China has moved a step closer to creating a supersonic submarine that could travel from Shanghai to San Francisco in less than two hours.
New technology developed by a team of scientists at Harbin Institute of Technology's Complex Flow and Heat Transfer Lab has made it easier for a submarine, or torpedo, to travel at extremely high speeds underwater.
Li Fengchen, professor of fluid machinery and engineering, said the team's innovative approach meant they could now create the complicated air "bubble" required for rapid underwater travel. "We are very excited by its potential," he said.
Water produces more friction, or drag, on an object than air, which means conventional submarines cannot travel as fast as an aircraft.
However, during the cold war, the Soviet military developed a technology called supercavitation, which involves enveloping a submerged vessel inside an air bubble to avoid problems caused by water drag.
A Soviet super-cavitation torpedo called Shakval was able to reach a speed of 370km/h or more - much faster than any other conventional torpedoes.
In theory, a super-cavitating vessel could reach the speed of sound underwater, or about 5,800km/h, which would reduce the journey time for a transatlantic underwater cruise to less than an hour, and for a transpacific journey to about 100 minutes, according to a report by California Institute of Technology in 2001.
However, super-cavitation technology has faced two major problems. First, the submerged vessel has needed to be launched at high speeds, approaching 100km/h, to generate and maintain the air bubble.
Second, it is extremely difficult - if not impossible - to steer the vessel using conventional mechanisms, such as a rudder, which are inside the bubble without any direct contact with water.
As a result, its application has been limited to unmanned vessels, such as torpedoes, but nearly all of these torpedoes were fired in a straight line because they had limited ability to turn.
Li said the team of Chinese scientists had found an innovative means of addressing both problems.
Once in the water, the team's super-cavitation vessel would constantly "shower" a special liquid membrane on its own surface. Although this membrane would be worn off by water, in the meantime it could significantly reduce the water drag on the vessel at low speed.
After its speed had reached 75km/h or more the vessel would enter the super-cavitation state. The man-made liquid membrane on the vessel surface could help with steering because, with precise control, different levels of friction could be created on different parts of the vessel.
"Our method is different from any other approach, such as vector propulsion," or thrust created by an engine, Li said. "By combining liquid-membrane technology with super-cavitation, we can significantly reduce the launch challenges and make cruising control easier."
However, Li said many problems still needed to be solved before supersonic submarine travel became feasible. Besides the control issue, a powerful underwater rocket engine still had to be developed to give the vessel a longer range. The effective range of the Russian super-cavitation torpedoes, for example, was only between 11 km and 15 km.
Li said the super-cavitation technology was not limited only to military use. In future, it could benefit civilian underwater transport, or water sports such as swimming.
"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.
Besides Russia, countries such as Germany, Iran and the United States have been developing vessels or weapons using super-cavitation technology.
