Why DARPA thinks a submersible aircraft could be feasible has become a little clearer with the posting of a presentation on the agency's website. Basically DARPA believes that, rather than asking for a flying submarine, by specifying an aircraft that can submerge to shallow depths for short periods it might be possible to reconcile the diametrically opposed design requirements. So it won't look like this...

DARPA is seeking concepts for an aircraft that can clandestinely insert and extract an eight-person special-forces team. The mission is to take off from a runway, fly 1,000nm as a conventional aircraft, fly another 100nm close to the surface, then travel the final 12nm to the coast underwater. Transit should take less than 8 hours, including any time required to land on water and reconfigure from aircraft to submarine - and the vehicle should be able to loiter near the coastline - on or under the surface - for three days in sea state 5. Then return the same way...

DARPA identifies five technical challenges. First is the diametrically opposed weight requirements between an aircraft that needs to be light to fly and a submarine than needs to be heavy to submerge. DARPA believes it should be possible to meet these requirements by controlling lift and volume - potentially by generating downforce on the wing and flooding spaces in the airframe to submerge.

The second challenge is to design a shape that works in air and water despite the difference in density. Here the key is that flows are similar in both fluids if something called the Reynolds Number (Re) is the same. DARPA believes it is possible to design a platform that operates at the same Re, airborne and submerged, but at radically different speeds: 100-400kt in air and 5-18kt in water.

Challenge three is structural - aircraft are pressure vessels with thin skins; submarines have thick skins to withstand crushing loads: the forces are applied in opposite directions. But DARPA believes this problem is dramatically reduced by limiting operating depth to the minimum required to avoid perturbing the surface.

Fourth is wing location. Traditionally seaplane wings are placed high to avoid the waves and shield the engines. Lowering the wing increases ground effect while sea-skimming. But the wing needs to be below the water to generate the downforce required to submerge. DARPA sees a range of possibilities ranging from two separate wings, one for air and one for water, to a single morphing wing able to change its height, area and airfoil.

The final (?) challenge is the powerplant, which must work submerged as well as airborne. Aircraft engines are light; submarine powerplants are heavy. One option is a single engine with air-independent and air-breathing modes. Another is dual powerplants, one breathing air and one running off batteries, fuel cells or DARPA's Aluminum Combustor. The agency believes the shallow depth required will allow use of a snorkel to provide air to the engine when submerged.

Bidders for the Submersible Aircraft program face related challenges: they must provide not only a detailed description of their proposed concept, but a plan for computational or experimental work that will demonstrate the feasibility of their design in the areas of weight and volume, operation in air and water, structural design, wing geometry and power generation/energy storage. Sounds "DARPA-hard" to me.