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Typical antiaircraft fire control systems
PART G

 

G-1
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Analysis of the Antiaircraft Fire Control Problem

Now that you understand the factors involved in the solution of the fire control problem for fire against surface targets, you are in a position to understand more easily the problem of hitting a moving air target from a moving ship-the antiaircraft fire control problem.

All of the factors which entered into the solution of the surface fire control problem must also be considered in solving the antiaircraft fire control problem. We must determine target range, bearing, course and speed, and make corrections for own ship course and speed, wind, drift, level and crosslevel, etc. These factors will not be discussed further here. The antiaircraft problem is further complicated, however, by the essential differences between surface and air targets-the fact that the air target is moving at high speed above the surface of the ocean, and may be climbing or diving. These factors make the solution more difficult, and have led to the development of fire control systems similar in principle but different in detail from main battery systems.

The basic difference between surface targets and air targets

In this section of instruction sheets you will learn something about the solution of the antiaircraft fire control problem, without going into great detail. Then you will learn about the two types of systems used to solve the problem and the equipment which makes up each system.

 

G-2
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Analysis of the Antiaircraft Fire Control Problem (continued)

The problem of hitting a moving target from a moving ship and the steps involved in its solution for fire against a surface target were introduced earlier in these instruction sheets. The basic problem and its solution do not change for fire against air targets. The latter case requires only that additional corrections be made to compensate for an elevated target. Before analyzing the nature of these additional corrections, let's review the steps involved in the solution of the basic fire control problem.

Steps In Solution of the Fire Control Problem
1. Determine present target position in relation to own ship
2. Predict future target position in relation to own ship
3. Stabilize the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The differences between surface and air targets affect principally the first two steps in the solution. In the surface problem, present target position (step 1) can be determined by measuring only target range and bearing in the horizontal; in the air problem it is necessary to measure, in addition, target elevation above the horizontal.

Determining present target position for surface targets and air targets

In the same way, future target position (step 2) must be predicted to include not only future range and future deflection, but also future elevation. The antiaircraft fire control system must therefore be capable of measuring, predicting and correcting for the elevation of the target above the horizontal.

 

G-3
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Analysis of the Antiaircraft Fire Control Problem (continued)

In the surface fire control problem the target elevation angle (between L. O. S. and horizontal) was zero, because both own ship and target were on the ocean's surface, and the L. O. S. , for all practical purposes, was in the horizontal plane.

L.O.S. in horizontal plane
TARGET ELEVATION ANGLE IS ZERO IN SURFACE FIRE

Air targets, however, are ordinarily well above the ocean's surface, so that the L. O. S. makes an appreciable angle with the horizontal. In addition, if the target is approaching or leaving own ship in a horizontal plane, the L, O. S. must be elevated or depressed to keep the target in view. Thus target elevation angle continually increases or decreases and as it does so the gun elevation order must be correspondingly altered.

Increasing target elevation angle
EFFECT OF MOVING AIR TARGET ON TARGET ELEVATION ANGLE

 

G-4
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Analysis of the Antiaircraft Fire Control Problem (continued)

Target elevation angle is determined by measuring director elevation at the director and combining this with the stable element's measurement of level.

Target evalation angle
HOW TARGET ELEVATION ANGLE IS DETERMINED

If we continue to consider the air target approaching the ship in a horizontal plane, you can see in the illustration below that "slant range" from gun to target along the L. O. S. decreases as target nears own ship. The reverse is true when the target is moving away from the ship. It is slant range which is measured by the rangefinder or radar.

Target motion in the horizontal plane
EFFECT OF MOVING AIR TARGET ON RANGE

Both target elevation angle and range affect gun elevation order. They determine, along with the other ballistic corrections, what the "sight angle" (the angle between the L. O. S. and L. O. F.) must be in order that the projectile's trajectory will intercept the target's flight.

 

G-5
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Analysis of the Antiaircraft Fire Control Problem (continued)

You have seen how an air target moving horizontally above the ocean's surface affects the fire control problem for fire against aircraft. The second major difference between surface and antiaircraft targets is that the latter may be moving vertically up or down-climbing or diving-in addition to approaching or leaving the ship.

To illustrate the effect of the target's vertical motion on gun elevation order, let's consider a target which is moving only in a vertical plane, so that the horizontal distance from own ship is constant. The target elevation angle and the slant range both increase as the height of the target above the earth increases, and vice versa, as shown below.

Target motion in a vertical plane
HOW VERTICAL TARGET MOTION AFFECTS TARGET ELEVATION ANGLE AND RANGE

Of course, in actual practice, air targets will move both horizontally and vertically toward and away from own ship. Regardless of target motion, the antiaircraft fire control system must be able to predict the future target range, bearing and elevation, and make the necessary corrections to sight angle and sight deflection.

Two different methods are employed to measure target motion and compute the required gun settings. These methods are: (1) linear-rate fire control, and (2) relative-rate fire control. These methods will be discussed on the next few sheets.

 

G-6
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Linear-Rate and Relative-Rate Fire Control Systems

Linear-rate antiaircraft fire control systems determine changes in target position by means of linear rates in much the same manner as do main-battery systems. Relative target motion perpendicular to the L. O. S. in a horizontal plane is linear bearing rate RdBs. Relative target motion along the L. O. S. is range rate dR. In addition, however, linear-rate antiaircraft fire control systems must determine relative target motion perpendicular to the L. O. S. in a vertical plane through the L. O. S. This motion is called "linear elevation rate" RdE, and did not need to be considered in the surface problem.

Drawing showing Linear bearing, linear elevation rate and horizontal plane

In order to determine the components of air target motion-RdBs, dR and RdE-the system director must measure target range, bearing and elevation, and must estimate target speed, course and rate of climb. These quantities are transmitted to the computer which predicts future range, bearing and elevation in a manner similar to the main-battery system rangekeeper.

 

G-7
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Linear-Rate and Relative-Rate Fire Control Systems (continued)

Relative-rate antiaircraft fire control systems determine changes in target position by measuring the angular velocity of the line of sight. If you keep a finger pointed at an airplane, the rate at which your arm and finger must move to follow the flight of the plane is a rough measure of the angular velocity of the line of sight. Relative-rate systems measure this angular velocity and correct for time of flight and curvature of trajectory.

Relative-rate systems measure angular velocity of L.O.S.

As the operator keeps his sights on the target and introduces range by hand, the equipment automatically computes the elevation and bearing lead angles required to compensate for target motion. The guns are then automatically and continuously moved through these angles.

The major difference, then, between linear-rate and relative-rate systems is that the former measures linear target motion and the latter measures angular velocity of line of sight, to determine changes in target position.

 

G-8
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Linear-Rate and Relative-Rate Fire Control Systems (continued)

Linear-rate antiaircraft fire control systems are installed on destroyers, cruisers, carriers and battleships and are the primary means for control of 5-inch and 6-inch guns against both surface and antiaircraft targets. Guns of this size are normally used against distant, high-flying air targets, where corrections for wind, drift, parallax, level and crosslevel are required.

Various relative-rate fire control systems have been designed to control the fire of 20-mm, 40-mm and 3-inch guns against low-flying, short-range air targets. Because of the short range at which they are normally employed, corrections for wind, drift, etc. are not necessary since the errors they introduce are very small. An individual relative-rate system is provided to control each mount, whereas in linear-rate systems a single director may control the fire of all or any part of the 5-inch or 6-inch battery.

Linear rate systems-control fire against distant, high-flying targets.
Relative-rate systems-control fire against short-range low-flying targets

Beginning on the next sheet we will discuss a typical linear-rate systems, its components and operations. Later in this section of instruction sheets we will discuss several relative-rate systems.

 

G-9
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System

The fire control system discussed here is typical of the linear-rate antiaircraft fire control systems you will encounter in your later work aboard ship. The system is similar in many respects to the main-battery system discussed in the previous section of instruction sheets. The complete system consists of: (1) a gun director, (2) a stable element, (3) a computer, and (4) associated equipment at the guns. The various components and their primary connections are shown below.

A typical linear-rate fire control system

Note the similarity between this system and the main-battery system in primary fire control. On the following sheets you will learn about the various components and then you will see how the system operates as a whole.

 

G-10
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-Shipboard Installations

The components of this typical system are located aboard ship at stations similar to those occupied by main-battery system components. These stations are:

1. Gun director stations
2. Plotting rooms
3. Gun stations

The gun directors are located high above decks so that they may sight and track the target by means of radar or optical equipment. The plotting rooms are located well below decks to provide maximum protection for the computer and stable element. The guns and their associated fire control equipment are located above deck.

Destroyers and auxiliary ships have only one director and one plotting room (containing a single computer and stable element) for control of the dual-purpose (main) battery.

Fire control stations on a destroyer

Cruisers have two secondary-battery directors-one forward and one aft-for control of their dual-purpose guns. Some cruisers have only one plotting room while others have two, each containing two computers and two stable elements. On ships with two dual-purpose battery plotting rooms, either director can be connected to either plotting room to allow flexibility of control.

Secondary battery fire control stations of a cruiser

 

G-11
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-Shipboard Installations (continued)

Battleships have four interconnected secondary-battery directors and two secondary-battery plotting rooms. Here, too, any director and plotting room combination can control all or any part of the dual-purpose battery. In the illustration below, the port secondary-battery guns and director are hidden from view.

Secondary battery fire control stations of a battleship

Aircraft carriers, depending on their class, may have two or four directors and one or two plotting rooms for control of their 5-inch dual-purpose guns. In the case illustrated below, one gun director is located at each end of the carrier's "island" on the starboard side.

Fire control stations of an aircraft carrier

All of the components at the various fire control stations are interconnected through a secondary-battery switchboard located in the plotting room.

On the next few sheets we'll discuss each of the components of a single system and see what part each plays in the solution of the antiaircraft fire control problem.

 

G-12
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Director and Radar

The director is equipped to measure target range, bearing and elevation by means of radar or optical equipment. Estimates of target course, speed and climb (or dive) are made by the gun director control officer. The primary function of the director is to locate and track the target, thereby determining present target position and target motion. This is the information necessary for the solution of the first step in the fire control problem. The director's secondary function is that of control station from which battle orders are transmitted to the plotting rooms and gun positions to assure effective and coordinated control of the ship's fire power.

Steps in Solution of Fire Control Problem
*1. Determine present target position in relation to own ship
2. Predict future target position in relation to own ship
3. Calculate the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The gun director is similar in appearance and function to main-battery gun directors. A shield provides protection for operating personnel and equipment against the weather and enemy action. It also supports the radar antenna. The shield varies in thickness from the thin, weather-protective type for destroyers to the heavy, splinter-proof 1-1/2 inch thick type for battleships.

 

G-13
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Director and Radar (continued)

If we were to remove the shield and its attached radar antenna from the director, as we've done in the illustration below, we'd have a good view of the director interior and its equipment. Note that the director provides stations for a crew of six men-(1) control officer, (2) pointer, (3) trainer, (4) assistant control officer, (5) rangefinder operator and (6) rangereader and radar operator. Under certain conditions a seventh station is occupied by a control officer's talker who relays messages from other control centers to the director control officer.

Interior of a gun director

The control officer is in charge of the entire fire control system. He selects and designates the target and estimates its course, speed and climb. He controls when and how the guns are fired, and he may act as spotter. The assistant control officer aids the control officer and is in charge of illuminating the target in night firing by means of searchlights or "star" shells-which explode and give off intense light in the target area.

When optical ranging is being employed, the rangefinder operator keeps the rangefinder trained on the target and range is read by the rangereader. The pointer and trainer keep their sights on the target during optical aiming, thus measuring target bearing and elevation in the same manner as in main-battery directors.

 

G-14
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Director and Radar (continued)

During radar ranging, the rangereader operates the range-indicating radar equipment, and is assisted by the rangefinder operator. He also supervises the pointer and trainer in their use of the elevation and train radar scopes.

The entire director is moved in train to place the lines of sight (or radar beam) on the target. The director can be moved in train in any one of three ways: (1) automatic (remote) power drive, in response to signals from the computer; (2) local power drive, controlled by the trainer's handwheels; and (3) manual drive by direct gearing from the trainer's handwheels.

Elevation of the line of sight above the horizontal is accomplished by rotating the sight telescope prisms and by rotating the rangefinder and radar antenna about their longitudinal axes. The amount of rotation is the measure of director elevation. Positioning of the instruments in elevation can also be done in three ways: (1) automatic (remote) power drive; (2) local power drive; or (3) manual drive. You will learn more about these methods of positioning the L. O. S. when you study the system in operation a little later.

HOW DIRECTOR L. O. S. IS POSITIONED
How director L.O.S. is positioned

 

G-15
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Director and Radar (continued)

As you already know, target elevation is determined by adding or subtracting the level angle from director elevation. To get a true value of target elevation E in a vertical plane, both level and director elevation must be measured in a vertical plane through the L. O. S. Hence level angle for antiaircraft fire differs from that for surface fire, where it was measured in a plane perpendicular to the deck through the L. O. S.

To measure director elevation in a vertical plane, the optical equipment and radar antenna must be stabilized so that they elevate in a vertical plane. This is accomplished by continuously supplying the director with crosslevel as measured at the stable element. An automatic positioning device continuously rotates the telescopes, rangefinder and radar antenna in response to the crosslevel signal, to keep their axes horizontal. Thus target elevation is always measured in a vertical plane.

Crosslevel Signal keeps rangefinder and radar continuously stabilized

 

G-16
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Computer

The second major unit of the linear-rate system that we will consider is the computer. The computer can be considered as the counterpart of the rangekeeper in the main battery system, in that it performs the calculations required to solve steps 2 and 4 of the fire control problem. It is supplied with the same inputs as the surface rangekeeper, and in addition receives director elevation and target rate of climb. It computes gun train and elevation orders which are then transmitted to the guns.

Steps in Solution of Fire Control Problem
1. Determine present target position in relation to own ship
*2. Predict future target position in relation to own ship
*3. Calculate the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The computer varies considerably in external appearance from the range-keeper, but has essentially the same hand input cranks, indicating devices, basic mechanisms and electrical connections. It is located in the plotting room (main battery for some ships, secondary battery for larger ships), adjacent and mechanically connected to the stable element.

 

G-17
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Computer (continued)

The computer receives the following inputs from the sources and by the method indicated below:

INPUT TO
COMPUTER
INTRODUCED
BY
SOURCE
1. Initial velocity Hand Interior ballistics correction
2. Target course Hand Gunnery officer estimate
3. Target speed Hand Gunnery officer estimate
4. Target rate of climb Hand Gunnery officer estimate
5. Wind direction Hand Ship's meteorologist
6. Wind speed Hand Ship's meteorologist
7. Deflection spot Synchro Gun director
8. Range spot Synchro Gun director
9. Elevation spot Synchro Gun director
10. Ship's course Synchro Ship's gyro compass
11. Ship's speed Synchro Pitometer log
12. Director train Synchro Gun director
13. Director elevation Synchro Gun director
14. Range Synchro Gun director
15. Level Mechanical linkage Stable element
16. Crosslevel Mechanical linkage Stable element

Hand inputs, mechanical inputs and electrical inputs to the computer

 

G-18
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Computer (continued)

The computer is built in four sections for ease in handling and assembling. These sections are: (1) the control unit, (2) the indicator unit on which is mounted the star-shell computer, (3) the computer unit, and (4) the corrector unit.

Major units of the computer.

The control unit contains the mechanisms for computing rates (range rate, linear bearing rate, etc. ), and carries most of the knobs, cranks and dials. The indicator unit shows on dials and counters the end result of the ballistic calculations-sight angle, sight deflection, fuze order, and future range. The computer unit contains the mechanisms which make the ballistic calculations. Finally, the corrector unit computes and indicates gun train order, gun elevation order, parallax and other corrections. The outputs of the computer are not only transmitted automatically to the guns, but are indicated on the dials of the computer as well. This prevents the computer from becoming useless in the event of failure of the automatic transmission system, since dial readings could still be telephoned to the guns.

The principal hand input cranks and indicators are shown below.

Principal hand input cranks and indicators

 

G-19
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Computer (continued)

The star-shell computer, which is mounted on top of the computer indicator unit as shown on the preceding sheet, is designed to control the fire of dual-purpose guns when firing star-shells. Star-shells are used to illuminate the target during night firing. They contain a bright flare and a parachute which are expelled from the projectile by a small explosive charge at the proper time; as the parachute slowly lowers the flare it lights up the target area. The star-shell computer determines star-shell gun train, elevation and fuze orders which will cause the star-shell to release and light the flare at a point about 1500 feet above and 1000 yards beyond the target.

Drawing showing Star Sheel exploding above and behind target.

The star-shell computer receives gun train and elevation orders and target range by mechanical means from the gun order computer. It also receives elevation, train and range spots electrically from the director. These inputs are combined and the results are shown on the star-shell computer indicators and transmitted as gun orders to the guns firing the illuminating projectiles.

What the star shell computer does

 

G-20
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Stable Element

The stable element is the third major component of the typical linear-rate antiaircraft fire control system. In general, its part in the system is equivalent to that of the stable vertical in the main battery system. It measures level and crosslevel and can be used as a remote firing station. As does the stable vertical, the stable element stabilizes the various units of the fire control system, thus providing the solution to the third step in the fire control problem.

Stable Element
Steps in Solution of Fire Control Problem
1. Determine present target position in relation to own ship
2. Predict future target position in relation to own ship
*3. Calculate the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The stable element is located in the plotting room adjacent to the computer, to which it is mechanically connected by means of shafts. It receives own ship's course electrically from the ship's gyro, own ship's speed electrically from the pitometer log and target bearing (director train) mechanically from the computer through one of the coupling shafts. It transmits level and crosslevel mechanically to the computer, and cross-level electrically to the director so that the radar antenna and optical instruments can be stabilized.

 

G-21
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Stable Element (continued)

The most important difference between the stable vertical and the stable element is the manner in which level and crosslevel are measured. The stable element measures level in a vertical plane and crosslevel in a plane perpendicular to the deck, while the stable vertical measures level in a plane perpendicular to the deck and crosslevel in a vertical plane. The only reason for this difference is the manner in which computations are made by the computer and rangekeeper respectively.

On the top of the stable element are level, crosslevel and direction train indicators which show the value of the quantities being transmitted to the computer. A top view of the stable element, illustrating these dials, is shown below.

Top view of stable element

There are three hand-operated keys on the front of the stable element. The right-hand key is a manual firing key that may be put into the firing circuit if desired. It is in no way connected with the internal mechanism of the instrument. The middle key is the automatic firing key and operates in conjunction with the automatic firing contacts during selected level fire which was discussed in the previous section. The third key is the salvo-signal key. It is used to sound buzzers in the gun mounts as a warning just before the firing key is closed.

The linear-rate system was designed for use in continuous-aim fire and it is so used most of the time. Selected level fire is often used for shore bombardment or against surface targets in rough weather, but the rate of fire in selected level fire is too slow for use against air targets. In both methods of fire the stable element performs the functions of a remote firing station in the same manner as the stable vertical.

 

G-22
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Plotting Room

The plotting rooms are always located in protected positions within the hull of the ship. The computer, the stable element and the fire control switchboard are located in the plotting room, which also contains various auxiliary devices, indicators and interior communications equipment.

Principle plotting room equipment

The plotting rooms of typical dual-purpose battery fire control system installations were described earlier in this section of instruction sheets. All units of the system are cross-connected through a fire control switchboard located in the dual-purpose battery plotting room. The switchboard and other components are designed so that any director can be connected to any computer, and any computer can be used with any gun or group of guns. The limits of the possible cross-connections are the limitations of the switchboard and other shipboard wiring, and the fact that a director can control only those guns training in about the same arc of bearing as the director.

The plotting rooms for linear-rate antiaircraft fire control systems serve the same function as main-battery plotting rooms. They are central control stations from which the operation of the entire system can be regulated. They provide the means for protecting delicate equipment and for removing disabled components from the system.

 

G-23
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Houses

Fire control equipment installed in the gun houses includes devices which elevate and train the guns automatically in response to gun orders from the computer, as well as instruments and controls which allow the gun crews to check the automatic settings and to position the guns manually in the event of failure of the automatic positioning equipment. The illustration below is a plan view of a typical 5-inch gun twin mount which is the standard dual-purpose battery installation on destroyers, cruisers and battleships.

Gun house fire control equipment

Linear-rate antiaircraft fire control systems are designed for director-controlled operation, and you will note that there are no rangefinding or computing devices in the gun house. The fire control problem cannot be solved locally as was possible in main-battery systems. The gun crew must rely on information supplied by the director and computer. You will learn more about methods of operation a little later.

 

G-24
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The Gun Houses (continued)

The illustration on the preceding sheet shows the gun house equipment with which you will be mainly concerned.

The pointer's, trainer's, and checker's telescopes are located within hooded sight ports which project through the side of the shield. They are used only during local gun laying which will be discussed on the next few sheets.

The elevation and train indicator-regulators are used during automatic and indicator gun laying. Their functions are: (1) in automatic gun laying, to control the corresponding power drives by causing them to respond to electrical signals from the computer and (2) in indicator gun laying, to indicate the action needed to comply with the signals from the computer, as well as to indicate the response of the power drive.

The sight-setting indicator is operated manually. The sight setter keeps the indicator dials matched, in response to signals from the computer, by turning the sight angle and sight deflection handcranks, thus mechanically transmitting these quantities to the telescopes. This must be done continuously, in order to permit the pointer and trainer to use their telescopes immediately whenever it is necessary to switch to local gun laying.

The fuze-setting indicator-regulator controls the operation of the fuze setter. Normally, fuze setting is fully automatic and the fuze setter simply watches to see that the indicator dials match. If the fuze setter power motor falls to respond to the fuze orders from the computer, as evidenced by failure of the dials to match, the fuze setter is operated manually, matching the dial indexes by handcrank movement.

The fire control equipment at the gun mount allows some flexibility, and provides means for alternate methods of operation in the event of the failure of one or all of the various power drives. However, there is no provision for local solution of the fire control problem in the event the gun house is isolated from the rest of the system. The linear-rate antiaircraft fire control system is always dependent on its director and computer for effective operation.

Battleship underway

 

G-25
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The System In Operation

So far we have considered the various major components of a typical linear-rate antiaircraft fire control system used to control dual-purpose guns. You've seen what each major component's function is and what the inputs to and outputs from each unit are. You reviewed the part played by each component in the solution of the fire control problem and you've seen generally how the system operates under normal conditions. Now let's take a look at the various methods of control available and how the system operates under each method.

You will recall from your study of the main-battery system that there were four methods of fire control possible: primary, secondary, auxiliary and local. Secondary and auxiliary control depended on an auxiliary computer and stable element, which replaced the rangekeeper and stable vertical in an emergency. Local control required use of turret rangefinding and computing devices. The linear-rate system has no auxiliary equipment capable of solving the fire control problem, and relies entirely on its own or a substitute director-computer-stable element team. Thus if a system component is disabled, the guns which had been controlled by that system must be transferred to another director through the fire control switchboard.

When we discussed the fire control methods available for a main-battery system, we always considered only one director and its associated equipment. Cross-connections between the directors, plotting rooms and guns of the main battery, or control of the main battery by the secondary-battery system or vice versa, were not considered as alternate methods of control. Since linear-rate systems, however, contain no provisions for secondary, auxiliary or local control, they must depend on these methods for emergency control.

Secondary battery

 

G-26
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The System In Operation (continued)

Let's consider a cruiser employing its secondary battery of dual-purpose guns against air targets. Under normal conditions the forward and aft directors would control fire against separate targets and the connections would be as shown below:

Normal operation of cruiser secondary battery

If the aft director were hit and put out of action, control of the aft guns would be transferred to the forward director, connections being made through the fire control switchboards.

Aft director disables- All guns controlled by forward director

Control of the aft guns, in the above case, might also be transferred to the aft main-battery director for fire against surface targets only. This director might also take over if both forward and aft secondary-battery directors were knocked out.

This brief example of alternate methods of control of the linear-rate antiaircraft system serves to show how flexible the system is. On the following sheets you'll see how the components of the system function under normal operating conditions.

 

G-27
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The System In Operation (continued)

Let's take another look at the overall system and its connections, and see how various quantities flow between components under normal conditions.

Flow of quantities in a typical linear-rate anti-aircraft fire control system

This is the method of operation employed, regardless of cross-connections between alternate directors and computers. The only variations occur in the manner in which information is transmitted and how the guns and the director are positioned. If the synchro transmission system becomes inoperative, information must be telephoned between stations and put into the equipment by means of handcranks. If the power elevating and/or training mechanisms fail, the guns must be positioned manually.

On the next sheet we will discuss the operation illustrated above.

 

G-28
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The System In Operation (continued)

The director uses its radar or optical equipment to track the target and measure range, bearing and elevation. This information is transmitted to the computer by synchro transmission. In addition, the director control officer estimates the target's course, horizontal speed and rate of climb. These values are telephoned to the plotting room and put into the computer by means of handcranks. The computer also receives own ship's speed from the pitometer log and own ship's course from the ship's gyro, both by synchro transmission. The stable element supplies level and crosslevel to the computer through direct mechanical connections. Crosslevel is supplied to the director by synchro, where it is used to stabilize the radar antenna and optical instruments. Wind speed, wind direction and initial velocity are put into the computer by hand, and range, elevation and deflection spots are received from the director by synchro transmission.

The computer combines all of the above inputs and calculates gun elevation and train orders, fuze orders, sight angle and sight deflection, all of which are transmitted to the guns by synchro.

The computer also transmits predicted changes in target bearing and elevation to the director. These signals actuate equipment which automatically trains the director and elevates the radar antenna and optical equipment to keep the L. O. S. on the target. If the predicted changes are accurate, the target is tracked automatically. The director pointer and trainer need only watch to see that the L. O. S. stays on the target. If it drifts off, they turn their handwheels to correct the positioning of the L. O. S., at the same time introducing corrections to the computer.

Should any of the synchro transmission systems used to transmit the above quantities fail, provision is made to use telephoned orders to convey information.

Now let's see what happens in the gun houses when the gun orders are received.

Silouhette of ship.

 

G-29
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-The System In Operation (continued)

At the gun mounts, the pointer, trainer, sight setter and fuze setter have alternative functions, depending on how the quantities from the computer are received and employed.

In automatic gun laying, the train and elevation indicator-regulators have complete control of gun movement, positioning them in response to the gun elevation and train orders received from the computer. The pointer and trainer merely observe to see that the indicator-regulator dials are continuously matched. If the remote control fails, the pointer or trainer (or both) changes to indicator gun laying by moving a selector lever, and turns his handwheels to position the guns and keep the indicator dials matched. If the indicator-regulator fails, he shifts to local gun laying and turns his handwheels to keep his telescope on target. Since sight angle and sight deflection are continuously fed into the telescopes by the sight setter, which receives them from the computer, the guns are continuously positioned as long as the telescopes are kept on target.

Laying the Guns

 

G-30
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

A Typical Linear-Rate System-Conclusion

Now you've been introduced to a typical linear-rate antiaircraft fire control system. You've seen what the components of a typical system are, what their inputs and outputs are, how they are interconnected, and how the system functions under various conditions. From what you have read and discussed you can see how similar the linear-rate system is to the main-battery system previously discussed. The major difference is that the antiaircraft system measures and corrects for target motion above the surface-that is, target elevation and rate of climb.

You may say: "What happened to the radar?" We have explained the operation of this system, as we did the main-battery system, assuming that the optical instruments were being employed. The function of each piece of equipment and the relationships between various units are basically the same, whether optical or radar equipment is being used. Images on radar scopes and indicator readings simply replace images in the telescopes and rangefinders. Bear this in mind, and remember that you will learn about the actual operation of the radar equipment a little later.

Radar and optical equipment do the same job - determining present target position

 

G-31
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Relative Rate Systems

Increased aircraft speeds during World War II made necessary the development of fire control systems capable of solving the antiaircraft fire control problem in less time than required by the linear-rate systems already discussed. This was particularly important for the control of machine guns, which are used primarily against low-flying short-range targets. As a result, a family of fire control systems was developed, called "relative-rate systems," which differ from linear-rate systems in that they determine target motion by measuring the angular velocity of the line of sight instead of measuring the target's linear velocity.

Linear-rate systems measure linear target motion.
Relative-rate systems measure angular velocity of L.O.S.

Although originally used only for the control of machine guns, systems of this type have now been developed to control guns as large as 6-inch at close and intermediate ranges. At short ranges such factors as wind, drift, earth's curvature and rotation, etc., have very little effect on a projectile's trajectory. Therefore certain relative-rate systems, designed for control of short-range weapons, do not consider these factors in determining the required gun settings.

 

G-32
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Relative-Rate Systems (continued)

Every hunter who shoots ducks on the wing knows that he must "lead" the duck by an angle sufficient to compensate for the duck's travel while the load of shot is in the air. A moment's thought will convince you that if the duck is climbing, the hunter must lead in elevation; if it is flying across the hunter's line of sight, there must be a traverse lead angle. In the same way, in antiaircraft fire control, the total lead angle required to compensate for the target's motion is the sum of the elevation and traverse lead angles. Lead angles, then, are the same as sight angle and sight deflection-the angular displacement of the gun bore axis away from the L. O. S.

Lead angle for a moving target

The train or bearing lead angle corrects only for target motion. The total elevation lead angle includes a superelevation lead angle which compensates for the curvature of a trajectory, as well as the elevation lead angle to correct for vertical target motion.

 

G-33
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Relative-Rate Systems (continued)

There are several types of relative-rate systems in use, ranging in complexity from the simple lead-computing gun sight mounted directly on 20-mm gun mounts, to the complex system used to control 3-, 5-, and 6-inch guns at ranges up to 12,000 yards. This latter system includes radar equipment, a director, two computers and various related equipment. The more elaborate systems make corrections for wind, drift, air density, etc., since at the ranges at which these systems are used the effect of these factors cannot be ignored. But all relative-rate systems, regardless of their complexity, are classified as such because they measure target motion by determining the angular velocity of the line of sight.

All relative-rate systems measure changes in the position of the line of sight in much the same way that the stable element measures level and crosslevel-by gyroscopic action. The lead-computing sights and the directors in more elaborate systems contain gyros of a special type called "rate of turn" gyros, which measure the rate of turn of the line of sight by utilizing the precession effect, which is characteristic of all rapidly spinning bodies. It is not necessary that you learn just how these rate-of-turn gyros work. But remember that this is the distinctive feature of all relative-rate fire control systems: motion of the line of sight is measured by gyroscopic action.

Lead-computing sight and stable element both use gyroscopic action

 

G-34
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Relative Rate Systems-Disturbed and Undisturbed Line-of-Sight Systems

Some relative-rate systems offset the fore-and-aft axis of the gun sight from the L. O. S. by the amount of the computed lead angle, so that the sight itself is aligned with the gun bore axis. These systems are known as "disturbed-line-of-sight" systems. Most lead-computing (relative-rate) systems are of this type. Other systems, however, measure the lead angles and transmit them to a computer, which makes up the gun orders. These systems move the fore-and-aft axis of the gun sight with relation to the gun bore axis, leaving the line-of-sight fixed within the gun sight. The latter systems are called "undisturbed-line-of-sight" systems.

Disturbed-line-of-sight systems, undisturbed-line-of-sight systems

On the following sheets you will learn more about several typical types of relative-rate antiaircraft fire control systems. Some of these are disturbed-line-of-sight systems, and others are undisturbed-line-of-sight systems. In general, the simpler systems are of the former type, and the more elaborate systems are of the latter type.

 

G-35
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Short-Range Lead-Computing Sights

The first type of relative-rate fire control system you will study is the short-range lead-computing sight, designed primarily for the control of antiaircraft machine guns against rapidly moving targets at short ranges. When used to control 20-mm machine guns, the gun sight is mounted directly on the gun mount, as shown below. The lead angles in train and elevation are established by the motion of the sight in tracking the target. The gunner tracks the target by moving the gun in train and elevation to keep the sight's crosshairs constantly centered on the target. As he does this, the sight's gyro mechanism determines the required lead angles, and offsets the line-of-sight by the necessary amount. Thus by keeping the L. O. S. on the target, the gunner automatically moves the guns through the lead angles required to score a hit. The only manual operations required are range setting, smooth tracking, and the introduction of spots. The way in which the sight is mounted and used eliminates the necessity for level and crosslevel corrections, and parallax corrections are unnecessary since the sight is located very near the gun. Since 20-mm guns are used only against targets at ranges where the effects of drift, wind, air density, earth's rotation, etc., are insignificant, these factors are not considered by the short-range sight.

Short-range lead-computing sight mounted on 20-mm gun
SHORT-RANGE LEAD-COMPUTING SIGHT MOUNTED ON 20-MM GUN

 

G-36
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Short-Range Lead-Computing Sights (continued)

One type of short-range lead-computing sight uses two rate-of-turn gyros to measure the angular velocity of the line of sight. One gyro measures motion in traverse and the other in elevation. Both gyros are driven by compressed air, which is supplied by the air power unit shown attached to the gun mount on the preceding sheet. This type of gun sight is illustrated below. The operator sights the target through the rear window, and the range setter estimates or receives range from other sources and sets it in on the range knob of the sight. The elevation and deflection-spot knobs are used only to correct for consistent errors observed during firing.

Air-driven short-range lead-computing gun sight

A later type of short-range lead-computing sight uses one electrically-driven gyro, which measures the total required lead angle both in train and elevation. This type has several advantages over the air-driven type, one being that the electrical gyro can be brought up to operating speed in one-eighteenth of the time required by the air-driven gyros used in the earlier type sight. The two types are similar in principle and operation, both being disturbed-line-of-sight instruments.

Electrically-driven short-range lead-computing gun sight

 

G-37
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Short-Range Lead-Computing Sights (continued)

When short-range lead-computing sights are used to control the fire of 40-mm or heavier guns, they are usually mounted on a gun director which is located as close as possible to the guns it controls. Either the air-driven or electrically-driven sights can be so mounted; the air-driven type is illustrated below, director-mounted.

Air-driven short-range gun sight mounted on a director stand

The director consists of three main parts: the pedestal, the head, and the gun sight with its air power unit. The pedestal is bolted to the deck and supports the head on ball bearings. The head supports a carriage which has a platform for mounting the gun sight. The handle bars attached to the carriage move the carriage and sight in elevation and the entire head in train. Counterweights are provided to balance the weight of the gun sight. As the sight is elevated and trained in tracking the target, elevation and train transmitters in the director send the lead angles measured by the sight's gyros to the gun positioning equipment at the guns. Thus the guns are continuously positioned even though the sight is some distance away from the weapons.

 

G-38
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Medium-Range Lead-Computing Sights

The advantages of individual mount control by light topside directors, together with the speed and simplicity of the solution of the antiaircraft fire control problem by gyroscopic means as in the short-range lead-computing sights just discussed, led to the extension of these ideas to the control of dual-purpose guns at medium ranges. To accomplish this it was necessary to develop a gun sight with greater stability and providing more accurate corrections for gun ballistics and wind. Such a sight was developed, and is used: (1) mounted on the light director, previously discussed, for the control of 40-mm guns, or (2) as a component of more elaborate fire control systems for control of 3-inch and 5-inch guns. The illustration below shows the medium-range sight mounted on a director which is part of a fire control system designed to control 3-inch and 5-inch guns. You will learn more about this system a little later.

Medium-range lead-computing gun sight installed on director for control of 3-inch and 5-inch guns

 

G-39
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Medium-Range Lead-Computing Sights (continued)

The medium-range lead-computing gun sight is in principle much the same as the short-range sight already discussed. However, as the ranges at which it is designed to control the fire of heavy machine guns and dual-purpose guns are greater than those at which the short-range sights are employed, certain important differences exist, as follows:

1. The gyros are heavier and more stable.

2. The optical system includes a telescope of sufficient magnifying power to view targets at greater ranges.

3. Means are provided to correct for drift, wind, and initial velocity changes.

4. For use with certain fire control systems, modifications provide train and elevation radar scopes which permit blind tracking.

Medium-range lead computing-gun sight

The optical system of the medium-range sight is different from the short-range instrument in that the optical line of sight of the telescope is fixed. Thus the axis of the sight is aligned at all times with the line of sight, and the gun sight is of the undisturbed-line-of-sight type.

A knob is provided for introducing corrections to I. V. settings. Range is put in mechanically after being determined by radar or other means. A weight attached to the train gyro shaft corrects for drift. Corrections for wind in train and elevation are made up in a wind transmitter and applied automatically to the gyros. Details of different medium-range fire control systems vary considerably, but in all of them gun train and elevation orders are determined by the gun sight, using, when necessary, inputs from various other sources.

 

G-40
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Fire Control Systems

The short-range lead-computing sight mounted on and used to control the fire of 20-mm guns is the simplest type of fire control device used in the Navy. As the size, number and range of the guns a fire control system is designed to control increases, so does the complexity of the system. Thus the medium-range lead-computing sights used to control 40-mm guns, are more complex and make more corrections than the short-range sights. So far we have considered the short- and medium-range lead-computing gun sights primarily by themselves, and not as a part of a more complex fire control system. Let's go on to see how these sights are used as components of various relative-rate antiaircraft fire control systems designed to control 40-mm and larger guns. You'll also see how different systems employ other elements to solve the fire control problem more rapidly and accurately. But bear in mind that all of these systems are relative-rate systems, and measure target motion by determining the angular velocity of the line of sight, using rate-of-turn gyros.

Gun sight can be used alone - or as part of a more complex relative-rate fire control system

 

G-41
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Fire Control Systems (continued)

At the present time there are four general types of relative-rate fire control systems in use aboard Navy ships-not considering the lead-computing sights by themselves as systems. For purposes of discussion, we will call these systems types I through IV. There are many similarities and differences among the four types, but they are all designed to control the fire of heavy machine-guns and dual-purpose guns against air targets. They are all equipped with radar to permit blind tracking, and all can also be used to control fire against surface targets. The components of each type of system vary, however, and so do the performance characteristics as shown in the table below. Range means the maximum range at which firing may be commenced. Gun size indicates the size of the guns the system is designed to control. Target speed refers to the maximum target speed at which the system can track and solve the fire control problem.


Fire Control System-Range in yards-Gun Size-Target Speed in knots
Type I - 7,000 - 40 mm-3 inch - 800
Type II - 7,500 - 40 mm-3 inch-5 inch-6 inch - ---
Type II - 7,500 - 3 inch-5 inch - 350
Type IV - --- - 3 inch-5 inch-6 inch - ----

Note that type I has the lowest range but is capable of controlling fire against very rapidly moving targets. Hence, its most important application is defense against close-range dive attacks by enemy planes which have got through the larger caliber antiaircraft fire. Types II and III have greater range, but lower target speed. Their primary application is control of 3-inch and 5-inch guns against approaching planes at intermediate range. Type IV has the greatest range, and is used to control fire against distant targets moving at high speeds.

On the following sheets you will learn more about each of the four types of systems and how they work.

 

G-42
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type I

The first type of relative-rate system we will consider is designed to control the fire of 40-mm and 3-inch guns against air targets at ranges of from 800 to 7,000 yards. Targets may be tracked either optically or by radar at target speeds not exceeding 800 knots when target is approaching ship and 350 knots when target is leaving ship. The radar antenna is carried on the gun mount. The system uses a disturbed-line-of-sight, since the gun sight housing remains aligned with the gun bore axis.

The major units of the system are: (1) lead-computing gun sight, (2) director pedestal, (3) radar antenna and mount, (4) radar equipment, (5) wind transmitter, and (6) target acquisition unit (TACU). These units are shown in the illustration below.

Major units topside and bleow decks, type I relative-rate fire control system

A crew of six is required. Topside personnel are the control officer, the director pointer, the director range setter, and the gun control talker. The target acquisition unit (TACU) operator and the radar operator are stationed below decks.

 

G-43
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type I (continued)

Here is a simplified diagram showing the transmission of quantities between the various major units of the system.

Flow of quantities between units of type I relative-rate fire control system

A medium-range lead-computing gun sight employing air-driven gyroscopes is mounted on the director pedestal, as shown on the preceding sheet. The director is swung manually by the director pointer in train and elevation. If the target is visible, the gun sight optical system is used for optical tracking by keeping the target image centered in the gun sight telescope. If the target is obscured as in night firing, the target is tracked by radar. The radar equipment introduces a light spot representing the target into the optical line of sight of the gun sight, and the director pointer keeps this spot centered in the telescope. Corrective data from other units in the system are transmitted to the director and applied as inputs to the gun sight automatically or by hand. The gun sight then determines the lead angle required to correct for wind, drift, effect of gravity, etc. Gun train and elevation orders are then transmitted to the gun mounts.

On the next sheet we will consider the function of each component in greater detail.

 

G-44
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type I (continued)

The following description of the major units of the system is arranged to follow, in general, the flow of data from director to gun as shown in the illustration on the preceding sheet.

The director pedestal supports the gun sight, moving it in train and elevation as the director handles are moved by the director pointer. It transmits gun train and elevation orders to the elevating and traversing mechanisms at the gun mount, gun train order to the wind transmitter and director elevation to the target acquisition unit. The director stand is bolted to the deck and supports the head assembly, which rotates on the stand and in turn supports the gun sight.

The radar antenna mount receives from the TACU and gun sight, the antenna elevation and traverse lead angle signals which keep the radar beam on target. Radar signals from the mount enable the below decks radar equipment to determine target range, which is transmitted to the gun sight and put in by the range setter. During radar tracking the radar equipment sends target bearing and elevation signals to the sight which position the light pot simulating the target in the director pointer's field of view.

Director pedestal and gun sight.
Optical tracking.
Radar Tracking.

Note in the above illustration that the gun sight used in this system employs a small circle in place of crosshairs.

 

G-45
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type I (continued)

The wind transmitter computes the lead angle corrections made necessary by the effect of wind on the projectile and sends wind corrections to the gun sight. To accomplish this the wind transmitter receives own ship speed, wind speed and wind direction through hand input knobs, and own ship's course electrically from the ship's gyro. In addition, the transmitter supplies director train to the TACU by combining own ship's course with gun train order received from the director.

Wind Transmitter

The target acquisition unit (TACU) helps the director pointer to pick up designated targets and aids in locating targets when visibility is poor. The unit compares director train and elevation as received from the wind transmitter and director pedestal with information received from search radar or other lookouts. If there is a discrepancy, it is transmitted to the gun sight and put in as a correction so that the guns are brought against the designated target.

Target Acquistion Unit (TACU)

 

G-46
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type I (continued)

Let's see how this system's components fit in with each step in the solution of the fire control problem. A great deal of the work is done at the gun sight itself by means of its gyros. Stabilization of the line of sight is accomplished within the gun sight by a cross-roll gyro which makes corrections for motion of the ship's deck. Here is how the various units are related to the five steps in the solution of the fire control problem.

Steps In Solution of the Fire Control Problem
*1. Determine present target position in relation to own ship
*2. Predict future target position in relation to own ship
*3. Stabilize the various units
*4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

Transmission of data, as in the main battery and linear-rate systems, is accomplished by a combination of synchro transmissions, telephoned verbal orders and mechanical linkages.

Now let's go on to learn about the second type of relative-rate antiaircraft fire control system in use aboard Navy ships at the present time.

 

G-47
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type II

The second type of relative-rate system we will consider is designed to control the fire of 40-mm, 3-inch, 5-inch and 6-inch guns against air targets at ranges up to 7500 yards. Targets may be tracked optically or by radar.

The system uses a manually operated undisturbed-line-of-sight director, which carries the radar antenna above decks, and various computing units and radar equipment below decks. In this system the sight telescope is separated from the case containing the gyros, which is designated as a lead angle computer. Both the telescope and the computer are mounted on the director and positioned by moving the director head. While the system is primarily designed for use against aircraft, it may also be used against surface targets.

The major units of the system are: (1) the gun director, (2) sight telescope, (3) lead angle computer, (4) gun order computer, (5) wind transmitter, and (6) radar equipment.

Type II relative-rate fire control system

 

G-48
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type II (continued)

This system can be used to locate, range, and track enemy air and surface targets before they are within firing range. The radar may be used to locate and determine range of targets up to 15,000 yards. However, since the maximum range of the gun sight computer is only 7500 yards, firing may not be commenced until an approaching target reaches this range.

Here is a simplified diagram showing the transmission of quantities between the various major units of the system.

Flow of quantities between major units of the type II relative-rate fire control system

The director pointer keeps on target either by means of the optical telescope, or by means of a radar spot indicator in the same manner as was done in the previously studied system. In this system, however, the radar spot appears on a radar indicator at the director instead of being introduced in the gun sight's optical line of sight. As the target is being tracked, the gyroscopes in the lead angle computer generate lead angles as in the medium-range lead-computing gun sight. The lead angle computer includes a cross-roll gyro which corrects for deck motion as is done in the gun sight of the previous system.

 

G-49
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type II (continued)

Ballistic corrections for gravity, drift, wind, air density and variations in I. V, are applied to the target-motion lead angles to obtain the total lead angles in train and elevation. Most of these corrections are computed and applied in the lead angle computer, which receives inputs from other units.

The lead angles received by the gun order computer include ballistic corrections. The mechanical and electrical devices in the computer combine the train and elevation lead angles with director train and elevation received from the gun director to give gun train and elevation orders, which are transmitted to the gun mount.

As the target is being tracked, the radar antenna transmits signals to the below decks radar equipment, which computes target range and sends it to the lead angle computer for use in its determination of lead angles.

The target acquisition unit receives target information from search radar or other sources. It provides the director pointer with information which enables him to train and elevate the director to the target's position and commence tracking.

The wind transmitter receives inputs of own ship's course and speed, wind speed and direction, and gun train. It computes lead angle corrections required to compensate for the effects of cross wind and range wind, and sends this information to the lead angle computer.

The gun order computer used with this system differs from other computers and the main-battery rangekeeper previously studied in that it does not use any of the basic mechanisms-such as mechanical multipliers, differentials, integrators, component solvers and cones. Its computing devices are all electronic, consisting of vacuum tubes, transformers, capacitors and synchros and servos. This type of computer has many advantages over mechanical computers, and is being used in an increasing number of fire control systems. You will learn more about it when you study in detail the specific systems in which it is used.

Drawing of ship firing upon othe ship and aircraft.

 

G-50
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type II (continued)

Let's take a closer look at the gun director of this system and the various units which are mounted on it.

Director pedestal and mounted parts - Type II relative-rate systems

Probably the first thing you will notice is that the radar antenna is mounted on the tracking yoke, instead of at the gun mount as in the system previously studied. Thus the antenna is positioned directly by the director pointer as he tracks the target. It is not necessary to transmit antenna lead angle signals to the antenna mount as was done in the previous system.

The lead angle computer, as you know, contains the gyros for computation of lead angles. The sight telescope, used for optical tracking, is mounted next to the radar indicator, which is used for blind tracking. Note that these three units-telescope, radar indicator and computer-are separate in this system, whereas they were all combined in the gun sight of the first system considered.

The air supply unit, mounted near the base of the director pedestal, supplies air under pressure to operate the air-driven gyros within the computer. As the director pointer manually trains and elevates the director, synchro generators in the tracking yoke transmit director elevation and train to the gun order computer.

 

G-51
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type II (continued)

To conclude your study of the second type of relative-rate system, here is how each system component functions in the solution of the fire control problem.

Steps In Solution of the Fire Control Problem
1. Determine present target position in relation to own ship
2. Predict future target position in relation to own ship
3. Stabilize the various units
4. Calculate required corrections to gun train and elevation
5. Transmit data to guns

The target acquisition unit, sight telescope and radar equipment (including the director-mounted indicator and antenna) work together to determine the target's present position. The lead angle computer predicts the future target position by measuring the elevation and train lead angles, and stabilizes the L. O. S. by means of the cross-roll gyro. Exterior and interior ballistics corrections are calculated by the wind transmitter and the gun order computer. Finally, the gun orders are transmitted to the guns by means of synchro transmission or telephone.

Now you've seen and compared the operation of the first two types of relative-rate antiaircraft fire control systems. Let's go on and see how the third type of system operates, and compare it with the first two types.

 

G-52
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type III

The third type of relative-rate system to be considered is designed to control 3-inch and 5-inch guns on small combatant ships, landing ships, fleet auxiliaries and Coast Guard vessels. It is intended primarily to control fire against medium speed (under 350 knots), medium range (under 7500 yards) incoming air targets; however, it may also be used against surface targets.

The system compensates for motion of own ship and target, the effects of gravity, drift, changes in I. V. , wind and horizontal parallax. Its major component units are: (1) a gun director, on which is mounted (2) a medium-range lead-computing gun sight, (3) a sight telescope, and (4) radar antenna. The below-decks components are (5) radar equipment, (6) two computers, (7) wind transmitter, and other auxiliary equipment.

The director employed with this system employs the medium-range lead-computing gun sight previously discussed. The director, in conjunction with the rest of the system, is capable of several methods of operation, including function as either a disturbed- or undisturbed-line-of-sight system. The method of operation used depends on the type of target against which fire is being controlled.

Gun director for type III relative-rate fire control system

 

G-53
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type III (continued)

For use against air targets, this system is utilized as a disturbed-line-of-sight system. The illustration below shows the units employed and how information flows during this method of operation.

Flow of quantities for antiaircraft fire - Type III relative-rate system

This system is similar to type I in that the gun sight contains the gyros and performs most of the computations for antiaircraft fire. It resembles type II in that the radar antenna is mounted on the director, not at the gun mount. Fuze setting order is calculated in the computer, which receives range from the radar equipment and target speed as a hand input. Gun train and elevation orders are determined in the gun sight itself, as was done in the first type of system.

Notice that only one computer is used during antiaircraft fire control. The other computer is used during surface fire, at which time it takes over some of the functions of the gun sight computing devices. On the next sheet you will see how the system operates against surface targets.

 

G-54
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Type III (continued)

There are two different ways in which this system may be utilized against surface targets. The method employed depends on the type of target engaged.

The primary method of surface control utilizes the system as an undisturbed-line-of-sight system, and is shown in the illustration below. During this method of operation the gun sight gyros are stopped and the gun sight itself is not used. The target is tracked by sighting through the sight telescope or by radar, and estimates of target course and speed are fed to the auxiliary computer which determines sight angle and sight deflection. The gun order corrector combines sight angle and sight deflection with director train and elevation, and determines gun orders. In effect, the system is no longer a relative-rate system, but is operating as a linear-rate system.

Primary surface fire control type III relative-rate system

The important differences between this system and a linear-rate system are that this system has no stable element to compensate for roll and pitch of the ship, and no provision for optical range determination.

The secondary method for use against surface targets is basically the same as that used against air targets as described on the preceding sheet.

 

Sheets 55, 56, 57, and 58 of Section 50, Part G, have been deleted for security reasons
 

G-59
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Typical Relative-Rate Systems-Review

TYPE I - This system employs a medium-range lead-computing gun sight to determine gun orders. Its maximum range is 7000 yards, and it is used to control the fire of 40-mm or 3-inch guns against high speed (up to 800 knots) air targets. Tracking is accomplished optically or by radar by manually positioning the director. The radar antenna is located at the gun mount, and radar signals are introduced into the gun sight's optical line of sight.

TYPE II - The second type of system does not use a lead-computing gun sight. The gyros are located in a separate lead angle computer. The radar antenna is mounted on the director, and radar tracking is accomplished by following signals sent to a radar indicator at the director. The system is manually operated and has a range of 7500 yards. It is designed to control 40-mm, 3-inch, 5-inch and 6-inch guns.

TYPE III - Designed to control 3-inch and 5-inch guns against air targets at speeds up to 350 knots and ranges to 7500 yards, this system employs a lead-computing gun sight to calculate gun order! for fire against air targets. When used for surface fire, the gun sight is not used. The system then operates as a linear-rate system, employing an auxiliary computer to determine gun orders.

TYPE IV - Security restrictions do not permit a complete description of this system. It is a comparatively recent development and is capable of controlling the fire of 3-inch, 5-inch, and 6-inch guns against air targets at speeds over 500 knots and at ranges in excess of 7500 yards. One of its outstanding features is the automatic radar tracking equipment, which keeps the radar beam on the target automatically just as accurately as the best human operator. No lead-computing gun sight is employed, but gyros in the director measure deck tilt and target motion.

Drawings of type I, II, III systems

 

G-60
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

Review of Linear-Rate and Relative-Rate Systems

Linear-rate systems are installed on combatant ships to control the fire of their 5-inch and 6-inch dual-purpose guns against long-range, highflying targets or against surface targets anywhere within the range of the guns. Linear-rate systems measure target motion by determining its linear velocity along and across the line of sight.

Linear-rate systems measure target linear motion

Relative-rate systems are installed on combatant ships to control the fire of their 20-mm, 40-mm and 3-inch machine guns, and on certain types of ships to control 5-inch and 6-inch dual-purpose guns. They are designed primarily for use against short-range air targets, but can also be used against surface targets. Relative-rate systems determine target motion by measuring the angular velocity of the line of sight. These systems may be very simple, as is the lead-computing gun sight mounted on the 20-mm gun; they may also be very complex, as is the fourth type of system discussed.

Relative rate systems - measure angular velocity of L.O.S.

 

G-61
 

TYPICAL ANTIAIRCRAFT FIRE CONTROL SYSTEMS

What You've Learned So Far

Now that you've completed your study of typical antiaircraft fire control systems and of a typical main battery system, you've found out all you need to know to introduce you to the fire control systems used to control the fire of Naval guns.

You saw, in the preceding section of instruction sheets, that main battery systems are used to control fire of large combatant ships' heavy guns against surface targets. Then, in this section of sheets, you've seen how antiaircraft systems are used against enemy aircraft. You have not learned any details about the operation of specific equipment within any one system, but have had typical systems described and have been told generally how they work. This information will serve you well when you do study specific equipments later on.

Do not think, now that you've come this far, that you can go out right now and operate or troubleshoot any main battery or antiaircraft fire control system. If you tried you'd be greatly surprised and disappointed, for there is much yet to be learned. The purpose of these sheets is not to make you a qualified technician, but merely to make you generally familiar with the problems, terms and types of equipment involved in fire control.

Now let's go on to find out about some of the other aspects of the basic fire control problem. The next problem we will consider, in the following section of instruction sheets, is that of antisubmarine fire control.

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Surface Fire, Antiaircraft fire
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