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 and this makes supercavitation an attractive technology; drag is normally about 1,000 times greater in water than in air.

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From cavitation to supercavitation

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 though 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 to badly designed rotating propellers.

A supercavitating object uses this phenomenon in a much larger (and sustained) manner (hence the name supercavitation). A supercavitating object's main features are a specially shaped nose, typically flat with sharp edges, and a streamlined, hydrodynamic and aerodynamic shape. When the object is traveling through water at speeds of above roughly one-hundred miles per hour, the nose deflects the water outward so fast that it flies free of the surface. Water pressure takes time to collapse the wall of the resulting cavity, hence the nose opens an extended bubble of water vapor. Given sufficient speed, or by the injection of gas into a partially-developed bubble, the cavity can extend to envelop the entire body of the object. A supercavitating object quite literally 'flies' through the surrounding gas.

Various underwater methods of propulsion have been proposed to reach the necessary speed, with a possible concept being a rocket engine burning aluminum with water. A conventional rocket engine is used to propel the Shkval supercavitating torpedo.

Current applications

The supercavitation principle is being used for very high performance supercavitating 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 velocities at water entry of approximately 150 to 180 m/s. Those values were corresponding with underwater paths of the warhead-projectile of about 60 to 80 meters.

After World War II and until 2004, Russian Shkval torpedoes were the only publicly known application of supercavitation technology applied to an entire underwater vessel. The gas bubble these torpedoes fly in is formed partly due to the shape of the torpedo body, partly by rocket exhaust diverted to a nozzle on the front of the torpedo.

In 2004 the German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo called the Barracuda. (English translation)

Beginning in 1994, the US Navy began developing a sea mine clearance system known as RAMICS (Rapid Airborne Mine Clearance System), based on a supercavitating projectile, invented by C Tech Defense Corporation, that was stable in both air and water. These have been produced in .50cal (12.7mm), 20mm (.79"), and 30mm (1.19") [1]. The terminal ballistic design of the projectile allowed it to cause explosive destruction of sea mines as deep as 45 m (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 [2], 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.[3]

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 that the opposing navy doesn't 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.

As of early 2006, Russia and China are known operators of supercavitating weapons. Germany and the United States have in-development weapons and may be fielding operational models secretly.

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 [4] [5] [6]. Russian Foreign Minister Sergei Lavrov denied supplying Iran with the technology [7]. Iran called this weapon the Hoot (Whale).

German press reports of an underwater anti-torpedo missile named Barracuda that allegedly reaches 800 km/h [8].

Alleged applications

Josef Papp claimed in 1966 to have built an underwater propulsion system which took advantage of supercavitation to achieve incredibly high speeds.[9] However this is generally regarded as a hoax, due to circumstances surrounding his claim.

References

• Office of Naval Research (2004, June 14). Mechanics and energy conversion: high-speed (supercavitating) undersea weaponry (D&I). Retrieved April 12, 2006, from http://www.onr.navy.mil/
• Savchenko Y. N. (n.d.). CAV 2001 - Forth Annual Symposium on Cavitation - California Institute of Technology Retrieved April 9, 2006, from http://cav2001.library.caltech.edu/159/00/Savchenko.pdf
• Hargrove, J. (2003). Supercavitation and aerospace technology in the development of high-speed underwater vehicles. In 42nd AIAA Aerospace Sciences Meeting and Exhibit. Texas A&M University.
• Kirschner et al. (2001, October) Supercavitation research and development. Undersea Defense Technologies
• Ashley, S. (2001, May). Warp drive underwater. Scientific American
• Miller, D. (1995). Supercavitation: going to war in a bubble. Jane's Intelligence Review. Retrieved Apr 14, 2006, from http://www.janes.com/
• Graham-Rowe, & Duncan. (2000). Faster than a speeding bullet. NewScientist, 167(2248), 26-30.
• VA-111 Shkval. (2006, April 7). In Wikipedia, The Free Encyclopedia. Retrieved April 11, 2006, from http://en.wikipedia.org/wiki/VA-111_Shkval