Submarines are a silent and difficult to detect underwater weapon. Their main armament, torpedoes, is generally of two sizes: 533 and 650 millimeters, they are generally equipped with high explosives. While there is a surprisingly wide variety of propulsion systems used by torpedoes, some, such as the torpedoes Mark 48 e Mark 46 of the US Navy use a piston engine and a specially formulated propellant. They are faster than most surface ships and submarines, but cannot travel faster than 80km per hour. Super-cavitation torpedoes travel much faster.
Supercavitating torpedoes travel much faster: at hundreds of kilometers per hour. They overcome the resistance caused by friction with the water by traveling in a sort of bubble. The compressed gas stored inside the torpedo is expelled from the nose and the torpedo “slips” through the water inside the bubble, remaining relatively dry. Control surfaces, usually fins, pierce the bubble and maintain contact with the water, allowing the torpedo to steer.
One of the design challenges is target acquisition. Supercavitation torpedo technology limits target acquisition because these torpedoes, powered by a rocket engine, are very noisy. And then, as mentioned, there is the problem of maneuverability.
How do I move a bubble?
With conventional torpedoes, steering is simple: just adjust the flaps. But with supercavitating torpedoes not only do the torpedo's fins have to be moved, but also the bubble sheath surrounding the torpedo. The rotation distorts the bubble surrounding the torpedo and can cause the torpedo to contact the water at high speed. To avoid the problem, more of the bubble-forming gas must be moved to the side of the torpedo facing the curve. And it's not easy at all.
Superkavitierender Unterwasserlaufkörper
In the early 2000s, a German company collaborated with the German Navy (Deutsche Marine) to develop the Superkavitierender Unterwasserlaufkörper, or supercaviting underwater body.
The German torpedo had a single cone nose that protruded forward of the torpedo's body. The speed of the torpedo was supposedly 400 kilometers per hour. The Superkavitierender Unterwasserlaufkörper could also control the depth by injecting more gas into the bubble to compensate for the compression that occurs when going deeper.
Other countries also produce supercavitation torpedoes. The Soviet Union produced supercavitating torpedoes in the 70s, and Iran claims to have developed a torpedo that no vessel can avoid. The USA may also have developed a similar torpedo (is registering incredible patents), but the details are scarce.
