HOME          INDEX
A. General
B. Analytical solution
C. Graphic rangekeeping
D. Mechanical solution-general
E. Basic mechanisms
F. Mechanical solution-basic rangekeepers
G. Mechanical solution-establishing the horizontal plane
                                  SURFACE FIRE CONTROL PROBLEM

A. General

19A1. Introduction

The fire control problem requires for its solution the collection of essential data, computations based on these data, and arbitrary correction of the results.

The Navy uses mechanical or electrical computing devices, organized into fire control systems, to perform the detailed computations. These devices provide accurate solutions to the fire control problem, with a great saving in both work and time. In a fire control system, we are more interested in saving time than in saving work. It is desirable that we solve the fire control problem correctly and deliver our weapons before the enemy can solve the problem and deliver his. Failure to do so might result in loss of the ship, and of the life of every man aboard.

The computing devices you will work with are extremely complex; you are not expected to master the details of their construction. But these machines can not think; they must be controlled by human intelligence. The solution provided by a computing device can be no more accurate than the information you put into it. To use these machines intelligently, you must have a complete understanding of their operating principles.

In general, computing devices can be divided into two broad categories:

Digital. The inputs and outputs of a digital computer are digits, and all its computations are carried out in terms of digits. The abacus is a simple digital computer that has been in use for hundreds of years. Adding machines and desk calculators are more recent examples. The principal operations performed by a digital computer are adding, subtracting, and counting.

To multiply, for example, the multiplicand is set into the computer, which is then made to add the multiplicand repeatedly, while counting the number of additions. When the number of additions equals the multiplier, the sum is the product of the multiplier and multiplicand. To divide, the dividend is set into the machine. The divisor is subtracted repeatedly from the dividend, until the number remaining is less than the divisor. The number of subtractions required is the quotient of the two machines.

Analog. The analog computer deals with continuously variable quantities, rather than with separate digits. It operates by setting up, within the machine, conditions analogous to the various factors of the problems. Thus a given quantity might be represented within the machine by a voltage, a distance, or an angle of shaft rotation. An ordinary slide rule is a simple analog computer, in which distances along the rule and the slide are analogous to numbers.

Analog computers are most useful when a given problem, represented by a given set of equations, must be solved repeatedly. This is true of the fire control problem, in which the equations remain the same although the quantities they deal with may vary. The fire control problem deals with continuously variable quantities (such as speeds and distances, and gun train and elevation angles), rather than with digital quantities. And, with an analog computer, the solution to the problem is available as soon as the inputs have been completed. For these reasons, most of the computing devices used in fire control systems are analog computers, even though some units of such systems may include certain features of digital computers.

19A2. Fire control systems

There are two broad classes of gun fire control systems:

1. Surface systems, utilizing estimated values of target course and speed and measured values of range, relative bearing and own-ship speed and course; computing necessary corrections including deck-tilt and trunnion-tilt compensation; resolving ship and target motion into linear rates; predicting the future position of the target on the basis of these rates; and computing and transmitting gun orders.

2. Dual-purpose systems, which are similar to the surface systems except that, since they fire at air as well as surface targets, they measure, in addition, target elevation as an original input.

Relative-rate method can be used with either surface or dual-purpose system, in which relative motion of own ship and target is measured by tracking the target and by using the torque generated in tracking to precess a system of gyroscopes, and modifying the gyro output by such corrections as may be necessary to obtain correct gun orders.

Angular-rate method is used with dual-purpose GFCS Mk 56.

In practice, the solution of the problem by a specific fire control system is specialized within certain limits dictated by the primary use of that system. A single-purpose surface battery system which is designed primarily for surface fire can operate on the assumption that all targets are on the surface, and it can disregard the effects of a small angle of elevation or depression of the line of sight. With this system it is perfectly possible to fire at elevated targets in shore bombardment, or even at aircraft within the elevation limits of the guns, but such action requires arbitrary corrections or interconnection with the dual-purpose computer. Since heavy machine guns are designed primarily for shooting at incoming air targets, there is no concern about the point of fall in the conventional sense, and the machine-gun fire control system can function well without some of the units which are absolutely essential to the surface or dual-purpose system. The dual-purpose fire control system limits the accuracy of its fire against surface targets, in order to be effective against aircraft and still stay within necessary limitations of space and weight.

The next few chapters are concerned with a functional explanation of the solution of the fire control problem. The surface-problem solution being simpler than that for an air target, the present chapter explores the basic nature of the surface problem and the principles required for its analytical solution. It also introduces the student to a typical array of fire control mechanisms which have been developed to perform certain of the necessary computations. In succeeding chapters, a fairly detailed investigation is made of the design and function of representative types of the three classes of fire control systems mentioned above. Definitions of terms and symbols are to be found in appendices E and F.