INTRODUCTION TO SONAR 



minutes to the time of explosion, leaving 2-1/2 

 minutes as the time to fire the weapon after 

 the decision to fire is reached. Bear in mind 

 that time to fire and time of explosion are 

 individual items. Although the weapon will be 

 fired in 2-1/2 minutes, it will explode in 3 

 minutes. 



Destruction Phase 



Destruction of the target is the ultimate goal 

 of the submarine hunter. During this phase the 

 computed ballistic solution is used so that the 

 ordnance may effectively be fired on the target. 

 Although the surface ship's primary A/S weapon 

 is the homing torpedo, the methods of submarine 

 destruction by surface ships are many and varied. 

 Included are ramming, direct hits with hedgehogs, 

 hull-rupturing near-misses with depth charges 

 and depth bombs, and damage of less serious 

 nature that forces the submarine to the surface 

 where it can be finished off with gunfire. Anti- 

 submarine submarines, of course, use their A/S 

 torpedoes on their submarine targets. In short, 

 the destructive phase includes action on the part 

 of the attacking unit that leads to a kill — directly 

 or indirectly. 



FUNDAMENTAL PROBLEM 



The fundamental problem of fire control is to 

 deliver fire on the selected target. A direct hit 

 is not required with some weapons. A depth charge, 

 for example, can cause the destruction of a sub- 

 marine if the charge explodes close enough to it. 

 All attacks are made with the intention of hitting 

 the target, but with some weapons, a near-miss 

 is good enough. 



Solving the fundamental problem is no easy 

 matter. The difficulty is due to such factors as 

 target speed, maneuvers, range and speed of the 

 weapon, all of which play important roles in 

 delivering fire on the selected target. The solu- 

 tion to the problem varies greatly with given 

 situations. A duck hunter, for example, "leads" 

 the duck along the path of flight. If the hunter 

 aimed directly at the duck as he fired, the shot 

 would pass through the area at which he aimed; 

 but, because the duck is moving, it no longer 

 would be at that spot when the shot reached there. 

 Consequently, the hunter makes an estimate of the 

 future position of the duck, aims for that spot, 

 and fires his gun. If he estimated correctly, 

 both the shot and the duck will arrive simulta- 

 neously at the predicted position. The fire control 

 problem is similar to the duck hunter's problem, 

 but on a much larger scale. 



REFERENCE PLANES 



Assume that a gun, rigidly fixed to a ship's 

 deck, fires while the deck is level, and its 

 projectile hits the target. The same gun, if fired 

 at a time other than when the deck is level, 

 will miss the target. If the gun is free to train 

 (rotate) and elevate, however, compensation can 

 be made for the ship's roll and pitch. The gun 

 then will remain reasonably steady, thus im- 

 proving the chances of hitting the target, regard- 

 less of the ship's motion. A fire control system, 

 in conjunction with a stable element, attempts 

 to make the necessary adjustments to keep the 

 weapon steady. 



Deviations from the level attitude are meas- 

 ured by a gyromechanism (the stable element), 

 which transmits to a computer in the fire control 

 system signals that indicate variation of the 

 position of the deck with respect to the horizontal. 



Based on signals from the stable element, 

 the fire control system computes values of 

 train and elevation for the weapons launchers 

 to compensate for deviation from the level 

 attitude. The various attitudes are measured 

 with reference to flat, two-dimensional surfaces 

 called planes. (See fig. 7-2.) An underwater 

 fire control system computes solutions in the 

 horizontal, deck, and vertical planes. 



Horizontal Plane 



A horizontal plane is tangent to the surface 

 of the earth. Visualize this condition by lajdng 

 a playing card on an orange. The card represents 



Figure 7-2.- 



71.123 

 ■Reference planes. 



114 



