Boeing and Aurora Flight Systems have tested a Ford-developed hydrogen engine planned to power future high-altitude long-endurance unmanned air vehicles (UAVs).

The almost four-day test run included three days at a simulated 65,000ft (20,000m) altitude. The gasoline version of the four-cylinder engine powers Ford's hybrid vehicles.

The hydrogen version is planned to power both Aurora's single-engined Orion HALL and Boeing's twin-engined HALE UAVs. Fuel economy during the test was better than expected, says Boeing.

Aurora's Orion, being built as a demonstrator for the US Army, will fly next year on conventional power, but is expected to have a four-day endurance when ultimately fitted with the hydrogen engine.

Boeing's HALE would have an endurance exceeding seven days, and the company believes the successful engine test will boost its chances of finding a customer for the vehicle.