Cavitation is the rapid formation and collapse of vapor-filled cavities in a fluid flowing under very low pressure. Cavitation occurs around propellers and in pump systems and is a frequent cause of structural damage to these components.
Supercavitation is the use of a cavitation bubble to reduce skin friction drag on a submerged object and enable high speeds. In supercavitation, the small gas bubbles produced by cavitation expand and combine to form one 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 workable design for the Soviet navy during the Cold War, the concept of supercavitating torpedoes has fascinated military engineers ever since, although little practical headway seems to have been achieved subsequently, aside from a number of stalled projects and aborted attempts over the years.
The fundamental idea behind supercavitation is surprisingly simple. When water is forced around an object, such as a ship’s propeller, at high speeds the pressure drops around the trailing edge, and if it drops below the water’s vapor pressure, bubbles are formed in a process known as cavitation. Traditionally, it has been a problem for engineers because when the bubbles strike the propeller itself they then implode, damaging the material and leading to serious cavitation erosion over time.
However, in the late 1940s Soviet scientists began to wonder if by deliberately manipulating this effect to create a huge, sustainable mega-bubble, and then encasing a torpedo body within it as it hurtles through the water, hydrodynamic drag could be largely overcome. Two decades and six prototypes later, their work was to see practical supercavitation realized, and the emergence of a new weapon class, capable of remarkable submerged speeds.