guns are trained and elevated to fire a projectile on a fixed, predetermined trajectory calculated to carry the projectile to the target's future position. The only thing which distinguishes the two attacks is the way in which the missile is delivered to the target. The gun's projectile travels on a curved trajectory though the air, while the torpedo travels in an essentially straight line just below the surface of the ocean. In addition, of course, the projectile receives all of its driving force from the gun, and thus proceeds as a free missile, while the torpedo is self-propelled.

Surface and torpedo fire are closely related

You can see that there is much similarity between the torpedo fire control problem and the basic surface problem. Now let's go on to learn more about torpedoes, how they are used and the fire control system which solves the torpedo problem.




The Construction and Operation of Torpedoes

There are various types of aimed torpedoes in use by the Navy today, but they are all basically the same in that they contain an explosive charge, a drive mechanism, and gyroscopic control devices.

Aimed torpedoes may use either an air-steam or an electric drive mechanism. The air-steam type was the "tin-fish" of World War II fame. It has the disadvantage, however, of leaving a wake of foamy white sea water behind it due to its exhaust. This wake can often be spotted by the target ship soon enough to maneuver out of the torpedo's path. To overcome this disadvantage, the electric torpedo was developed. This type is driven by an electric motor which receives power from storage batteries. In other respects it is similar to the air-steam type. Let's take a look inside a typical air-steam torpedo and see how it works.

Parts of a typical air-steam aimed torpedo

The warhead contains the explosive charge and the exploder, which is a device which detonates the explosive charge when the torpedo reaches its target. The air flask section includes flasks which contain the air, water and fuel required to propel and guide the torpedo. The air, fuel and water are combined and the resultant high pressure gas is fed to and spins the engine's turbines. The turbines turn the propeller shafts which drive the propellers in the torpedo's tail.

The afterbody contains, in addition to the engine, the gyro mechanism and depth and steering devices. These devices are driven by high pressure air from the air flask, and by making use of gyroscopic action and a water pressure gauge, keep the torpedo traveling on the desired course and at the desired depth beneath the surface. All of these mechanisms are set in motion when the torpedo is fired. The electric torpedo is very similar to the air-steam type, with storage batteries replacing the air flask and an electric motor replacing the air-steam engine. A smaller air tank carries air to drive the control devices.

Now let's see how torpedoes are used by destroyers and what the torpedo fire control problem is.




How Destroyers Use the Torpedo

The typical torpedo carried by a destroyer is 21 inches in diameter, 24 feet long and weighs 3850 pounds. It is carried, aboard ship, in one of several torpedo tubes which are mounted side by side on a torpedo tube mount. The tube mount can be moved in train within certain limits. When fired, the torpedo is ejected from its tube by means of compressed air or a small charge of black powder with sufficient force to clear the firing ship. As it leaves the tube, its internal mechanisms are set in motion so that it is under its own power by the time it hits the water. The mount is trained, after solution of the fire control problem, to start the torpedo in the direction which, with the torpedo's gyro settings considered, is calculated to score a hit.

The functions of the torpedo tube mount.

The modern destroyer carries one or more torpedo tube mounts, each mount consisting of five tubes. Thus in battle a single destroyer can fire five or more torpedoes simultaneously or in rapid succession at an enemy ship.

Now that you know a little about torpedoes and the mechanics of firing them from destroyers, let's find out about the problems involved in directing this powerful weapon against the enemy-the torpedo fire control problem.




Methods of Firing Torpedoes

A destroyer torpedo attack will usually be detected by the target ship in the early stages; therefore the torpedoes must be fired at comparatively long ranges. Unfortunately, this gives the target time to maneuver so as to avoid being hit. For this reason, destroyers usually fire several torpedoes in rapid succession to form a spread or fan-shaped pattern. This procedure greatly increases the possibility of obtaining a hit, and is similar to the method used in antisubmarine warfare in that the fire control problem is solved with respect to the center torpedo in the pattern. A group of destroyers in battle usually fires its torpedoes as a unit, with each ship firing a spread. This combination of spreads produces a pattern which is almost certain to result in some hits. With spread firing it is possible to neglect certain errors which would have to be considered in the fire control problem if a single torpedo were fired to hit a given point of aim.

The two methods of firing torpedoes in a spread are: (1) straight fire, and (2) curved fire. In both methods the spreads are produced by setting small angular offsets into the gyro mechanisms of the individual torpedoes, thus causing each to follow a slightly different course. Curved fire is accomplished by setting an additional uniform offset angle in all the torpedoes in the mount to the angle it is desired for the torpedoes to turn right or left after being launched. Curved fire is used when the desired torpedoes' course is such that straight fire would cause the torpedoes to strike some part of the ship's superstructure.

Methods of Torpedo Fire
Straight fire (seldom used)
Straight fire with spread
Curved fire with spread




The Nature of the Torpedo Fire Control Problem

The torpedo fire control problem is very much like the surface fire control problem. Its solution requires the determination of present target position and prediction of future target position. There are several important differences, however. Instead of traveling in a curved trajectory through the air at very high speed, the fired torpedo travels in a straight line just below the surface of the ocean at relatively low speeds. It is not necessary to correct for wind, drift, level, crosslevel, earth's curvature, etc. since these factors do not affect the travel of the missile. Due to the nature of the weapon no computations for gun elevation and fuze orders are necessary. Thus the fire control problem for torpedoes is much simpler than for surface or antiaircraft fire.

It is necessary to determine only the torpedo course and speed required to have it intercept the target at its future position. The tubes are then trained to start the torpedoes on the required course, and the weapons are fired at the proper time to cause the torpedo to intercept the target.

Basic elements of the torpedo fire control problem

The torpedo track and the target track represent the travel of the torpedo and target respectively from the instant of firing until the time of impact. The above presentation of the problem is much simplified, but serves to illustrate the nature of the basic elements of the problem.




A Typical Torpedo Fire Control System

A typical torpedo fire control system designed to control the fire of non-homing torpedoes from destroyers consists of two torpedo directors, one at each end of the signal bridge, and torpedo course indicators and torpedo-tube mount sights at each tube mount. The two kinds of control used for torpedo fire are bridge control and local control. In bridge control, the primary method of control on modern destroyers, the fire control problem is solved at the torpedo director. Torpedo course (tube mount train angle) and torpedo gyro settings are transmitted automatically from the director to the torpedo course indicators at the tube mounts. The mount is trained by the mount operators, who turn their handcranks to match dials on the indicator, thus introducing the proper sight angle.

Local control is accomplished at the mount itself, and is normally used only when the torpedo director is inoperative. In this method the torpedo-tube mount sight is offset by the required sight angle and the mount is trained until the sight is on the point of aim.

The torpedo fire control system




The Torpedo Director

As already mentioned, most destroyers are equipped with two torpedo directors, one on each side of the signal bridge. Either of these directors may control the fire of the single quintuple tube mount usually installed, or either or all mounts if the ship has more than one. The director performs the following functions: (1) measures relative target bearing automatically as the director trainer keeps the line of sight on the target or matches pointers in response to radar signals; (2) computes the required sight angle, using inputs of torpedo speed, target speed and target course; (3) combines relative target bearing with sight angle to obtain the required torpedo course.

In solving the torpedo problem the initial director set-up is made by (1) introducing target speed, torpedo speed, and target course by hand, (2) receiving own ship's course electrically, and (3) keeping the director telescope line of sight on the target. After certain corrections are applied, the details of which will not be discussed here, the director automatically transmits gyro angle order and torpedo course order to the torpedo course indicator at the torpedo tube mount.

Torpedo director controls and indicators

The torpedo director is not much different from the computers you have already studied. It is basically a mechanical computer equipped with a sight telescope to make tracking possible. The hand inputs it requires are received from the Combat Information Center by phone. Because you are already familiar with similar computers, it will not be discussed here further.




The Torpedo Course Indicator and Tube Mount Sight

The torpedo course indicator receives torpedo course order and gyro angle order electrically from the torpedo director. When straight fire is being employed, the gyro angle is zero and the torpedo course equals the tube train. However, during curved fire the tube train is the sum or difference of the required torpedo course and the gyro angle, as shown below.

Relationships of tube train, torepdo course and gyro angle

By turning the corresponding handwheels on tile tube mount, the indicator operator matches the torpedo course and gyro angle dials, at the same time moving the mount through the correct tube train angle and setting the torpedo gyro controls to the proper value.

The torpedo-tube mount sight is used only during local control. The sight is a combination simple director and aiming device, and when the proper inputs are introduced and the sight is aimed at the target, the mount is correctly trained to lead the target by the proper amount. This type of control is not, of course, as accurate as director control, but is sufficiently accurate for emergency use when the director is inoperative.

This, then, is briefly how a typical destroyer torpedo fire control system operates. With the development of homing torpedoes, the non-homing type is being used less and less, but you should know a little about how these weapons are employed.

Now let's go on to the final specialized type of fire control system to be considered-the systems used to control rockets and guided missiles.

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