The US Defense Advanced Research Projects Agency (DARPA) has demonstrated that future high-altitude, long-endurance (HALE) aircraft will be capable of refuelling in flight.
DARPA’s two-year Autonomous High-Altitude Refueling (AHR) programme, which concluded on 30 September, explored the ability to safely conduct the fully autonomous refuelling of unmanned aerial vehicles (UAVs) in high-altitude flight conditions.
During its final test flight, two modified Global Hawk aircraft flew in close formation, 100ft (30m) or less between refuelling probe and receiver drogue, for most of a two-and-a-half-hour engagement at 44,800ft.
According to a statement, this demonstrated for the first time that HALE-class aircraft can safely and autonomously operate under in-flight refuelling conditions.
The flight was the ninth test and the first time the aircraft flew close enough to measure full aerodynamic and control interactions.
Flight data has been analysed over the past few months and fed back into simulations to verify system safety and performance through contact and fuel transfer, including the effects of turns and gusts up to 20 knots.
At the start of the programme, it was thought that only one of six attempts would achieve positive contact as HALE aircraft are designed for endurance at the expense of control authority. The final analysis, however, indicated that 60 per cent of the attempts would achieve contact.
According to DARPA, multiple autonomous breakaway contingencies were triggered in advance of potentially hazardous conditions.
Similarly, fuel systems were fully integrated and ground tested, reportedly demonstrating a novel ‘reverse-flow’ approach with the tanker in trail — an advance that will allow future developers to choose between various fixed and modular implementations of probe and drogue hardware.
‘The goal of this demonstration was to create the expectation that future HALE aircraft will be refuelled in flight,’ said 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 lessons from AHR certainly extend beyond the HALE flight regime, and insights into non-traditional tanker concepts may offer further operational advantages.’