Tech

GUILLEMOTS and gannets do it. Cormorants and kingfishers do it. Even the tiny insect-eating dipper does it. And if a plan by the Pentagon's Defense Advanced Research Projects Agency (DARPA) succeeds, a remarkable airplane may one day do it too: plunge beneath the waves to stalk its prey, before re-emerging to fly home.

The DARPA plan, announced in October 2008, calls for a stealthy aircraft that can fly low over the sea until it nears its target, which could be an enemy ship, or a coastal site such as a port. It will then alight on the water and transform itself into a submarine that will cruise under water to within striking distance, all without alerting defences.

That, at least, is the plan. The agency is known for taking on brain-twistingly difficult challenges. So what about DARPA's dipper? Is it a ridiculous dream? "A few years ago I would have said that this is a silly idea," says Graham Hawkes, an engineer and submarine designer based in San Francisco. "But I don't think so any more."

DARPA, which has a $3 billion annual budget, has begun to study proposed designs. In the next year or so it could begin allocating funding to developers. Though the agency itself is unwilling to comment, Hawkes and others working on rival designs have revealed to New Scientist how they would solve the key problems involved in building a plane that can travel under water - or, to put it another way, a flying submarine.

The challenges are huge, not least because planes and submarines are normally poles apart. Aircraft must be as light as possible to minimise the engine power they need to get airborne. Submarines are heavyweights with massive hulls strong enough to resist crushing forces from the surrounding water. Aircraft use lift from their wings to stay aloft, while submarines operate like underwater balloons, adjusting their buoyancy to sink or rise. So how can engineers balance the conflicting demands? Could a craft be designed to dive into the sea like a gannet? And how will it be propelled - is a jet engine the best solution, both above and below the waves?

According to Norman Polmar, former adviser on naval strategy and technology to the US government, the starting point must be to find a way to make an aircraft that can sink in water. "Submarines cannot fly," he says, "but seaplanes can submerge." This was the thinking behind what was probably the first stab at a flying submarine. In 1934 Boris Petrovich Ushakov, a student engineer at a Soviet military academy, devised a flying underwater boat - a three-engined floatplane designed to scout out enemy ships and then ambush them. Ushakov envisaged his craft flying ahead of the target, landing on the sea and then flooding its fuselage so that it could lie in wait beneath the surface and torpedo the ships as they sailed past. Ushakov submitted his radical design, which included a conning tower and periscope, to senior officers in 1936. But the concept was never put into practice, being deemed too heavy to be effective.

It took another three decades before a flying sub appeared for real. This was a craft built in 1962 by Donald Reid, an engineer at aircraft manufacturer North American Aviation. The Reid Flying Submarine (RFS-1) was a true mongrel, constructed by Reid in his spare time using leftover parts from other aircraft and, like Ushakov's design, it was a floatplane. The craft proved able to dive to a depth of a few metres in tests, but was so heavy it could only make short hops into the air. Though this was at the height of the cold war, the US navy showed little interest in Reid's machine.

That may have been because the navy had already commissioned another aircraft manufacturer, Convair, to build what became known as the "subplane". It dispensed with heavy floats, relying instead on its streamlined fuselage, like the hull of a flying boat, to land on the water. In a paper in the September 1964 issue of Naval Institute Proceedings (p 144), hydrodynamics engineer Eugene Handler at the US Bureau of Naval Weapons claimed this flying sub would be ideal for attacking Soviet shipping in the Baltic, Black and Caspian seas. Convair drew up detailed designs and even built scale models which were tested in water tanks. Though the results looked promising, the project never made it any further; it was cancelled by Congress in 1966.

So is DARPA's new project destined for a similar fate? "What the Americans want sounds incredibly ambitious," says UK Royal Navy commander Jonty Powis, head of NATO's submarine rescue service. "If they achieve half of what they want from this machine they will be doing well." Others are more optimistic, especially in the light of advances in engineering and materials science since the last attempt - notably in lightweight carbon fibre composites and energy-dense batteries. "There's probably no reason why it can't be done," says Hawkes.

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