Boeing has tested a hydrogen-fuelled propulsion system for a high-altitude, long-endurance (HALE) unmanned air vehicle that would stay aloft for 10 days. Although fuel-cell propulsion has also been investigated, conventional piston engines modified to burn liquid hydrogen are the favoured powerplants, says George Muellner, president, advanced systems, Boeing Integrated Defense Systems.

The HALE UAV, which uses the wing design from the 61m (200ft)-span Condor flown by Boeing in the late 1980s, would fly at 65,000ft. Analysis of the number of aircraft needed to maintain a continuous orbit shows that 10 days is the optimum endurance, says Muellner. "Fuel cell is the most efficient energy conversion, but does not enable a large enough aircraft," he says. In addition to modifying existing piston engines, liquid hydrogen fuel requires the development of lightweight composite tanks, he says, adding: "It is our intent to proceed when we get the propulsion system nailed down."

In 1989 the 80h-endurance Condor, powered by two 175hp (130kW) Continental piston engines, reached almost 67,000ft and flew autonomously from take-off and landing. Being pursued as a private venture, the new HALE UAV is a candidate for a US Department of Defense advanced concept technology demonstration, says Muellner.

Aerovironment made the first liquid hydrogen-fuelled UAV flights in May 2005 when it flew the fuel-cell-powered Global Observer. The 15m-span UAV is designed to fly at up to 65,000ft for over a week.