Torpedo Tube (thing)

A device for launching torpedos, shown here is the tube used on submarines.
Some crude ASCII art to show the component parts;

     
           Pressure in
          _| |________________________________________________
         /         ____________________________               \
Inner>   |      |_/ Torpedo                    \              |  Outer
Door     |      | \____________________________/              |    Door
(Breech) \_   ________________________________________________/    (muzzle)
           | |
           Drain  

The tube is drained, and the breech (inner door, at left in the diagram) opened. A torpedo is inserted into the tube and the breech is shut. The tube is then flooded from the sea (via a connection not shown on the diagram), the outer (muzzle) door is then opened. Pressure is introduced behind the torpedo, which pushes the torpedo out of the tube. The outer door is then closed and the cycle can begin again.

Torpedo tubes on surface vessels are similiar, but don't require flooding or draining if they are located above the waterline.