Submarines are a silent and difficult to detect underwater weapon. Their main armament, torpedoes, are generally of two sizes: 533 and 650 millimeters, generally equipped with high explosives. While there is a surprisingly wide variety of propulsion systems used by torpedoes, some, such as the i 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.
Supercavitation torpedoes travel much faster - hundreds of kilometers per hour. They overcome the resistance caused by friction with water by traveling in a kind of bubble. The compressed gas stored inside the torpedo is ejected from the nose and the torpedo "slides" through the water inside the bubble, remaining relatively dry. The control surfaces, usually the 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 supercavitation torpedoes not only the fins of the torpedo 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 come into high-speed contact with water. To avoid this problem, more of the bubble-forming gas must be moved to the side of the torpedo facing the bend. And it's not easy at all.
In the early 2000s, a German company partnered 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 from the torpedo body. The speed of the torpedo was allegedly 400 kilometers per hour. The Superkavitierender Unterwasserlaufkörper could also control depth by injecting more gas into the bubble to compensate for the compression that occurs as you go 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 US may also have developed a similar torpedo (is registering incredible patents), but the details are scarce.