TORPEDO TUBE FIRING PROCEDURE:
With the muzzle door shut, the breech door is opened and the torpedo is loaded into the tube. The breech door is then shut and the muzzle door can then be opened safely.
At any depth the sea pressure prevents the muzzle door from opening. To offset this external pressure an equal pressure is obtained within the tube by admitting water from a tank (displaced air is vented inboard) and then opening a valve to the sea to equalize the pressure.
When the tube is flooded and the pressure is equalized, the muzzle door can be opened and the torpedo is ready to fire. In actual practice the tube is flooded from tanks within the ship rather than the sea itself.
The compressed air valve is opened and the torpedo is ejected from the tube. The air is not permitted to completely fill the tube, but is vented off inboard so that a bubble does not escape into the sea and rise to the surface giving away the position of the submarine.
The compressed air is shut off and the tube fills with sea water. This offsets the lost weight of the torpedo and keeps the submarine in trim.
The muzzle door is then shut and a valve to a drain tank is opened to allow the tube to drain. The breech door can now be opened and the tube reloaded.