For ships traveling faster than 60 miles per hour, propeller-induced cavitation is unavoidable. Supercavitation offers a solution.
In supercavitation, the small gas bubbles produced by cavitation expand and combine to for mone large, stable, and predictable bubble around the supercavitating object. The bubble is longer than the object, so only the leading edge of the object actually contacts liquid water. The rest of the object is surrounded by low-pressure water vapor, significantly lowering the drag on the super-cavitating object. Modern propellers intentionally induce supercavitation to reap the benefits of lower drag.
A super cavity can also form around a specially designed projectile. The key is creating a zone of low pressure around the entire object by carefully shaping the nose and firing the projectile at a sufficiently high velocity. At high velocity , water flows off the edge of the nose with a speed and angle that prevent it from wrapping around the surface of the projectile, producing a low-pressure bubble around the object. With an appropriate nose shape and a speed over 110 miles per hour, the entire projectile may reside in a vapor cavity.
Since drag is proportional to the density of the surrounding fluid, the drag on a super-cavitating projectile is dramatically reduced, allowing supercavitating projectiles to attain higher speeds than conventional projectiles. In water , a rough approximation predicts that a supercavitating projectile has 200,000 times less skin friction than a normal projectile. The potential applications are impressive.
Water has 1000 times more drag than air. Supercaviation has the potential for an enclosed object in water to attain higher speed. The speed of sound is 5 times higher in water than in air.
Propulsion of Supercativation submarines
“Getting up to supercavitation speeds requires a lot of power,” says researcher Savchenko. “For maximum range with rockets, you need to burn high-energy-density fuels that provide the maximum specific impulse.” He estimates that a typical solid-rocket motor can achieve a maximum range of several tens of kilometers and a top speed of perhaps 200 meters per second. After considering propulsion systems based on diesel engines, electric motors, atomic power plants, high-speed diesels, and gas turbines, Savchenko concluded that “only high-efficiency gas turbines and jet propulsion systems burning metal fuels (aluminum, magnesium or lithium) and using outboard water as both the fuel oxidizer and coolant of the combustion products have real potential for propelling supercavitating vehicles to high velocities.”
Aluminum, which is relatively cheap, is the most energetic of these metal fuels, producing a reaction temperature of up to 10,600 degrees Celsius. “One can accelerate the reaction by fluidizing [melting] the metal and using water vapor,” Savchenko explains. In one candidate power-plant design, the heat from the combustion chamber would be used to melt stored aluminum sheets at about 675 degrees C and to vaporize seawater as well. The resulting combustion products turn turbine-driven propeller screws.
This type of system has already been developed in Russia, according to media reports there. The U.S. also has experience with these kinds of systems. Researchers at Penn State’s Applied Research Laboratory are operating an aluminum-burning “water ramjet” system, which was developed as an auxiliary power source for a naval surface ship. In the novel American design, powdered aluminum feeds into a whirlpool of seawater occurring in what is called a vortex combustor. The rapid rotation scrapes the particles together, grinding off the inert aluminum oxide film that covers them, which initiates an intense exothermic reaction as the aluminum oxidizes. High-pressure steam from this combustion process expands out a rocket nozzle or drives a turbine that turns a propeller screw.
Tests have shown that prop screws offer the potential to boost thrust by 20 percent compared with that of rockets, although in theory it may be possible for screws to double available thrust, Savchenko says. Designs for a turbo-rotor propeller system with a single supercavitating “hull propeller,” or a pair of counterrotating hull props that encircle the outer surface of the craft so they can reach the gas/water boundary, have been tested. He emphasizes, however, that “considerable work remains to be done on how the propeller and cavity must interact” before real progress can be made.