Faster than a speeding bullet
YOU'RE SITTING on Concorde, zooming comfortably over the Atlantic at Mach 2, sipping your free glass of champagne. Feeling smug? Little do you know that a few kilometres down, under the sea, a slim, grey, pencil-shaped vessel is overtaking you. Yes, it's a supersonic submarine.
Actually, this vehicle is still a fiction, but the technology that could produce it is real enough. At least one Russian submarine is armed with torpedoes that exploit it. The same idea is being used to make guns for destroying underwater mines, and it could double the speed of surface vehicles such as hydrofoils. Most remarkably, it could produce an underwater vessel capable of travelling at thousands of kilometres an hour while remaining almost entirely dry. For the key to this technology lies in that glass of champagne--and in particular, the humble bubble.
Like a lot of other bizarre ideas, the supersonic sub came out of the Cold War. In the 1960s, the Soviet Union had relatively slow, bumbling torpedoes that left its subs at a serious disadvantage. Rather than push conventional torpedo technology a bit further, the Soviets decided to try to leapfrog the Americans with a radical solution. The problem holding back ordinary torpedoes was drag. Any object, no matter how streamlined, suffers resistance as it moves through a fluid. One source of drag is skin friction, the force required to shear the thin layer of fluid lying against the moving body's surface. This happens in air too, but water, being about a thousand times as dense as air, generates a thousand times as much drag. What's more, the power needed to overcome drag is proportional to the cube of an object's speed. So each incremental improvement in propulsion technology produces only a meagre increase in speed.
In the early 1960s, Mikhail Merkulov at the Hydrodynamics Institute in Kiev, realized that the solution lay in a phenomenon called cavitation. It was a daring idea, because naval architects usually see cavitation as a menace, rather than something that works to their advantage. When a body moves rapidly through a fluid, the pressure at various points on the body -- at trailing edges, for example -- is reduced, explains Rudra Pratap, a dynamist who works on supercavitating bodies at the Indian Institute of Science in Bangalore. The faster the body moves, the lower the pressure becomes. "When the pressure reduces enough to equal the vapour pressure of the fluid, the liquid state is no longer sustainable," says Pratap.
With not enough pressure to hold them together, the liquid molecules vaporize and form cavities, or bubbles, causing pitting and erosion. In pumps, turbines and propellers, cavitation leads to two problems. The bubbles distort flow patterns, which reduces efficiency. And eventually the bubbles reach regions of higher pressure and collapse, creating shock waves violent enough to dig pits in exposed metal.
But supercavitation is a different matter. Under certain conditions, a single bubble or supercavity can be formed, enveloping the moving object almost completely. Newton alluded to the basic principles in his Principia Mathematica of 1687, but supercavitation is hard to achieve. For a start, the body has to be going pretty fast -- at least 180 km an hour, or 50 metres per second, according to Pratap. That's far faster than ordinary torpedoes. The shape of the nose also has to be right, says John Castano of the supercavitation project at the US Naval Undersea Warfare Center (NUWC), in Newport, Rhode Island. Rather than being streamlined, a supercavitating object needs a flat nose (see Diagram). Then, at high speeds, the fluid is forced to flow off the edge of the nose with such speed and at such an angle that it can't wrap around the surface of the body. A supercavitating body has extremely low drag, because its skin friction almost disappears. Instead of being encased in water, it is surrounded by the water vapour in the supercavity, which has much lower viscosity and density. So in a supercavitating vehicle, only the nose of the craft causes significant drag, because this is the only part of the body actually in contact with the water.
However, there is a trade-off here, says Pratap. The blunter the nose, the higher its drag. So the best nose is usually slightly curved. The overall drag reduces enormously once you reach the supercavitating regime, according to Pratap and then increases only linearly with speed. "Now if you ask me why, I would not be able to answer. It is complicated, and I am not sure if the fluid mechanics community understands it yet." But regardless of why it happens, the effects are undeniable. With much of the drag eliminated, very high speeds become possible.
The idea that supercavitation could lubricate marine vehicles was proposed by Marshall Tulin, who is now director of the Ocean Engineering Laboratory at the University of California at Santa Barbara. Tulin's idea was to use it to reduce drag on the foils of hydrofoils, enabling them to double their speed. As hydrofoils never made the big time, this high-tech add-on also sank -- though not without trace. When Merkulov saw Tulin's work, he realized that supercavitation could create a super-fast torpedo. There was one problem: With only the nose of the craft actually touching the water, conventional propellers wouldn't work, so an entirely new form of underwater propulsion was called for. The solution was simply to mount a rocket motor on the back. Rockets work in empty space, so having no water to push against isn't a problem. They also provide an immensely powerful kick. But while the ideas are simple, turning them into a working torpedo was hard. In particular, there were problems with stability and finding materials strong enough to stop the nose from buckling under the extreme pressures. And at the speeds achievable, the cavity was not long enough to envelop the whole torpedo. So the Russian torpedo was designed to generate an artificial cavity by feeding part of its exhaust out through the nose.
"If the speed of the object is not high enough to travel through the vapour cavity before it collapses, then artificial ventilation into the cavity can keep it open until the object moves past," says Castano. According to Mark Galeotti, a defence expert at Keele University in Staffordshire, prototypes of the torpedo appeared in the 1980s, but there was still plenty of work to be done. "It was only in the early 1990s that the Russians were in a position to produce a proper working torpedo," says Galeotti. Called Shkval, meaning squall, it is said to be capable of speeds as high as 500 km an hour. It is fired like an arrow from the submarine, possibly with a mechanical catapult. That gets it moving fast enough for the cavity to form, so that its rocket can be lit. The Russians had built themselves a formidable weapon. Shkval leaves enemy torpedoes standing, and can knock out an opposing sub before it has time to react. It could even be used defensively to intercept enemy torpedoes. But Shkval is a slowcoach compared with what was to follow.
By the time it had appeared in the early 1990s, the US had established its own supercavitation programme. To begin with, it concentrated on unpowered projectiles -- underwater bullets. When conventional projectiles are fired into water, they are dragged to a halt before they have penetrated more than a metre or so. Researchers at the NUWC knew that supercavitating munitions ought to be able to go a lot further and at very high speed too. In 1997, they proved it.
Just a few years after Shkval's debut, NUWC researchers announced they had gone supersonic. An unpowered projectile, with a carefully designed flat nose and fired from an underwater gun, broke the sound barrier in water. That's nearly 5,400 km per hour -- or 1.5 km per second. Lacking any onboard power to sustain its motion, the shell slowed rapidly, but this was still a vivid demonstration of the speeds that supercavitation makes possible. Already they aren't very far off the 2.5 km-per-second speed record for conventional munitions in air and NUWC scientists have calculated that their supercavitating projectiles should be able to match or even surpass this. Even without reaching such dizzying speeds, supercavitating bullets are being put to good use. The US Navy would like to be able to clear mines at sea by simply shooting at them from the air, but conventional shells don't penetrate deep enough to reach most mines. So a group at the Naval Air Warfare Center Weapons Division, in China Lake, California, is blowing bubbles at them.
In the Rapid Airborne Mine Clearance System (RAMICS), projectiles are shot from a standard 20-millimetre Gatling gun. With their blunted cone-shaped noses, the laser-targeted bullets will be fired from more than 350 metres above the water, travel 12 metres through it and still be able to zap a mine. "We have to penetrate a steel wall and still have enough residual kinetic energy to ignite the explosive," says Doug Todoroff, project sponsor of RAMICS at the Office of Naval Research (ONR). The system has so far only been test fired on the ground, but airborne testing has also been done. Todoroff sees the project as a cost-effective way of neutralizing a dangerously cheap weapon.
So how about that slippy submarine going faster than Concorde? Firing a bullet into water is one thing, driving a rocket-powered underwater vessel at high speed and keeping it wrapped in a dry bubble is a lot harder. What is the practical speed limit? Military scientists are reluctant to discuss the speeds they have reached in propelled vehicles, but there is no fundamental reason why they can't go as fast as bullets. "The Russians certainly see Shkval as the start, not the finish," says Galeotti. "There is no reason why this technology cannot be applied to a manned vehicle," adds Castano.
But there are plenty of technological hurdles. One thing still needed is a powerful and compact propulsion system. An aluminum-burning rocket is one answer. It would use water as its oxidizer and so wouldn't need to carry oxygen. The problem with aluminum has been that unreacted fuel quickly becomes coated with aluminum oxide, inhibiting any further reaction. To avoid this, powdered aluminum can be injected into a vortex of water, which keeps the burning, molten drops apart. Aluminum-burning rockets may be fine for short ranges, but what about long-distance supersonic underwater travel? For this, probably only a nuclear reactor is a compact enough power source. Travelling at 2.5 km per second, such a craft could make the journey from London to New York in well under an hour, easily outpacing Concorde. Assuming it didn't hit a whale on the way, of course.
There's also the problem of steering. "Shkval is a straight shooter," says Kam Ng, of the ONR. "There are no control capabilities whatsoever." So much effort had gone into stabilizing the projectile that it can only travel in a straight line. Once launched, it is beyond anyone's control. "A high-speed vehicle with control capability is the challenge," says Ng. His team is trying to gain some control by mounting fins along the vessel's body. It was considered undesirable to have any part of the craft besides the nose touching water, because that increases the drag. But this, says Ng, is a small price to pay for the ability to control the vessel. There is also a risk of destabilizing the cavity, but Ng and his team are working on solving this -- though he won't reveal how.
Supercavitation could completely change the nature of undersea warfare, says Galeotti, turning the traditional cat-and-mouse game, with vessels sneaking around as quietly as possible, into a cacophonous dogfight. "If we do get supercavitating vehicles, we're not going to be talking about big submarines," he says. Small supercavitating craft might instead be sent out by a mother ship on short-range attacks, wheeling about to get a good line of sight for their underwater machine guns. "It is like a shift into airborne warfare," says Galeotti.
There's yet one more problem facing crewed, supercavitating craft, which no one appears to have an answer to. How will such a vehicle reach supercavitating speeds in the first place? After all, who wants to sit in a craft as it is fired from a gun, or risk getting stranded in the middle of the ocean if it slows down too much? Until this is sorted out, it seems unlikely that there will be many volunteers to fly the Spitfires of the seas.
© New Scientist