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.
There is general agreement that Convair's hull design was sound. Landing on a flying-boat-style fuselage and doing away with cumbersome floats should make the craft lighter and faster both in the air and under water. But once the craft is on the water, how best to get it to dive?
Simply flooding the fuselage with water is one solution, but this means the crew would have to be kitted out with scuba equipment. Housing the crew in a watertight cabin is obviously preferable, and to counter its buoyancy Polmar suggests borrowing another idea from Convair's design - floodable fuel tanks. If the fuel in the tanks is held in a rubber bladder, the craft can be submerged by letting water into the void vacated by fuel used on the outward trip. When it's time to surface, the water can simply be pumped out.
For propulsion under water, electric power is the preferred option, according to Ian Poll, an aerospace engineer at Cranfield University in the UK. "Using batteries to drive electric motors when submerged could have another benefit," he says: their weight would help counter the craft's buoyancy.
Unfortunately batteries could severely undermine the sub's airworthiness. In a report titled "Conceptual Design of a Submersible Tactical Insertion Aircraft", published last year by the American Institute of Aeronautics and Astronautics, a team of engineering students at Auburn University in Alabama calculated that the batteries required for a sub capable of travelling 44 kilometres under water - a distance specified by DARPA - will weigh as much as all the other components of the vessel combined, making it too heavy to fly.
So rather than using electric power, the Auburn team favours propelling the vessel with a gas turbine fed by air drawn in through a 10-metre snorkel. That means the sub will have to stay close to the surface. While DARPA has yet to specify at what depth the flying sub should operate, being restricted to a limited depth might not matter. "As long as it is not visible, there's not much reason to dive far below the surface," says Bob Allwood, engineer and chief executive of the Society for Underwater Technology in London. "The problem is that the craft has still got to be slightly denser than water to submerge."
Hawkes, however, does not see this as a problem. In fact he doesn't accept that the craft has to be made heavier to sink beneath the waves, any more than a normal aircraft has to become more buoyant to take off. "You can't build an aeroplane that is also a balloon, and an aeroplane can't go under water in the same way a sub does. You're mixing two fundamentally different modes of operation."
Hawkes already builds submarines that are lighter than water (New Scientist, 12 February 2000, p 36). To overcome their natural buoyancy and keep them below the surface, they are equipped with wings that generate downward "lift". "Think about it as flying under water," he says. "It can be done. It just needs a lot of work."
To operate below the waves as well as above them, these wings will have to be a bit out of the ordinary. "One important thing is that the craft's wings will need a symmetrical aerofoil, unlike the asymmetrically curved wing that gives aircraft lift," he says. So when the craft is airborne, the wing will need a positive "angle of attack": in other words, it will need to be angled upwards relative to the airflow. To achieve this, the craft will have to fly in a nose-up attitude. Conversely, when under water it will need a negative angle of attack, so the craft will travel nose-down (see diagram).
Hawkes has already built a stubby-winged submersible called the Super Falcon that can "fly" down to 300 metres, about 10 times deeper than a scuba diver. Redesigned with aero engines and larger wings, it could be made to fly at about 900 kilometres per hour with its nose angled up by about 5 degrees, Hawkes says. Under water it should manage around 10 knots (18 kilometres per hour). At these speeds, the characteristics of the air and water flow - defined by a parameter known as the Reynolds number - are roughly the same, so the craft's control surfaces should work in both environments.
Hawkes admits that an awful lot of power will be needed to get the Super Falcon airborne, and only jet engines have enough oomph to do the job. Polmar agrees, and points out that the piston engines used in conventional light planes are ruled out for other reasons: they would fail if any water leaked into the cylinders. "You cannot immerse a reciprocating engine and expect it to work," he says. But protect a jet engine against saltwater corrosion and position it high on the craft so the spray doesn't enter the intake during take-off and landing, and it will work fine. Russian aircraft maker Beriev has proved this with its Be-200 amphibious plane.
In fact, Hawkes foresees jet engines playing a dual role, propelling the plane through the water as well as through the air. There's no reason why the compressor and turbine blades in a jet engine can't be driven by an electric motor to generate thrust under water, he says. It should be possible to build an engine that runs on kerosene in air and switches to electricity when submerged.
Others are already thinking along these lines. Last year, aircraft manufacturer Airbus patented a hybrid electric jet engine for airliners which can be powered by both conventional kerosene and electricity. Most jet engines have an electric starter motor, and this motor could spin the turbine's shaft under water, Hawkes suggests. The blades would rotate more slowly than normal, he says, and the engine won't be particularly efficient. "But I believe this could work perfectly well."
The Auburn students came up with much the same strategy in their design, opting for a type of gas turbine called a turboshaft to get the best performance. Equipped with large rotor blades and gears to adjust its speed, a turboshaft unit offers "acceptable efficiency" in both air and water, they say. Alternatively the air could be fed to a fuel cell to generate electricity to spin the blades.
But there is one stumbling block to Hawkes's scheme for using a conventional jet engine for propulsion in air and water. "You can't let cold seawater get at a hot engine because the thermal shock will blow it apart," warns engineer Jim McKenna of the UK Civil Aviation Authority, who has previously researched submersible systems. "It takes a long, long time to cool down a jet engine: the turbine runs at somewhere between 500 and 600 °C," he says. In other words, without some innovative thinking, a jet-powered sub might have to wait hours on the surface before its engines are cool enough for it to dive.
Should Hawkes's buoyant design win out, getting it to sink low enough in the water for its wings to start creating downward forces could also be a problem. Hawkes has a dramatic solution: copy what diving birds do. "You might have to put the nose down and literally dive, smack, into the water," he says. Taking inspiration from birds would put submersible-aircraft engineers in illustrious company: 19th-century glider pioneer Otto Lilienthal and the early 20th-century inventors of powered flight, the Wright brothers, are among those who did so - though it's no guarantee of success in this case. Whatever happens, says Hawkes, "it would certainly be spectacular".