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.

Supercavitation exploits the phenomena of cavitation, something that submarine designers usually try to avoid. It would work as follows. When the nose of a torpedo or submarine is shaped in a certain way, usually flat with sharp edges, it creates an excessive amount of drag through the water. But the design of the nose accelerates the water it moves through at quick speeds, causing it to lose pressure and vaporize into bubbles. This follows from the well-known Bernoulli's principle, which states that fluid velocity and pressure/density are related. When the velocity increases, the density drops. When the density drops below the vapor pressure of the water, the water vaporizes until it slows down enough to recondense. By purposefully vaporizing as much water as it can, a supercavitating submarine or torpedo can create a bubble of air so large that it encompasses the whole vehicle. This is a positive feedback process - the more water is vaporized by the specially designed nose, the less drag on the vehicle, the easier it gets to move even faster and vaporize more water. The primary downside is that a supercavitating object is extremely loud. It churns water with such abandon that a shock wave would emanate out from the vehicle wherever it went. This, along with technological challenges, is the reason why supercavitating submarines have not yet been developed, although they are in the works. Supercavitating torpedoes have experienced limited use.

Supercavitating Submarines and Possible Future Supercavitation Vehicles
http://nextbigfuture.com/2009/08/supercativating-submarines-and-possible.html



The possibility of supercavitating manned vessels will be discussed in a future article.

The greatest limiting factor to the effectiveness of conventional torpedoes is the very medium through which they travel. The density of water induces a great deal of drag on even the sleekest aquatic projectile. More, push a torpedo too fast, and it will inevitable veer off-course as water drag builds up on it exponentially.

Thus, torpedoes are typically designed for maneuverability and endurance rather than speed, zooming through the water and homing in on its target. Most modern torpedoes have ranges limited to a few thousand meters, though some can reach over 30 miles, and are limited to speeds at best of about 60 mph. The nose of a supercavitating torpedo uses gas nozzles that continually expel an envelope of water vapor around the torpedo as it speeds through the ocean. This bubble of gas--a 'super cavity'--prevents the skin of the torpedo from contacting the water, eliminating almost all drag and friction and allowing the projectile to slide seamlessly through the water at great velocity. Some people have described supercavitating torpedoes as the first true underwater missiles.

The first such weapon in this class, the Shkval ("Squall"), was in development by the Soviet Union throughout the latter half of the Cold War but was not recognized in the West until the 1990s. Using powerful solid rocket motors, the Shkval is capable of speeds exceeding 230 mph, over four times the velocity of most conventional torpedoes. The Shkval also has a reported 80% kill rate at ranges of up to 7000 meters.

The US navy is seeking to build its own version of the Shkval, but one with a much higher velocity. This is mostly in response to Russia selling stripped down versions of the Shkval on the open international weapons market. However, a US combat-ready version is not expected for at least another 10+ years.

The technology does have one great weakness--maneuverability. The bubble of water vapor generated by the gas nozzles tends to become asymmetrical and breaks up along the outer side of the turn if the torpedo alters its course significantly. At the speeds such a torpedo would typically be travelling, the sudden re-assertion of water pressure and drag on it could not only severely knock it off course, but may even rip the projectile apart.

A new, improved version of the Shkval has been reported in use by the Russian Navy, one that can maneuver and track its intended target. However, it was also reported that in order to do so, this improved Shkval had to slow down significantly once in the general area of the target so it could scan and home in on its prey like a normal torpedo. While a genuine improvement, the true goal of current research is to have the torpedo maneuver and home in on a target without the need to decrease its velocity. Both Russian and US Navy researchers are striving toward this end.

One means of making sure the gas bubble does not wear down upon a turn would be by having the gas-ejection nozzles pump more water vapor into the side of the bubble that's on the outside of the turn, to provide the torpedo with a thick enough "buffer" for the turn without any more parts of it exiting the cavity. Another option might be to magnetically charge the vapor used in the torpedo�s bubble, and use a magnetic field to hold the bubble cohesive while it turns.

Another weakness of the technology is that the Shkval is both very noisy and shows up very readily on sonar. Whereas some long-range conventional torpedoes might be able to stealth relatively close to their targets before going active, the target of a supercavitating torpedo will know right away if they're in the bullseye. However, the supercavitating torpedo may also be travelling fast enough to give its intended victim much less time to take effective countermeasures.

A drawback that had been pointed out in several articles is that the Shkval and its peers only have ranges of several kilometers, whereas a number of modern torpedoes, like the US Mark 48, has a range of over 30 nautical miles. Its possible that a US submarine could just sit outside of Shkval-equipped submarine's range and pound on such an enemy with impunity.

The downside to that strategy is, of course, that most subs are unlikely to be equipped only with supercavitating projectiles. Like most modern combat subs, they will likely carry a variety of different weapons for different purposes, and the Shkval will just be one of the weapons it has in its arsenal. One can assume at long ranges they will likely employ conventional torpedoes, but once within the effective kill-range of a Shkval, they will use their supercavitating weapons to fullest possible effect. Also, it is almost a certainty that all parties engaging in research are striving to increase the weapon's range as much as possible.

Submarines, even with minimal warning, can evade a supercavitating torpedo by blowing some ballast and quickly ascending. However, an enemy submarine captain may anticipate this, and may launch a second or even a third Shkval simultaneously, aimed above the target submarine, in order to keep the enemy vessel from attempting this maneuver. The first US weapon system that is likely to employ supercavitating technology is the RAMICS (for Rapid Airborne Mine Clearance System), designed to be outfitted on naval helicopters for mine-clearing duties. The projectiles are designed to fly true both in air and water, and are fired from a specially-modified, 30 mm rapid-fire gun with advanced targeting assistance. Unlike torpedoes, the rounds have no independent propulsion system but rely on the velocity imparted to them by the gun. This would allow the aircraft to take out marine mines both on the surface and underwater, as the bullets would be able to enter the water without deflection or a significant loss in velocity. Surface ships may also deploy gun turrets using supercavitating bullets on their stern, to help defend against wake-tracking torpedoes. One non-military application for this technology is to use it for anchor mooring lines for deep-water structures such as oil rigs, aquafarms, buoys, anchored ships, and artificial islands.

Shooting a mooring line onto a seabed with a conventional projectile tends to be ineffective. A normal torpedo simply can't get enough momentum to penetrate too far into the seabed to make it more effective than just dropping the anchor and letting it drag until it catches. Sending divers or teleoperated remote vehicles down to drill the anchor in properly can be both expensive and time-consuming.

The answer is to use a smaller version of a supercavitating torpedo, as the missile would not only be travelling at high speed but would not slow down due to water pressure. It would be able to hit the ocean floor at hundreds of miles an hour, with a "warhead" designed for deep kinetic penetration of rock in order to imbed the anchor line it would be trailing. Using this technology, even large deep-ocean platforms would be able to securely anchor themselves in a very short amount of time, perhaps even within a few minutes. A proposed defensive system for submarines, these small mini-torpedoes are launched, either individually or in a small swarm, when an incoming enemy torpedo is detected. Equipped with high-efficiency, high-specific-impulse rocket motors, their main advantage is raw speed. They zip toward the incoming bogey, and either knock it out through direct kinetic impact, or explode in the incoming torpedo's path, creating a cloud of dense shrapnel designed to tear the enemy torpedo apart as it passes. Surface vessels may also use versions of these darts to take out mines. In 1997, the Navy tested a supercavitating projectile that reached 5,082 feet per second, becoming the first manmade vehicle to exceed Mach 1 underwater. As the supercavitating technology matures and the problems with maneuverability are solved, research will turn toward making the projectiles much faster and more lethal. And this will invariably mean trying to make them faster, up to the point that the torpedoes can routinely exceed Mach 1 and beyond. Greatly extended ranges as the projectiles use more sophisticated engines are also likely.

Supercavitation technology promises to greatly change the nature of modern naval warfare, for submarines especially. Instead of the patient, cat-and-mouse game reminiscent of that marked Cold War engagements, we would instead have heated, quick exchanges of transonic torpedoes underwater, making the confrontation much more akin to aerial combat. 21st Century Soldier, Popular Science Books, Time Inc, 2002http://www.periscope.ucg.com/mdb-smpl/weapons/minetorp/torpedo/w0004768.shtml http://en.wikipedia.org/wiki/VA-111_Shkvalhttp://www.popsci.com/popsci/technology/generaltechnology/de669aa138b84010vgnvcm1000004eecbccdrcrd.htmlhttp://www.stratmag.com/issueMay-15/page02.htmhttp://www.military.com/soldiertech/0,14632,Soldiertech_060420_shkval,,00.htmlhttp://www.sciam.com/article.cfm?articleID=000CA29B-0EA6-1C70-84A9809EC588EF21

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