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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.

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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 metres/sec) under the Advanced High-Speed Underwater Munition (Ahsum) programme.

To digress, supercavitation has also been applied to improve the penetration and stability of bullets used in big game hunting, as illustrated by the products of Michael Reichenberg Spezialgeschosse in Germany and GS Custom in South Africa.

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The US Navy's AWS-2 Ramics (Rapid Area Mine Clearance System) is being developed to arm the Sikorsky MH-60S, tasked with protecting surface vessels against floating and near-surface moored mines. These will first have been detected by other sensors, and then reacquired by the Northrop Grumman AWS-1 Almds (Airborne Laser Mine-Detection System) on the MH-60S. The Almds will cue the aircraft's gun turret, which has its own lidar, and fire the supercavitating ammunition.

The Ramics ATD (Advanced Technology Development) phases ran from FY95 to FY00 and employed 20 mm fin-stabilised, discarding-sabot sub-calibre ammunition developed by CTech Defense. In November 1998 in a lethality demonstration at Aberdeen Proving Ground, Maryland, the company's AMP (anti-mine projectile) fired from an M197 Gatling gun on a Cobra helicopter successfully neutralised a submerged Mk 6 mine by the required deflagration process (rapid burning of the explosive without detonation). C Tech Defense subsequently sold its 20 mm ammunition proprietary rights for airborne mine clearance applications to Hughes (now Raytheon).

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For the concept technology demonstration phase of FY00 to FY02 the US Navy changed to 30 mm ammunition, to be fired from an ATK Mk 44 Bushmaster II mounted on the MH-60S. Without competition, the service adopted its own patented ammunition design, based on the Raufoss Bushmaster II long rod penetrator, but with a supercavitating nose and slender tapering forebody added. This was cleared by the Norwegian Army as the NM225 and is produced by Raufoss for the Ramics development phase as the Mk 258 Mod 1. Raufoss is also in full production with the non-cavitating Mk 258 Mod 0 for use against surface targets.

This 30 mm Ramics ammunition fires a 150 gram sub-calibre projectile at 1430 metres/sec. Tipped with a carbide or tungsten insert (reports differ) to survive water entry pressures of over 20,000 kg/[cm.sup.2], it typically strikes the sea at 1200 metres/sec.

The Ramics projectile remains lethal to at least 43 metres and (in the absence of target impact) continues supercavitating to a total distance of 180 metres. Given that the mine may explode, the helicopter has a safe standoff distance of 45 metres for a shallow mine and 20 metres for a deep mine.

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Torpedoes

The largest and best-known supercavitating weapon is Russia's Shkval (Squall) rocket-powered torpedo, which is marketed by Tactical Missiles (TMC). The idea goes back to the 1950s, and took off in 1960, when the Scientific & Research Institute No24 (NII-24) was charged with developing a high-speed torpedo for use against nuclear submarines.

As projected in 1963, the Shkval was to have a range of up to 20 km at a speed of 100 metres/sec. It was to have an autonomous control system; receiving mid-course target updates via sonar from the launch submarine. It would be fired from a depth of 30 metres, then climb closer to the surface for the cruise phase before diving prior to detonation. This terminal manoeuvre, coupled with TMC's reference to a 'special tactical payload', indicates that the baseline Shkval was nuclear armed.

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First test fired in 1964, the Shkval came under the newly formed Scientific & Research Institute for Hydrodynamics (NIIPGM) from 1969. In state acceptance trials the M-5 version was tested seven times from a Project 613-RV submarine. There were around 300 further launches from a Project 671-RT (Victor-class hunter/killer) submarine of the Northern Fleet. In late 1977 the missile was accepted for service under the designation VA-111. It went into series production in 1978, and armed most Soviet nuclear-powered submarines of the second and third generations.

The 2.7-tonne Shkval has a length of 8.2 metres and a diameter of 534.4 mm. It employs a solid hydro-reactive propellant and has eight boost motors surrounding the cruise motor nozzle. Gas is bled from the main motor to augment the bubble generated by the nose cavitator. In 'flight' the torpedo is supported by lift on this cavitator and four aft-mounted spring-out fins, which also control the direction of motion. Range is now given as 10.0 km at a speed of up to 100 metres/sec.

In the 1980s the improved Shkval-15 and -15V appeared and, in 1992, the Russian Government decided to allow exports of some Shkval variants, to provide funds for further development. In 1995 the non-nuclear Shkval-E (which may slew for an acoustic-homing terminal phase) made its public debut at Idex in Abu Dhabi.

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According to TMC, China bought up to 40 Shkvals from Kazakhstan, and Russia and the Ukraine are the only other users. The Iranian Navy firings of two similar missiles named Hoot (Whale) from a fast patrol boat during an exercise in April 2006 are not explained. At the time, Russian defence minister Sergei Lavrov denied supplying Iran with Shkval technology.

In 1988 Germany launched the technology demonstration phase of a supercavitating torpedo for close-in defence of submarines and surface vessels against incoming torpedoes. The resulting Diehl Defence Barracuda has a solid propellant rocket, an autopilot, an inertial measuring unit and an articulated conical cavitator that also provides pitch and yaw control. Around a dozen test specimens have been fired successfully. Speed is given as over 400 km/hr (110 m/sec), but there are reports that the true figure is much higher. The Barracuda was unveiled at Imdex 2005 in Singapore.

The US Naval Undersea Warfare Center has a 171 mm, 90-kg Anti-Torpedo Torpedo (ATT) project, which is being studied by the Applied Research Laboratory of Penn State University. However, the ATT is not currently aimed at countering the Shkval.

Underwater Express

In December 2005 Darpa invited proposals from American industry (only) for the Underwater Express technology development and demonstration programme; this was designed to investigate the feasibility of a 60-tonne <<super-fast submerged transport>> with a 2.4-metre diameter and a cruise speed of 50 metres/sec.

Underwater Express came into the limelight in April 2006 when the Pentagon cancelled plans to buy more than the first example of the Advanced Seal Delivery System (ASDS). This 60-tonne miniature submarine was designed to deliver up to 16 Seals clandestinely and in relative comfort, having been transported to the launch area on the back of a Los Angeles class SSN. Only one vehicle (ASDS-1) of the six planned was accepted by the US Navy (in July 2003), and this was trialled from the USS Greeneville (SSN-772).

The ASDS suffered years of performance and reliability problems, and when it was put on hold in November 2005, it was said to be seven times over budget and twelve years behind schedule. Funding ASDS improvements is being weighed against switching to an alternative based on the Underwater Express. A decision is scheduled for mid-2008.

In November 2006 contracts were awarded to Northrop Grumman and General Dynamics' Electric Boat division for the 13-month first phase of the Underwater Express programme. The second and third phases will each cover 15 months. The latter will include construction of a Demonstration Super-fast Supercavitating Transport (DSST) able to operate at 50 metres/sec for up to ten minutes. The three phases are potentially worth $ 45.8 million to Northrop Grumman and 37.1 million to General Dynamics.

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In August 2007 a Darpa presentation appeared to indicate optimism over Underwater Express performance and propulsion aspects, while voicing concern over communications, GPS-less navigation and obstacle avoidance.

Other Applications

The gaseous lubrication of surface vessel hulls is a development of the 'stepped' floats and hulls traditionally used on seaplanes and flying boats. The concept is the subject of much research in Europe, Japan, Russia and the United States. For example, the Darpa Aircat (Air Cavity Drag Reduction) programme aims to achieve an 80% reduction

in hull wetted area.

The St Petersburg-based CMDB Almaz is probably Russia's leading designer of fast patrol boats. Its Project 12200V 'Sobol' (Sable) and Project 14310 Mirazh (or Mirage, seen in our title photograph) use a patented system of electrically-actuated ventral and stern flaps to generate ventral gas bubbles and thus achieve around a ten% increase in speed, while reducing pitch and roll motion.

In 2005 Lockheed Martin was reportedly attempting to extend supercavitation in solid materials (as thought to be achieved by the Ramics projectile) to larger calibre bombs in order to penetrate soil and concrete. If a blunt-nosed cavitator can be made strong enough, and it is moving fast enough, then (maybe) it can generate a cavity by vaporising the target material.

Referred to as the 'Kinetic Energy Cavity Penetrator Weapon' using 'terra-dynamic cavitation', this device was due to be tested before the end of 2005. However, nothing more has since been published, suggesting that it either does not work or is very successful. In a similar context, several American patents refer to supercavitating gun-fired projectiles and flechettes to destroy buried land-mines.
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Title Annotation:Technology
Author:Braybrook, Roy
Publication:Armada International
Date:Feb 1, 2008
Words:1938
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