Run Silent, Run Very Fast

Funding pushes development of 127 mph sub

February 18, 2007|By Allison Connolly | Allison Connolly,Sun reporter

Russia has it. And reportedly, so does Iran. Now the United States is taking another look at supercavitation, a technology first honed in Maryland more than 50 years ago that would allow mini-submarines to travel beneath the ocean's surface at more than 100 miles an hour.

The Navy wants to build a vessel that could deliver SEAL commandos underwater to enemy shores undetected at high speed. If successful, the technology could one day be used commercially to transport cargo around the world faster, promising a huge impact on commerce.

The Defense Advanced Research Projects Agency (DARPA), the Defense Department's research and development arm, awarded Northrop Grumman Corp.'s Undersea Systems in Annapolis a $5.4 million grant in November to design a concept vessel called the Underwater Express that can attain speeds of 110 knots, or 127 miles per hour. By comparison, the government says a Virginia-class submarine's cruising speed is 25 knots (29 mph). General Dynamics Corp.'s Electric Boat subsidiary in Groton, Conn., also won a contract to come up with a competing concept.

Likely long, cylindrical and 8 feet in diameter, the sub would have a blunted nose that would cut through the water. At high speed, the nose deflects water away from the surfaces of the vessel. A gas bubble or cavity forms around it, from the nose to the stern, which nearly eliminates drag and allows it to go faster.

Funding pulled

The initial award is small by defense contract standards, but if one of the teams develops a successful prototype, the total contract could be worth as much as $46 million.

It won't be easy, said Marshall Tulin, who DARPA credits with being the father of the technology and whose Howard County company, Hydronautics Inc., tried to bring the technology to market during the 1960s and 1970s.

Back then, Tulin said, the Navy was intent on developing a supercavitating vehicle that would "skim" the water on a cushion of air, but Congress pulled the funding. Hydronautics turned its focus to supercavitating foils or fins and propellers, including one for the U.S. Maritime Administration's 50-knot Denison hydrofoil. Work on it was discontinued.

The company also tested propulsion systems for the Navy and NASA in its 300-foot model basin, then the largest privately owned test basin in the country. Tulin and a partner sold the company in 1982 to Austin, Texas-based Tracor Inc. along with patent rights to the technology, though much of their work was not patented, he said.

Now 80 and a professor emeritus at University of California, Santa Barbara, Tulin said the technology has long been promising, but Northrop Grumman must overcome two big challenges: control and noise. He believes the latter is a big reason why the Navy didn't explore a supercavitating sub before.

Quiet operation

"The Navy was always focused on quiet operation underwater, and really excelled in that area," said Tulin, who said he wrote his first scientific paper on the flow dynamics of supercavitation in 1953. At that time he worked at the Navy's David Taylor Model Basin, now part of the Naval Surface Warfare Center's Carderock Division headquarters in Bethesda.

Cavitation is the physical process by which liquid changes to vapor in the form of bubbles. Unlike with supercavitation, where a bubble grows and envelops the object, cavitating air bubbles grow to a certain point and then collapse violently. For example, when water boils in a pot, it is "cavitating," Tulin said. But cavitation doesn't require heat: An object cutting through water at high speed can create low pressure pockets, allowing bubbles to form.

Normally, ships and submarines want to avoid cavitation, said Kenneth M. Kalumuck, a senior staff member in the Oceanic, Atmospheric & Remote Sensing Sciences Group at the Johns Hopkins Applied Physics Laboratory in Laurel.

In the movie The Hunt for Red October, he said, the crew heard loud clanking when the submarine sped up - the speed of the propellers caused low pressure pockets to form, triggering the physical process in which the liquid molecules turned into air bubbles, which then collapsed against the propeller blades as they whirled through the water, emitting an audible noise.

In such a case, the noise would have not only disclosed the submarine's position to the enemy, he said, but the collapsing bubbles could have caused damage to the blades.

However, if supercavitation were to occur, a bubble would form somewhere on a propeller blade and extend behind it, Kalumuck said, making it more or less stable and allowing the vessel to go faster. Tulin's supercavitating propellers worked in this way.

Russian scientist

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