Optimum Firing Position Within the TDZ
Barring flukes in the exploder mechanism, one ideally wants the torpedo’s path to cross the target’s path at a 90° angle. This means TA + AOB = 90° for an ideal shot. Such an interception ensures the maximum length of the target is exposed to the attack. Under actual conditions you will be doing well to get a shot having TA + AOB within ± 30° of the target’s beam.
If the target is stationary, we want to shoot from its beam (AOB = 90° , Port or Starboard) and SA = 0°. You will rarely get this opportunity. Most of the time you will need to fire from forward of the target’s beam in order to get a TA + AOB = 90° . Numerically TA = SA, so we can say SA + AOB = 90°. In the special case of SA + AOB = 90° , SA will equal the angle forward of target’s beam we need to be in order to get a crossing angle of 90° . See Figure 3.
Figure 3 – Special Case of 90° Crossing Angle
Just how much does SA need to be? That depends on the ratio of (Target Speed)/(Torpedo Speed), which I call K. The following table shows what SA must be to yield a 90° crossing shot for various values of K. You can see that a high torpedo speed brings many advantages – the higher the torpedo speed, the lesser the value of K, thus the lesser value of SA we need to get a more optimum shot. A higher torpedo speed also reduces time to target, which reduces the target’s ability to evade.