# Supercavitation

An object (black) encounters a liquid (blue) at high speed. The fluid pressure behind the object is lowered below the vapour pressure of the liquid, forming a bubble of vapour (a cavity) that encompasses the object.

Supercavitation is the use of cavitation effects to create a bubble of gas inside a liquid large enough to encompass an object travelling through the liquid, greatly reducing the skin friction drag on the object and enabling achievement of very high speeds. Current applications are mainly limited to projectiles or very fast torpedoes, and some propellers, but in principle the technique could be extended to include entire vehicles. This phenomenon can also be produced by the very fast strike of the appendages of the crustacean mantis shrimp Odontodactylus scyllarus, that uses it to attack and kill its prey.[1]

## Physical principle

In water, cavitation occurs when water pressure is lowered below the water's vapour pressure, forming bubbles of vapour. That can happen when water is accelerated to high speeds as when turning a sharp corner around a moving piece of metal such as a ship's propeller or a pump's impeller. The greater the water depth (or pressure for a water pipe) at which the fluid acceleration occurs, the lesser the tendency for cavitation because of the greater difference between local pressure and vapour pressure. (The non-dimensional cavitation number is a measure of the tendency for vapour pressure bubbles to form in a liquid, calculated as the difference between local pressure and vapour pressure, divided by dynamic pressure.) Once the flow slows down again, the water vapour will generally be reabsorbed into the liquid water. That can be a problem for ship propellers if cavitation bubbles implode on the surface of the propeller, each applying a small force that is concentrated in both location and time, causing damage.

A common occurrence of water vapour bubbles is observed in a pan of boiling water. In that case the water pressure is not reduced, but rather, the vapour pressure of the water is increased by means of heating. If the heat source is sufficient, the bubbles will detach from the bottom of the pan and rise to the surface as steam. Otherwise if the pan is removed from the heat the bubbles will be reabsorbed into the water as it cools, possibly causing pitting or spalling on the bottom of the pan as the bubbles implode.[citation needed]

A supercavitating object is a high speed submerged object that is designed to initiate a cavitation bubble at the nose which (either naturally or augmented with internally generated gas) extends past the aft end of the object, substantially reducing the skin friction drag that would be present if the sides of the object were in contact with the liquid in which the object is submerged. A key feature of the supercavitating object is the nose, which may be shaped as a flat disk or cone, and may be articulated, but which likely has a sharp edge around the perimeter behind which the cavitation bubble forms.[2] The shape of the object aft of the nose will generally be slender in order to stay within the limited diameter of the cavitation bubble. If the bubble is of insufficient length to encompass the object, especially at slower speeds, the bubble can be enlarged and extended by injection of high pressure gas near the object's nose.[2]

The great speed required for supercavitation to work can be achieved temporarily by a projectile fired under water or by an airborne projectile impacting the water. Rocket propulsion can be used for sustained operation, with the possibility of tapping high pressure gas to route to the object's nose in order to enhance the cavitation bubble. An example of rocket propulsion is the Russian VA-111 Shkval supercavitating torpedo.[3][4] In principle, maneuvering may be achieved by various means such as drag fins that project through the bubble into the surrounding liquid[5] (p. 22), by tilting the nose of the object, by injecting gas asymmetrically near the nose in order to distort the geometry of the cavity, by vectoring rocket thrust through gimbaling for a single nozzle, or by differential thrust for multiple nozzles.[2]

## Applications

In 1960, the USSR started developing a project under the codename "Шквал" (Squall) run by NII-24 (Kiev) to develop a high-speed torpedo, an underwater rocket, four to five times faster than traditional torpedoes capable of combating enemy submarines. Several models of the device were made, the most successful – M-5 – was created by 1972. In 1972 to 1977, over 300 test launches were made (95% of them on Issyk Kul lake), by 29 November 1972 VA-111 Shkval was put into service with mass production started in 1978.

In 2004, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, Barracuda, now officially named "Superkavitierender Unterwasserlaufkörper" or "supercavitating underwater running body" (English translation). According to Diehl, it reaches more than 400 kilometres per hour (250 mph).[6]

In 1994, the US Navy began developing a sea mine clearance system invented by C Tech Defense Corporation, known as RAMICS (Rapid Airborne Mine Clearance System), based on a supercavitating projectile stable in both air and water. These have been produced in 12.7 millimeters (0.50 in), 20 millimetres (0.79 in), and 30 millimetres (1.2 in) diameters.[7] The terminal ballistic design of the projectile allowed it to cause explosive destruction of sea mines as deep as 45 meters (148 ft) underwater with a single round.[8] In 2000, these projectiles were used to successfully destroy a range of live underwater mines when fired from a hovering Sea Cobra gunship at Aberdeen Proving Grounds. RAMICS is currently[when?] undergoing development by Northrop Grumman for introduction into the fleet. The darts of German (Heckler & Koch P11) and Russian underwater firearms,[9] and other similar weapons are also supercavitating.

In 2005, DARPA announced the 'Underwater Express program', a research and evaluation bid to establish the potential of supercavitation. The program's ultimate goal is a new class of underwater craft for littoral missions that can transport small groups of Navy personnel or specialized military cargo at speeds up to 100 knots. The contracts were awarded to Northrop Grumman and General Dynamics Electric Boat in late 2006.[citation needed] In 2009, DARPA announced progress via a new class of submarine.

The submarine's designer, Electric Boat, is working on a one-quarter scale model for sea trials off the coast of Rhode Island. If the trials are successful, Electric Boat will begin production on a full scale 100-foot submarine. Currently, the Navy's fastest submarine can only travel at 25 to 30 knots while submerged. But if everything goes according to plan, the Underwater Express will speed along at 100 knots, allowing the delivery of men and materiel faster than ever."[10]

Iran claimed to have successfully tested its first supercavitation torpedo on 2 April and 3 April 2006. Some sources have speculated it is based on the Russian VA-111 Shkval supercavitation torpedo, which travels at the same speed.[11] Russian Foreign Minister Sergei Lavrov denied supplying Iran with the technology.[12] Iran called this weapon the Hoot (Whale).

A prototype named the Ghost, designed for stealth operations by Gregory Sancoff of Juliet Marine Systems, uses supercavitation to propel itself atop two struts with sharpened edges. The vessel rides smoothly in choppy water and has reached speeds of 29 knots.[13]

Artist rendering of a supercavitating propeller in action

The supercavitating propeller is a variant of a propeller for propulsion in water, where supercavitation is actively employed to gain increased speed by reducing friction. They are being used for military purposes and for high performance boat racing vessels as well as model boat racing. The supercavitating propeller operates submerged with the entire diameter of the blade below the water line. Its blades are wedge-shaped to force cavitation on the whole forward face, starting at the leading edge, in order to reduce water skin friction. As the cavity collapses well behind the blade, the supercavitating propeller avoids the spalling damage due to cavitation that is a problem with conventional propellers.

## Alleged applications

The Kursk submarine accident was rumored to have been due to a faulty Shkval torpedo,[14] though later evidence points to a faulty 65-76 torpedo - see Kursk submarine disaster.