Supercavitation is the use of a cavitation bubble to reduce the skin friction drag of a submerged object and enable high speeds. In supercavitation, the small gas bubbles produced by cavitation expand and coalesce to form a large, stable, and predictable bubble around the supercavitating object.
Applications include torpedoes and propellers, but in theory the technique could be extended to an entire underwater vessel. First developed into a viable design for the Soviet Navy during the Cold War, the concept of supercavitating torpedoes has fascinated military engineers ever since, although little practical progress seems to have been made since then, aside from a number of stalled projects and aborted attempts over the years.
The basic idea behind supercavitation is surprisingly simple. When water is forced around an object, such as a ship’s propeller, at high speed, the pressure around the trailing edge drops, and when it drops below the vapor pressure of the water, bubbles form in a process known as cavitation. Traditionally, this has been a problem for engineers because when the bubbles hit the propeller itself, they implode, damaging the material and causing serious cavitation erosion over time.
In the late 1940s, however, Soviet scientists began to wonder if the hydrodynamic drag could be largely overcome by deliberately manipulating this effect to create a huge, persistent mega-bubble, and then encasing a torpedo body in it as it hurtled through the water. Two decades and six prototypes later, their work would lead to the realization of practical supercavitation and the emergence of a new class of weapons capable of remarkable underwater speeds.