From sea to sky: Submarines that fly

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?

via From sea to sky: Submarines that fly - tech - 05 July 2010 - New Scientist.