Aerial Refueling: Look, Ma, No Hands!
DARPA does some quietly interesting things. The latest is an autopilot that lets an aircraft perform air-air refueling – one of the most challenging routine flight tasks outside of carrier landings – without pilot intervention. While impressive on its own, such systems have special relevance because they offer the promise of unmanned fighters that can remain aloft far longer than aircraft which require human pilots. The result would be far longer strike reach and persistence, two areas that will be critical to the US Navy’s ability to keep its carrier force relevant and effective through the next 3 decades. It’s no coincidence that the J-UCAS program, which is now the Navy’s UCAS-D, was a DARPA initiative as well.
The Autonomous Airborne Refueling Demonstration program (AARD) aimed to demonstrate that unmanned aircraft can autonomously perform in-flight refueling under operational conditions. AARD uses precise inertial, GPS, and video measurements, combined with advanced guidance and control methods, to plug a refueling probe into the center of a 32-inch basket trailed behind a tanker. The test aircraft were a NASA F/A-18D Hornet with pilots on board for routine flight and safety backup, and a piloted 707-300 tanker (similar to the KC-135) from Omega Air Refueling Services. Each attempt was made from Edwards AFB in California across a range of turbulence conditions, the most challenging of which were characterized by up to 5 feet of peak-to-peak drogue motion. This approaches the limits of routine manned refueling operations – and that wasn’t all the AARD system was able to do…
Sierra Nevada Corp. led the AARD team. Their system improved over time, removing the need for pilot consent points and eventually matching the drogue’s motion – something human pilots are specifically taught NOT to do. In one case, the system followed the drogue through a full 3-foot cycle in the 2 seconds before making contact, never deviating more than 4 inches from the exact centerline while traveling at 250 miles per hour, 18,000 feet above the Tehachapi Mountai. The system further demonstrated the ability to join the tanker from up to 2 nautical miles behind, 1,000 feet below, and 30 degrees off heading.
Improved video processing eliminated troublesome dropouts, allowing the system to conduct 4 times as many plug attempts per flight. Several control techniques were tested, and the best was 100% effective over 18 attempted probe-and-drogue connections. Advanced control algorithms eventually gave AARD its ability to anticipate drogue motion, and even allowed contact attempts with the drogue during turns, another thing human pilots don’t generally do.
The first-ever fully autonomous refueling happened in August 2006, August. Since then, the Autonomous Airborne Refueling Demonstration (AARD) has completed 10 additional flights. DARPA release [PDF format]