Future UAVs will be able to refuel in flight
08 October 2012
DARPA has completed close-proximity flight tests of two modified unmanned aerial vehicles (UAVs), demonstrating technology for the autonomous aerial refuelling of these aircraft.
Increasingly, UAVs are conducting combat and ISR operations, but they are not designed to be refuelled in flight. In 2007, DARPA teamed up with NASA to show that high-performance aircraft can easily perform automated refuelling from conventional tankers, yet many unmanned aircraft can’t match the speed, altitude and performance of the current tanker fleet. The 2007 demonstration also required a pilot on board to set conditions and monitor safety during autonomous refuelling operations.
DARPA’s two-year Autonomous High-Altitude Refuelling (AHR) programme, which concluded at the end of last month, explored the ability to safely conduct fully autonomous refuelling of UAVs in challenging high-altitude flight conditions. During its final test flight, two modified Global Hawk aircraft flew in close formation, 100 feet or less between refuelling probe and receiver drogue, for the majority of a 2.5 hour engagement at 44,800 feet.
This demonstrated for the first time that High Altitude Long Endurance (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 the full aerodynamic and control interactions. Flight data was 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.
Since HALE aircraft are designed for endurance at the expense of control authority, the programme started with the expectation that only one of six attempts would achieve positive contact (17 percent). The final analysis, however, indicated that 60 percent of the attempts would achieve contact. Multiple autonomous breakaway contingencies were successfully triggered well in advance of potentially hazardous conditions.
Fuel systems were fully integrated and ground tested, demonstrating a novel “reverse-flow” approach with the tanker in trail. This approach opens valuable trade space for future developers to choose between various fixed and modular implementations of proven probe and drogue hardware.