Can A Stealthy Flying Submarine Escape the Madness?
Seaplanes and flying boats have been around for some time. But a flying boat that can also run submerged, is a bird of another feather. Multiple teams are now attempting to design such a bird ... er, fish ... er, loon. Loons, for example, are excellent swimmers, and can fly long distances at up to 70 miles per hour, and dive to 150 feet, staying submerged for several minutes (up to 15 minutes).
This is a DARPA project, in progress. Designers have been forced to abandon the deep dive capability of the flying sub, due to the excessive weight of the batteries necessary to maintain undersea life support and ship's functions. This may only be a temporary setback, however.
...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...
... 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."_NewScientist