How did they fire a torpedo? How were they aimed? What was the Torpedo Data Computer?
The first thing that was needed was to develop a firing solution. Therefore, it would help to explain that process first.
The captain and crew had to determine where the target was, where it was going and how fast it was moving. To do that, they used the Torpedo Data Computer, known as the TDC, in the conning tower as an aid.
The TDC was a large electromechanical computer that was developed in the 1930s. It was the size of a large American refrigerator and weighed about 1,500 pounds - somewhat larger than your laptop or tablet. The TDC was developed from a computer that controlled the big guns on surface ships. It had no software and did only the one thing –compute the final course for the torpedo so that it would collide with the target. It used gears, wheels, motors and servomotors. It accounted for the turning characteristics of two different torpedoes, one that ran at either of two speeds and the other that ran at a third speed. (It was clearly designed by a genius.)
The British, Germans and Japanese had similar devices that calculated the course for a given point in time. The TDC, however, provided a running solution. It updated itself constantly, based on the parameters that were entered. Therefore, if the approach officer had to wait a minute or so before firing, as long as there were no changes to the manual input parameters, there was no need to recalculate. The other machines had to recalculate in such a situation.
The target solution was developed using observations of the target and the TDC. The course and speed of the submarine were fed into the machine automatically, from the gyrocompass and the speed log. The approach officer, usually the captain but perhaps the XO, would sight the periscope on the target for the bearing and use a range finder to determine the distance to it. Those values could then be transmitted from the periscope, or from the Target Bearing Transmitter (TBT) on the bridge, to the TDC.
The crew still needed to determine the course and speed of the target. The course was estimated by the “angle on the bow” of the target. This provided from an observation by the approach officer. A zero angle on the bow means the target was headed directly toward the boat. A 90-degree angle on the bow, port or starboard, meant the target’s course was at a perpendicular angle and probably was starting to move away from the boat. A 180-degree angle on the bow means the target was headed directly away from the sub. This angle changed continually as the attack developed. Approach officers practiced determining the angle on the bow of the target and usually were very good at it.
The remaining parameter was the speed of the target. If the target was a merchant ship, most speeds in WW2 would be around 8 to 10 knots. That made it easier to come up with an initial estimate. However, combatant ships such as destroyers or cruisers could be steaming at anything from 10 to 30 knots. To get an initial estimate of the target’s speed, the approach officer checked to see if the target had “a bone in his teeth.” He looked to see if there was a tall white wake (the bone) at the bow of the target. If so, it was sailing at a higher speed. Late in the war, as sonar improved, the crew might have been able to get a “blade count” on ships. They could actually count the revolutions of the target’s propellors. Once they had that and identified the type of ship, they could determine the speed.
The TDC operator updated the estimates with each observation. When the approach officer wanted to make another observation, the periscope was brought up on the bearing where the TDC estimated the target to be. If the target was on that bearing, that meant that they likely had a good working estimate. However, if the target was at a different bearing, they needed to adjust. If, for example, the target was ahead of where it was expected to be, was it closer than estimated or going faster than estimated? Either could be true. If, on the other hand, the target was where it was expected to be for a few successive observations, it was probable that the TDC had a good solution.
All the time that the TDC was running during the approach, it was adjusting the gyroscopes in the torpedoes. This was done via servomotors. The TDC sent the signal from the output side of the machine to the small boxes on the side of each torpedo tube. That caused the spindle into the torpedo to set a new final course into the gyroscope. All of the torpedoes in the tubes in the torpedo room would be updated with the current course calculation.
Once the approach officer had a good solution and the target was in the optimum location, it was time to launch the torpedo(es). To fire a torpedo, first the tube had been flooded and pressurized to match sea pressure. Then the outer door was opened. This happened before the target was in firing range.
When the order was given to fire the torpedo, it would be fired electrically from the conning tower. There was a panel that allowed the operator to select the desired tube in the forward torpedo room or the after room. The panel provided status information such as whether the outer door was open and whether the gyroscope in the tube matched the setting from the TDC. When all was ready, there were two firing keys (actually plungers), one for the forward tubes and one for the after group. The torpedoes could also be fired from the torpedo rooms in case there were problems with the firing keys.
The torpedo would be launched with a small blast of air at 300 PSI. This would get the torpedo moving and the steam engine or electric motor would start right away. It would then swim the rest of the way out of the tube and speed towards the target. The launching air was then captured by a series of poppet valves above the tubes and brought back into the boat. They did not want a large bubble of air to escape from the tube to give away their position.
The torpedo normally dropped down about 30 feet in depth until it got up to speed. Then it recovered to the desired depth. It would run straight out of the tube for about 100 yards and then would turn to the course stored in the internal gyroscope. With luck it would continue on that course until it hit the target and exploded.
The TDC could be used to help plan the attack. One of the features of the output side of the TDC was that the operator could put in the desired spread, in degrees, of the torpedoes. Another feature was that the operator could determine the optimal course for the submarine so that the torpedo did not need to make the turn. It could indicate the course for a zero-angle shot. Torpedoes could turn up to 80 degrees off the firing axis, but large turns increased the chance of errors. If there were only the one target, eliminating the need for the torpedo to make a turn improved the odds of a successful firing.
Generally, the boat needed to be at medium speed or less to fire a torpedo. High surface speeds could cause damage to the torpedo tube outer doors. Submerged speed was rarely an issue since the boat wouldn’t go over 9 or 9.5 knots (10 or 11 mph).