U.S. Navy developing 'supersonic submarines'

Theorists say such a submarine could cross the Pacific Ocean in 2 hours.

Mark Prigg, London Daily Mail
28 June 2016
As swimmers know, moving cleanly through the water can be a problem due to the huge amounts of drag created – and for submarines, this is even more of a problem.
However, U.S. Navy funded researchers say they have a simple solution – a bubble.
Researchers at Penn State Applied Research Laboratory are developing a new system using a technique called supercavitation.
The new idea is based on Soviet technology developed during the cold war.
Called supercavitation, it envelopes a submerged vessel inside an air bubble to avoid problems caused by water drag.
A Soviet supercavitation torpedo called Shakval was able to reach a speed of 370km/h or more – much faster than any other conventional torpedoes.
In theory, a supercavitating vessel could reach the speed of sound underwater, or about 5,800km/h.
This 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, the technique also results in a bumpy ride – something the new team has solved.
'Basically supercavitation is used to significantly reduce drag and increase the speed of bodies in water,' said Grant M. Skidmore, recent Penn State Ph.D. recipient in aerospace engineering.
'However, sometimes these bodies 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.
However, sometimes the bubble will contract, allowing part of the vehicle to get wet.
The periodic expansion and contraction of the bubble is known as pulsation and might cause instability.
'Shrinking and expanding is not good,' said Timothy A. Brungart, senior research associate at ARL and associate professor of acoustics.
'We looked at the problem on paper first and then experimentally.'
The researchers first explored the problem analytically, which suggested a solution, but then verifying 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 report the results of their analytic analysis and experimentation online in the International journal of Multiphase Flow.
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 not 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.
'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 pulsation.
'Supercavitation technology might eventually allow high speed underwater supercavitation transportation,' said Moeney.
China is also developing a 'supersonic' submarine that could travel from Shanghai to San Francisco in less than two hours.
Researchers say their new craft uses a radical new technique to create a 'bubble' to surround itself, cutting down drag dramatically.
In theory, the researchers say, a supercavitating vessel could reach the speed of sound underwater, or about 5,800km/h.
The technology was developed by a team of scientists at Harbin Institute of Technology's Complex Flow and Heat Transfer Lab.
Li Fengchen, professor of fluid machinery and engineering, told the South China Morning Post he was 'very excited by its potential'.
The new sub is based on Soviet technology developed during the cold war.
Called supercavitation, it envelopes a submerged vessel inside an air bubble to avoid problems caused by water drag.
A Soviet supercavitation torpedo called Shakval was able to reach a speed of 370km/h or more – much faster than any other conventional torpedoes.
In theory, a supercavitating 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.
The Chinese system constantly 'showers' 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 supercavitation state, Li said.
However, Li admitted problems still needed to be solved before supersonic submarine travel became feasible.
A powerful underwater rocket engine still needs to
be developed.
The technique could even be used to aid swimmers, he believes.
'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.