There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units (also referred to as torpedo launchers) installed aboard surface vessels. Deck-mounted torpedo launchers are usually designed for a specific type of torpedo, while submarine torpedo tubes are general-purpose launchers, and are often also capable of deploying mines and cruise missiles. Most modern launchers are standardised on a 12.75-inch (324 mm) diameter for light torpedoes (deck mounted aboard ship) or a 21-inch (533 mm) diameter for heavy torpedoes (underwater tubes), although other sizes of torpedo tube have been used: see Torpedo classes and diameters.
A submarine torpedo tube is a more complex mechanism than a torpedo tube on a surface ship, because the tube has to accomplish the function of moving the torpedo from the normal atmospheric pressure within the submarine into the sea at the ambient pressure of the water around the submarine. Thus a submarine torpedo tube operates on the principle of an airlock.
The diagram on the right illustrates the operation of a submarine torpedo tube. The diagram is somewhat simplified but does show the working of a submarine torpedo launch.
A torpedo tube has a considerable number of interlocks for safety reasons. For example, an interlock prevents the breech door and muzzle door from opening at the same time.
The submarine torpedo launch sequence is, in simplified form: