Supercavitation

Supercavitation is the use of cavitation effects to create a large bubble of gas inside a liquid, allowing an object to travel at great speed through the liquid by being wholly enveloped by the bubble. 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 torpedos. Some experiments with small submarines also exist.

Cavitation happens when water pressure is lowered below its vapor pressure or vapor pressure is increased to water pressure. This often happens at extremely high speed although it can happen at any speed and even when still. 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 and/or 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 thus vapor pressure increases reaching water pressure, it vaporizes – typically forming small bubbles of water vapour (water in its gas phase). 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.

Current applications
The supercavitation principle is being used for very high performance propellers and also for control surfaces such as rudders.

In 1940, Herbert A. Wagner - Head of the Development Department for Guided Missiles at Henschel Flugzeugwerke (HFW), Berlin - started the development of two Guided Air-to-Sea Missiles: the Henschel Hs 293 and the larger Henschel Hs 294. Both missiles were supposed to be guided to a point in front of the water line of a ship. At water entry, the warhead would separate from fuselage and wings. Using its remaining kinetic energy, the (unguided) warhead-projectile would then follow an underwater path towards the ship target. The projectile body had a slender conical shape with an ogive nose. The underwater path could be curved slightly upward by means of a small ridge on the upper side of the ogive. The projectile had to be curved upward in order to achieve a nearly horizontal path at the point of impact. In order to stabilize the projectile under water within its supercavity bubble, a somewhat larger cone angle was used at the tail of the body. Tests with prototypes of the Hs 294 achieved water entry velocities of approximately 150 to 180 m/s, corresponding to underwater paths of the warhead of about 60 to 80 meters.

In 2004, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, Barracuda. (English translation) According to press reports, it reaches 800 km/h&lt;ref name="diehl"&gt;Diehl BGT Defence: Unterwasserlaufkörper&lt;/ref&gt;.

Beginning 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 mm (0.50 in), 20 mm (0.79 in), and 30 mm (1.19 in) calibers. The terminal ballistic design of the projectile allowed it to cause explosive destruction of sea mines as deep as 45m (140 feet) underwater with a single round. (C Tech) In 2000, these projectiles were used to successfully destroy a range of live underwater mines when fired from a hovering Sea Cobra gunship at the Aberdeen Proving Grounds in Maryland. RAMICS is currently undergoing further development under a contract to Northrop Grumman for introduction into the fleet.

The darts of German (Heckler & Koch P11) and Russian underwater firearms &lt;ref&gt;&lt;/ref&gt;, and other similar weapons are also supercavitating.

In 1999 the supercavitation technology was adopted to hunting projectiles. These "SuperPenetrator" bullets feature a very stable straight line penetration in aqueous media.&lt;ref&gt;&lt;/ref&gt;

To date, the main emphasis of research into supercavitation has been into the development of torpedoes, due to the fact supercavitating types can give an overwhelming advantage to a navy possessing them in quantity (assuming the opposing navy does not possess them).

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.

In 2009, DARPA announced that they have made much progress on the 'Underwater Express program' through 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 material faster than ever."&lt;ref&gt;http://www.popsci.com/military-aviation-amp-space/article/2009-07/darpa-readies-ultra-fast-mini-sub&lt;/ref&gt;

Iran has 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. Russian Foreign Minister Sergei Lavrov denied supplying Iran with the technology. Iran called this weapon the Hoot (Whale).

Alleged applications
Josef Papp claimed in 1966 to have built an underwater propulsion system which took advantage of supercavitation to achieve incredibly high speeds. However this is generally regarded as a hoax, due to circumstances surrounding his claim. The Kursk submarine accident is rumored to be due to a faulty supercavitation torpedo.