First Flight: Early 2011Boeing has just unveiled the hydrogen-powered Phantom Eye unmanned airborne system, and aircraft that should be able to stay up at 65,000 feet for up to four days without refueling. Of course Boeing's first customer will no doubt be the U.S. military, but the company also mentions potential uses of the Phantom Eye for "civil and commercial customers", and that's where things get interesting. Such a drone could send a 450-pound payload to high altitude more rapidly and more cheaply than a satellite and with more range and precision than a balloon, allowing scientists more flexibility to run certain experiments, helping us better understand our planet and its ecosystems.
Boeing writes: "Phantom Eye is powered by two 2.3-liter, four-cylinder engines that provide 150 horsepower each. It has a 150-foot wingspan, will cruise at approximately 150 knots and can carry up to a 450-pound payload." The internal combustion engines (ICEs) are provided by Ford and are fed liquid-hydrogen, giving the Phantom Eye "great fuel economy," according to Boeing.
Why Hydrogen? Are they Really Using ICEs?
Commenter Henrik on GCC provided insight into why hydrogen was chosen as a fuel, as well as some speculation about the nature of Phantom Eye's engines:
They are using liquid hydrogen because this is the best fuel available with regard to UAVs. The gravimetric energy density of liquid hydrogen is 143 MJ/kg versus only 42.8MJ/kg for kerosene jet fuel. Liquid hydrogen is not used for manned aviation because of its low volumetric energy density which is 10.1MJ/L versus 33MJ/L for jet fuel. The liquid hydrogen fuel tank needs to be so large that it is getting really difficult to add a passenger cabin for humans.
Also I don't believe for one minute that they use a combustion engine even though they write so. This is a military project so they are almost obliged to lie about it. My bet is that they use a PEM fuel cell in combination with cryogenic power electronics and electric motors. The PEM fuel cell will be 60% efficient and the electric system will be 100% efficient because it can use superconductive components cooled by the liquid hydrogen. This is much more efficient than any combustion engine can ever be. (source)