Hello Max:

Re torpedo launch from the torpedo tube:
1. The most common method used during WWII was by compressed air. The torpedo fit closely within the tube so that the torpedo itself was like a "piston" in the tube. The outer torpedo tube doors were opened and a pulse of compressed air was admitted behind the torpedo. The expanding air pushed the torpedo out of the tube somewhat like a BB being shot from an air rifle. This method of launch had a disadvantage in that a large bubble of rising air exiting from the tube creating a surface disturbance which could reveal the submarines position.

2. Later efforts were directed toward eliminating the air bubble problem. The most succesful system allowed the torpedo to "swim" out of the tube. This worked particularly well with electrically driven torpedoes. The torpedo tube was flooded and the torpedoes motor started. The thrust from the torpedoes propellor then drove it out of the tube.

Re depth control:
1. A torpedo is steered by a set of rudders and elevators located on the "fins" back on the tail end near the propellors. In this sense a torpedo "flys" through the water much like a rocket flying through the air. The rudders control the torpedoes direction in the horizontal plane. The elevators control its direction in the vertical plane. The manner in which the rudders and elevators were controlled depends upon what kind of torpedo we are dealing with. (a) Consider first the "strait runner" being a torpedo which runs at a fixed depth and on a straight pre-set course. The early "straight runners" carried both a gyro and a depth sensor. The gyro was spun up prior to launch and provided the directional reference used to control the rudders. Any deviation from the set course was detected by the gyro and a corresponding signal sent to the rudders which steered the torpedo in a direction such as to bring it back on course.

The depth was contolled by the elevators which received control signals from the depth sensor. The depth sensor itself was somewhat like an aneroid barometer. It sensed the hydrostatic pressure at which the torpedo was operating. It could be pre-set to order the torpedo to run at the desired depth. If the torpedo tended to rise above the pre-set run depth the depth sensor could detect the decrease in hydrostatic pressure and would then send a "down" signal to the elevators. Likewise if the torpedo was descending below the desired depth the depth sensor detected the increase in hydrostatic pressure and sent an "up" signal to the elevators.
(b) The acoustic homing torpedo was also controlled by a set of rudders and elevators but the manner in which they were controlled was a little different. A depth sensor was still used to control the depth through the action of the elevators. The rudders however were controlled by the acoustic sensing system. A hydrophone array was located in the nose section of the torpedo which could detect the noise being radiated into the water from a ships propulsion machinery. The acoustic system could also determine the direction from which the ships "noise" was coming. Control signals were sent to the rudders which directed the torpedo in the direction from which the noise was being received. If the ship changed course the torpedo would change course and follow the noise source.

However, a torpedo using this control system proved relatively easy to countermeasure. The acoustic sensing system was designed to direct the torpedo at the "loudest" noise source in the water. So ships towed a "noisemaker" behind them which produced more noise than the ship itself. The torpedo then tended to attack the "noisemaker".

(c) WW II saw the development of the first acoustic homing torpedo which was designed to attack a submarine. This torpedo did carry a depth sensor which was only used to take the torpedo down to a pre-set search depth. Once at search depth control was turned over to the acoustic homing system which could detect the noise of the submarine and then steer the torpedo in both the horizontal and vertical planes to follow the submarines sounds. (This was the MK 24 "Fido" - you can read a more detailed account of its development on this website under the "Fighting the U-boats" weapons section.)

Modern acoustic homing torpedoes carry very sophisticated acoustic signal processing systems which make them very difficult to countermeasure. Of course this technology was not available to torpedo designers during WW II.

I hope this will give you some of the information you were seeking.

Best regards,
Don Baker