Northrop Grumman made a major step towards autonomous aerial refueling between two UAVs. Two aircraft, the manned high altitude Proteus and one of NASA’s RQ-4 Global Hawks simulated refueling maneuvers while flying at 45.000ft. The two aircraft came as close as 40ft distance and they evaluated the flight control system response and engine performance at such high altitude. These tests are part of the KQ-X program by DARPA that will demonstrate autonomous aerial refueling between two Global Hawks in 2012. The refueling will take place at 60.000ft (double the usual flying altitude of a typical airliner) and at a speed of around 160kt, where thin air and low dynamic pressure makes for very challenging conditions. More information after the jump.

In this simulation only one aircraft was unmanned, the RQ-4. The Proteus had a flight crew although only for supervision as a risk reduction measure. It is worth noting that in contrary to air refueling practices both in this demonstration and on the KQ-X program the tanker follows the fuel receiving aircraft and sends fuel through its probe to the leading aircrafts drogue. This unusual arrangement is dictated by the need to retrofit the equipment to the existing Global Hawk fleet. It is easier (and cheaper) to install a drogue pod, so only a few Global Hawks will be converted to tankers; the rest will only be able to receive fuel. The air refueling will allow an RQ-4 to stay on air for around 120h (up from ~30h) and fewer aircrafts could perform the same mission.

Five years ago a similar demonstration took place when a Boeing 707-300 tanker refueled one of NASA’s F/A-18B. Even though both aircraft were manned the pilots as you can see in the video above weren’t touching the controls. The refueling maneuver was performed by a system designed by Sierra Nevada Corporation, the Autonomous Airborne Refueling Demonstration System (AARD) that fuses differential GPS data, inertial navigation and optical tracking for the final contact phase. This system was fully integrated into the F-18 and no modifications were made to the tanker. Northrop Grumman is also using the AARD and likewise the workload will be asymmetrical, the tanker RQ-4 will have the relative navigation system and will be responsible to rendezvous with the receiver. SNC aims to use it not only in unmanned vehicles but also as a safety aid for regular piloted aircraft while Northrop Grumman also develops the X-47B stealth drone for the US Navy that is also scheduled for aerial refueling at some point.

(via Ares blog, first video by Northrop Grumman, second video by Sierra Nevada Corporation)