Supercavitation

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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 (i.e., 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.

A cavitation forms behind an object passed by a rapidly streaming liquid
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A cavitation forms behind an object passed by a rapidly streaming liquid

In 1977, Soviet engineers developed the VA-111 Shkval ("Squall") torpedo. This can travel at 230 mph (100 m/s) underwater, compared to the top speed of about 80 mph (35 m/s) for conventional aquatic craft, but the first version of the Shkval is not steerable. Even faster speeds of about 310 mph (ca. 140 m/s) and higher have also been rumored. News of the device reached the West in the 1990s.

The US Defense Advanced Research Project Agency (DARPA) began the "Underwater Express" program in 2005 to research applications of supercavitation. DARPA awarded contracts under the program to General Dynamics and Northrop Grumman in 2006. Research is now taking place at the Naval Undersea Warfare Center (Newport, Rhode Island, USA) and Space and Naval Warfare Systems Center (San Diego, California, USA).

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[edit] From cavitation to supercavitation

To hydroengineers, cavitation is a known phenomenon. Cavitation happens when water is forced to move at extremely high speed, e.g. inside of a pump or around an obstacle, such as a rapidly spinning propeller. The pressure of the fluid drops due to its high speed (Bernoulli's principle) and when the pressure drops below the vapor pressure of the water, it vaporizes — typically forming small bubbles of water vapour, i.e. of 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 fast-rotating propellers.

A supercavitating object uses this phenomenon in a much larger (and sustained) manner, hence the name. 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 aluminium with water. A conventional rocket engine is used to propel the Shkval supercavitating torpedo.

[edit] 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 Hs 293 C and the larger 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 hitting point. 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 pathes 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 .50 cal, 20 mm and 30 mm[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 for introduction into the fleet.

The darts of German (HK 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 that supercavitating torpedoes can give an overwhelming advantage to a navy possessing them in quantity (assuming that the opposing navy doesn't possess them).

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 a domestically-produced copy of an early-model Shkval in April of 2006 [4]. They call this weapon the "hoot", which means "Whale".

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.

[edit] Alleged applications

Josef Papp claimed in 1966 to have built an underwater propulsion system which took advantage of supercavitation to achieve incredibly high speeds.

[edit] Drawbacks

Though increasing the speed of a torpedo does theoretically confer a significant tactical advantage to the combatant employing them, supercavitation has its drawbacks as well. In short, naval tactics have evolved to the point that the use of supercavitation is not of undisputed benefit, and the navies that employ it tend to be those that have not perfected sophisticated guidance systems for their torpedoes. Naval combat frequently occurs over significant distances; the Mark 48 torpedo, the staple torpedo of the US navy, has a range of five miles with unconfirmed reports of ranges in excess of twenty miles. The maximum speed of at least 32 miles per hour, (unconfirmed reports indicate speeds upwards of 60 mph) meaning it can reach its maximum range in a matter of minutes (ten to twenty, depending on the figures used). A supercavitating torpedo traveling at 230mph would still take over a minute to reach a destination five miles away, and around six minutes to reach a target twenty miles away. This is more than ample time for a target to dodge. But whereas conventional torpedoes are capable of homing in on a target using either wires connected to the launching ship or active sonar, the nature of supercavitation precludes either method of guidance. The supercavitating engine would sever any wires attached to the torpedo, and the bubble of vapor surrounding the torpedo both enables it to travel at very high speeds and prevents the use of sonar. Supercavitation also produces an incredible amount of noise, which alerts the target to both the torpedo and the location of the launching sub. A common submarine tactic is to quietly launch a torpedo, but not to activate it until the firing sub has moved a few hundred yards away, then activating it and guiding it to the target. Supercavitating torpedoes are only capable of being "dumb-fired" directly from the launching vessel, which would immediately reveal its position, a lethal mistake in modern naval warfare. As the torpedoes produced by the American navy - and sold through arms contracts to allied navies - have incredible guidance systems, interest in supercavitation has been rather muted in the Western naval community.

[edit] 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/
  • 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.

[edit] External links

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