# Supercavitation

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Supercavitation is the use of cavitation effects to create a large bubble of gas inside a liquid. The cavity (the bubble) reduces the drag on the object, since drag is normally about 1,000 times greater in liquid water than in a gas. Current applications are mainly limited to very fast torpedoes.[citation needed]

Cavitation happens when water pressure is lowered below its vapor pressure or vapor pressure increases to equal water pressure. This often happens at extremely high speed although it can happen at any speed and even when not moving. Cavitation occurs inside a pump or around an obstacle, such as a rapidly spinning propeller or in a body of liquid (such as a kettle) due to temperature/pressure change. The pressure of the fluid can drop due to its high speed (Bernoulli's principle) and when the pressure drops below the vapor pressure of the water or the temperature increases. When vapor pressure increases to water pressure, it vaporizes–typically forming small bubbles of water vapor (water in its gasphase). In ordinary hydrodynamics, cavitation is a mostly unintended and undesirable phenomenon: the bubbles are typically not sustained but implode as they and the water around them suddenly slows down again, with a resulting sudden rise in ambient pressure. These small implosions can even lead to physical damage, for instance spalling damage to badly designed rotating propellers, pumps, and piping.

Various underwater methods of propulsion have been proposed to reach the necessary speed, with a possible concept being a rocket engine burning aluminium with water. As an example, a conventional rocket engine is used to propel the Russian Shkval supercavitating torpedo.[1]

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### Applications

In 2004, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, Barracuda, now officially named "Superkavitierender Unterwasserlaufkörper" or "supercavitating underwater running object". (English translation) According to Diehl, it reaches more than 400 kilometres per hour (250 mph).[2]