Submarines are currently the slowcoaches of the travel world, with record speeds of 83kph. However, the Harbin Institute of Technology has told the South China Morning Post (SCMP) they want to raise that by two orders of magnitude, producing something that would outrace jet airplanes.
Is this some sort of April Fools Joke? Well there doesn't seem to be anything published in the scientific literature, so whether speeds of almost 6000kph—as required for the claims of “Shanghai to San Francisco in 100 minutes”—are realistic will have to wait. Nevertheless, the idea Harbin Institute is working on certainly has the potential to improve current speeds substantially.
The central idea is called supercavitation, which the Russians are using to fire torpedoes at speeds of hundreds of kilometers an hour, and it is also being explored for surface vessels.
Supercavitation occurs when the small bubbles caused by the motion of a solid object through water join together to form one bubble that is longer than the object itself. This leaves most of the solid surrounded by gas rather than liquid, and low pressure gas at that. Drag drops dramatically, so it becomes possible to go much faster. We already use supercavitation to enable our propeller blades to spin faster and mantis shrimps have been using it for millions of years to kill their prey.
The main challenge is to reach speeds at which supercavitation can occur, and then keep it going. That's not an issue for the mantis shrimp – the shockwave when their bubbles collapse can kill their prey even when they miss the target – but it is a big problem for anyone seeking a fast form of underwater transport.
By pushing gas out the front of a submarine it is theoretically possible to sustain supercavitatio, but past efforts have not found this viable as a method of transport, as opposed to a weapon of war.
The Harbin researchers claim they have developed a liquid membrane which, when showered over the front of the vessel, would slash drag and allow the submarine to reach the necessary speeds in the first place.
"Our method is different from any other approach, such as vector propulsion," Professor Li Fengchen told the SCMP. "By combining liquid-membrane technology with supercavitation, we can significantly reduce the launch challenges and make cruising control easier." It's not clear how Li plans to keep supercavitation going, and he admits there are still many technical problems to solve to achieve the speeds claimed, which are bounded by the speed of sound in water, which is almost five times that in air.
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