Chapter 7 — BASIC FIRE CONTROL 



related factors change constantly. As a result of 

 the target motion analysis phase, we can establish 

 the true motion components of the target (course 

 and speed). There are several ways of arriving 

 at a course and speed solution. A discussion of 

 the methods follows. 



The target's true course and speed can be 

 established on the dead-reckoning tracer (DRT) 

 plot maintained in CIC by Radarmen from infor- 

 mation supplied by the sonar operators over 

 sound-powered telephone circuits. The DRT uti- 

 lizes own ship's course and speed inputs to cause 

 a lighted "bug" to follow own ship movements. 

 Sonar target ranges and true bearings are plotted 

 from the bug, thus establishing the submarine's 

 position. Course and speed are then determined 

 by the plotter. The DRT plot is used by the CIC 

 officer to aid him in conning the ship when CIC 

 has control of the attack, and to supply search 

 arcs to the sonar operators whenever contact 

 is lost. 



A maneuvering board may also be used to 

 determine contact course and speed, but this 

 method is not so rapid nor so accurate as the 

 DRT. The direction and distance of contact 

 movement are transferred to a vector represent- 

 ing own ship's course and speed, thus establishing 

 the target's course and speed vector. Another 

 drawback to the maneuvering board method of 

 plotting is that the tai'get's plotted track shows 

 only apparent movement (relative motion). 



Target true course and speed also can be 

 read from dials on the attack director (discussed 

 later) . 



On surface ships the underwater fire control 

 systems usually have to compute a liorizontal 

 sonar range from the measured slant range and 

 estimated target depth information. This computa- 

 tion is necessary because the fire control system 

 solves for target course and speed in the hori- 

 zontal plane on the surface in which own ship 

 operates. Slant range is transmitted to the attack 

 director from the sonar console. Depth is set 

 manually into the director. 



The attacidng submarine may use different 

 methods to determine true direction of target 

 motion. One method is by direct observation, 

 when possible, of the angles on the bow. Angle 

 on the bow was discussed in chapter 3. 



Ballistic Solution Phase 



It is difficult to say when one phase of 

 solving a fire control problem ends and another 

 begins. One phase usually overlaps another; 

 often they are concurrent. The start of the 



ballistic solution phase, for example, practi- 

 cally coincides with that of the tracking phase. 

 An antiaircraft fire control system presently 

 in use provides a solution in only 2 seconds, 

 once the target is acquired by radar. Yet, during 

 that time, the ta-rget is tracked, its motion is 

 analyzed, and the ballistic solution is computed. 

 Most underwater fire control systems, however, 

 take longer to develop a solution. The reason 

 is that certain inaccuracies in target informa- 

 tion are provided by underwater sound. Addi- 

 tionally, range limitations of the weapons require 

 the attacidng unit to be at a definite point to 

 launch nontrainable weapons, and within a specific 

 area to fire its trainable weapons. 



After determining the target's course, speed, 

 and depth, two items must be considered in order 

 to complete the problem. One is how to close the 

 target. The other is when to fire. 



To close the target, the course to steer for 

 the optimum firing point must be Icnown. In the 

 fire control problem the course to steer is indi- 

 cated as a correction to own course. Own sldp's 

 present course must be changed by the amount 

 of correction necessary to intercept the target. 

 The best intercept course is a collision course, 

 which means the target is closing on a constant 

 bearing. Computing course correction is an auto- 

 matic process in underwater fire control systems. 



The second consideration — when to fire — is a 

 decision reached from a compilation of many 

 items. If you use two different types of weapons 

 in the same attack, one of them to be fired ahead 

 and the other to be di'opped astern, it stands to 

 reason that the ahead-thrown weapon must be 

 fired first. What the problem develops into is a 

 calculation of two time periods: the time of 

 explosion and the time to fire. The problem is 

 solved by computing the time of explosion after 

 the initial classification of the target, then sub- 

 tracting (1) dead time (how long it takes, after 

 the command to fire, to actually fire the weapon), 



(2) time of flight (the time required, after 

 firing, for the weapons to strike the water), and 



(3) sinking time (length of time for the weapon, 

 after strildng the water, to reach the depth of 

 the target). When these subtractions are made, 

 you have the time to fire. The values subtracted 

 have been calculated from testing weapons and 

 from previous experience. 



Assume that, after detecting a target, explo- 

 sion time of 3 minutes is calculated. If the 

 weapon used has a dead time of 5 seconds, a 

 time of flight of 7 seconds, and a sinldng time 

 of 18 seconds, then a total of 30 seconds 

 (5 + 7 + 18 = 30) is subtracted from the 3 



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