Multibillion-dollar carriers are still at risk from thousand-dollar weapons. The Navy's newest defense: clever hydrodynamic bullets, traveling through the water are super speed.

Mines are the forgotten threat of naval warfare – simple low-tech floating bombs that destroyed and damaged more ships in the 20th century than gunfire, torpedoes and air attack combined. A tethered mine is invisible from the surface, a few fathoms down where it will hole a ship below the waterline. These mines are exported by twenty nations (including China) and can be dropped from a simple fishing boat to wait for their target. Some view them as the maritime version of IEDs, an asymmetric threat to the big navies. The most modern warships are vulnerable – during Operation Desert storm the USS Princeton and *USS Tripoli *were damaged by mines within a few hours of each other.

(Mines go way back, too. When Farragut issued his famous order "Damn the torpedoes, full speed ahead!" during the Civil War, the "torpedoes" were tethered mines.)

Tackling mines is a time-consuming business. Minesweepers can cut the tethering cable; the mine floats free - and is easy to spot on the surface and destroy with gunfire. But this is a dangerous game. Otherwise, mines can be destroyed in situ, either by divers or by a Mine Neutralization System (MNS), which uses a remote-controlled robot to place destructive charges. The Navy only has 28 of the systems to cover the world, and the process of destroying a single mine can take hours.

In the latest Popular Mechanics, I take a look at a faster alternative being developed by Northrop Grumman: the Rapid Airborne Mine Clearance System(RAMICS). Mounted on a helicopter, it combines a sensor to see through water with a 30mm cannon, which can take out a mine with a single shot.

The difference is in the ammunition. When a RAMICS round strikes the water, the pressure wave from blunt tip produces a bubble which surrounds the projectile. This reduces friction so, unlike other projectiles, the bullet keeps its velocity underwater.

This "supercavitating" technology has been used before. The Russians were the first to develop supercavitating weapons, notably the VA-111 Shkval. This is an underwater rocket with a maximum speed of two hundred knots – four times the speed of a conventional torpedo German company Diehl announced their own demonstrator, the Barracuda, in 2004 and Iranian work on a supposedly supercavitating torpedo was leaked in 2006.

These superfast torpedoes are a new type of challenge. The initial version of the Shkval was unguided and realtively easy to evade. But later versions (and the Barracuda) can home in on the target. Even defenses still under development, like the Navy's Common Very Light Weight torpedo may have trouble against such a fast opponent.

However, the success of RAMICS in tests suggests another alternative. If you can have a Phalanx gatling gun on the deck of a ship to defend against missiles, why not have something similar to destroy incoming torpedoes? RAMICS shows that it's possible to detect a torpedo underwater using a laser-based sensor, and a 30mm supercavitating round which can destroy a mine can certainly destroy a smaller, think-skinned torpedo. The main question would be accuracy – but RAMICS has proved more accurate than expected in tests, and only needs one shot.

In fact, this is exactly the approach taken by Darpa's Very High Speed Torpedo Defense project. Testing of the sensor, targeting mechanism and projectiles are being carried out this year in a variety of sea states. (Elsewhere, it's been suggested that an underwater gun pod, firing supercavitating rounds may be used rather than one above the water).

So a supercavitating projectile is not only the answer to one of the oldest threats to warships, it may be the answer to one of the newest threats, too.

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