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WHY THIS MATTERS IN BRIEF
The world’s military powers are pushing the boundaries of speed – underwater.
Water is a thousand times more dense than air and as any of you who swim will know this difference in density creates a huge amount of drag. But for submarines the problem is amplified. Now though researchers from the US and China, operating particularly in the military sector, think they have a simple solution – a bubble. More specifically millions of small gas bubbles produced by cavitation that expand and combine to form one large, stable and predictable bubble around the supercavitating object.
The idea though is a new variation on an old idea that the Russians first developed during the cold war in an attempt to create hypersonic torpedoes and it helps eliminate the effects of drag on an underwater vehicle by wrapping it inside an air bubble. Using the technique a Russian supercavitation torpedo called the VA111 Shakval was able to reach a speed of 250mph or more – much faster than any other conventional torpedoes and then in 2005 Diehl Defence demonstrated an even faster missile, the Barracuda Mk 50 which reached speeds of over 450mph.
In theory at least, a vessel using supercavitation could reach speeds of about 3,600mph.
A 2012 video demonstrating supercavitation in action
Putting this into perspective, according to the California Institute of Technology, a torpedo, or submarine equipped with this technology could cross the Atlantic in under an hour and the Pacific in under one and a half hoursthis would reduce the travel time for a transatlantic underwater journey to less than an hour, and for a transpacific journey to about 100 minutes almost matching some of the new supersonic and hypsersonic aircraft systems.
However, and as you might expect, it’s not that easy to wrap a submarine in a bubble and there are numerous other challenges as well.
“Basically supercavitation can be used to significantly reduce drag and increase the speed of bodies in water,” said Grant Skidmore, a Penn State Aerospace and Engineering Ph.D., “but sometimes these bodies of air can get locked into a pulsating mode.”
To create the bubble around a vehicle, air is introduced in the front and expands back to encase the entire object but sometimes the bubble will contract allowing part of the vehicle to get wet – this is called Pulsation. Now you might not think that a submarine getting wet would be that much of an issue, after all they cost billions to build and it’s not as though they’re that dainty, but at those speeds any part of the submarine that’s exposed to the water will get ripped off. Whether that’s a conning tower or a bulkhead.
“Shrinking and expanding is not good,” said Timothy Brungart, senior research associate at ARL and an associate professor of acoustics, “we looked at the problem on paper first and then experimentally.”
The researchers first explored the pulsation problem analytically and the results suggested a solution but verifying them with an experiment was not simple. The ideal outcome for supercavitation is that the gas bubble forms, encompasses the entire vehicle and exits behind, dissipating the bubble without pulsation. The researchers reported the results of their analytic analysis and experimentation online in the International Journal of Multiphase Flow.
Next the ARL researchers decided to use the Garfield Thomas Water Tunnel facility’s 12 inch diameter water tunnel to test their numerical calculations.
“The water tunnel was the easiest way to observe the experiment,” said Brungart.
“But it wasn’t the easiest place to create the pulsation.”
Creating a supercavitation bubble and getting it to pulsate in order to stop the pulsations inside a rigid walled water tunnel tube had not been done before.
“Eventually we ramped up the gas really high and then way down to get pulsation,” said Jules W. Lindau, senior research associate at ARL and associate professor of aerospace engineering.
They found that once they had supercavitation with pulsation, they could moderate the air flow and, in some cases, stop it.
“Supercavitation technology might eventually allow high speed underwater supercavitation transportation,” said Moeney.
Meanwhile, 2 hours across the water in Shanghai Chinese Li Feng Chen, a professor at the Harbin Institute of Technology, earlier this year announced a breakthrough in its laboratory. By constantly showering a special liquid membrane onto the submarine’s surface it becomes easier to maintain the bubble, allowing the vehicle to achieve supercavitation more easily and – furthermore, make it controllable.
Now, you might be forgiven for thinking that when Li Feng Chen means controllable you’re thinking perhaps that the submarine is more stable. But this then brings us to the next problem. If you’re travelling underwater steering might be advantageous but, again, if the rudder meets the water – which is needs to after all in order to do it’s duty, then it’ll get ripped off and this is one of the problems that it looks like the Chinese researchers are getting closer to solving.
“By combining liquid membrane technology with supercavitation, we can significantly reduce the launch challenges and make cruising control easier,” Prof Li told the South China Morning Post.
Although this membrane would eventually be worn off by water it could significantly reduce the water drag on the vessel at low speed and after its speed had reached 75km/h or more the vessel would enter the supercavitation state, Li said.
However, Li admitted problems still needed to be solved before supersonic submarine travel became feasible such as the creation of a powerful underwater rocket engine.
Bringing the subject back, or is it up, to earth Li also believes that the new technique could even be used to aid swimmers.
“If a swimsuit can create and hold many tiny bubbles in water, it can significantly reduce the water drag. Swimming in water could be as effortless as flying in the sky,’ he said.
Other countries are thought to be actively researching the technology as well though but details are hard to come by because it is highly classified. However, one vehicle already using supercavitation above the water is Juliet Marine System’s Ghost, a high speed, lightweight stealth craft designed for the infiltration and surveillance of enemy waters. The Ghost isn’t a fully supercavitating vessel though because just its propellers are supercavitating, meaning it can travel at high speeds when it is raised out of the water on two blade-like pontoons.
So far, it has been tested at speeds up of 34mph but Juliet Marine Systems promises future versions of the vehicle will be capable of up to 57mph and for a warship, that is incredibly fast.
The prototype vehicle is currently on show at arms fairs around the world, with several governments understood to have expressed interest.
“It’s a revolutionary programme,” said Gregory Sancoff, chief executive of Juliet Marine Systems.
“Nothing like this has ever been built by anybody, not even the navy.”
Flying, swimming – it all seems to be the same these days but if you’re swimming at those speeds you better make sure that your trunks are tied on tight…