The US Defense Advanced Research Projects Agency (DARPA) has completed autonomous refuelling trials with two Northrop Grumman RQ-4 Global Hawks, demonstrating that unmanned air vehicles could refuel in mid-air.
The success of the experiment has the potential to revolutionise large UAV manufacturing and operations.
During the tests the two aircraft flew in close proximity in nine test flights. While the NASA-owned high-altitude, long-endurance (HALE) aircraft were never physically linked and no fuel was passed, DARPA's analysis shows that the aircraft could have done so on up to 60% of the attempts. The agency had anticipated a 17% success rate given the nature of the aircraft - capable of great altitude and endurance, but not manoeuverability - and the difficulty of precision flight at 44,000ft (13,400m), where the tests took place.
"The goal of this demonstration was to create the expectation that future HALE aircraft will be refuelled in flight," says Jim McCormick, DARPA programme manager. "Such designs should be more affordable to own and operate across a range of mission profiles than systems built to satisfy the most stressing case without refuelling."
The unusual refuelling system consisted of a hose-and-drogue used worldwide, however in this case the receiver deployed the hose, while the tanker would push fuel upwards. The fuel system was tested on the ground and certified as fully functional, but there are no imminent plans to actually transfer fuel in mid-air using these vehicles.
Autonomous refuelling was preceded by low-altitude tests using a modified Boeing F/A-18.
Plans are moving forward to use the Northrop Grumman X-47, a testbed for unmanned combat vehicle technologies, for autonomous refuelling from manned tankers following the completion of aircraft carrier landing trials.