Scientists at the Naval Undersea Warfare Center in Newport, Rhode Island demonstrated in 1997 a fully submerged launch of a supercavitating projectile (with air injected in its nose) with a muzzle velocity of 5082 feet (1549 meters) per second, making it the first underwater weapon to break the sound barrier. More recently the U.S. unveiled supercavitating bullets. That program was inspired by the menace posed by harbor mines during the Gulf War. The slow and dangerous job of disarming mines often falls to divers because bullets lose momentum and direction after traveling a few feet through water, which is thousands of times denser than air. But supercavitating bullets fired from planes or helicopters could pierce and detonate mines from a safe distance.
The Russians, who during the Soviet era took an early lead in supercavitation in the 1960s thanks to Ukrainian experiments, remain extremely covetous of their advances in the field. Ask Edmond Pope, an American businessman and retired Navy captain imprisoned in Russia last year amid that government’s accusations that he spied on its supercavitation program. Rumors linger that Russia hesitated over seeking foreign aid in the aftermath of the Kursk submarine disaster because it carried experimental supercavitating missiles. But the Russians are selling one of their inventions the Shkval-E (Squall) High-Speed Underwater Rocket. According to their military sales brochure, the missile can top 200 miles (320 kilometers) per hour. But, that model Shkval is a "straight shooter," notes Kam, meaning that it can't maneuver or home in on a target.
That brings up the point of steering even without a bogey on your tail, the ocean is crowded place to go rocketing around compared to the relatively empty atmosphere. While Kam says collisions with fish won't pose the kind of threat to a supercavitating vessel that flocking birds pose to jets today, few admirals would commit their best and brightest officers to a sub that moved only in straight lines. But turning is tricky. "If the bubble is distorted and hence air layer coverage is destroyed, the body becomes 'wetted' -- or there is no air layer to provide the low drag," Kam wrote. The answer is to rapidly adjust the orientation of the cavitator disk or cone at the craft's nozzle and control ventilation to make sure surfaces under pressure are getting coverage. This is where mechanical engineering may need a helping hand from digital magic: using high-tech sensors and controls to maintain that air bubble through sharp turns, acceleration and deceleration, constantly changing pressures, altered body geometry, vibration, contact with foreign matter and a host of other unpredictable variables.
The way it looks now, an experimental ramjet thruster engine called a vortex combustor will probably power a supersonic sub, according to Kam. Propellers would be useless because they wouldn't be touching water. This engine reacts powdered aluminum with water in a contained whirlpool to produce heat that powers the ramjet turbine. "It has 2.5 to 3 times the specific impulse of solid rocket propellants," Kam notes.
Supercavitating missiles could also surprisingly revolutionize the more peaceful art of ocean farming. A supercavitating torpedo with a mooring line fired down from the water's surface could maintain the force needed to slam an anchor deep into the sea floor, whereas such a remote system in deep seas using existing technology would slow and then simply clunk onto the sediment below. A report by Stanley Associates Inc. and Designers and Planners USA proposed using the technology to moor open ocean platforms for aquaculture instead of using divers or extensive underwater operations as with traditional drag embedment anchors.
Erik Baard, Technology Correspondent
(originally posted at space.com)