Like wire through cheese: supercavitation: suprcavitation occurs when a submerged moving body is covered in water vapour, with consequent reduction in density-dependent friction. Already applied to torpedoes and special-purpose bullets, the concept may be extended to submarines, surface vessels and (possibly) bunker-busting munitions.

Author:Braybrook, Roy
Position:Technology
 
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An underwater supercavitating missile can be likened to a wire cutting through cheese. Compared to a knife, a wire accepts some increase in pressure drag in order to achieve a massive reduction in wetted area and thus viscous drag.

Supercavitation was first considered as a means to increase the underwater range of shells fired to strike ships below their waterlines. However, anti-ship projectiles with blunt noses (as used to generate supercavitation) were developed to avoid ricochets from the water surface.

During World War I the renowned Thomas Edison proposed a 'Pagoda Head' shell for this purpose. A 1921 US patent by Wilkinson refers, as does a 1932 patent by Germany's Professor Herbert A Wagner, who was later employed by Junkers and subsequently Henschel. The Japanese (at least) used a blunt-nosed anti-ship shell during World War II.

The Henschel Hs293/294 anti-ship guided missile (also of WW II), using an SC500 warhead with Kopfring (a device fitted to German and Russian bombs to limit penetration in soil), may have achieved supercavitation on entering the water at 150 to 180 metres/sec.

The 1940s brought cavitation problems with high-speed ship propellers, reducing thrust, while increasing sonar-emission, vibration and blade erosion. Western hydrodynamic research then concentrated on reducing propeller blade drag by supercavitation, while the Soviets also used it to reduce the drag on non-rotating bodies.

In the 1970s the Soviets introduced two underwater firearms for special forces: the 4.5 mm SPP-1M handgun, and the 5.66 mm APS assault rifle. Both weapons fire supercavitating projectiles and are used to attack divers and defend against sharks. A conventional bullet will penetrate through only a metre or two of water, but these weapons are lethal to 17 and 30 metres respectively, at a depth of five metres.

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The dimensions of the supercavitation bubble vary with ambient pressure, as does the force on the projectile nose, hence effective range reduces as depth increases. In the case of the APS rifle, range is reduced to 20 metres at a depth of 20 metres, and to ten metres at 40 metres depth.

The West's leading small-calibre underwater weapon is the Heckler & Koch 7.62 mm P11 pistol, which fires rocket-powered darts, reportedly lethal to 15 metres distance at 50 metres depth. The P11 is absent from the H&K website, although it entered service in 1976, and was used by Angelina Jolie in the 2003 movie Tomb Raider--The Cradle of Life.

In 1997 the US Navy's Naval Undersea Warfare Center developed a supersonic bullet (exceeding 1500...

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