Aerial refueling is no simple task, involving a complex choreography of tanker, receiver and fuel-transfer mechanism, whether using flying boom or hose and drogue. It is a challenging job for manned aircraft, and now the US Air Force and Navy want to refuel unmanned aircraft. It could be argued the Air Force faces a slightly easier task, as the boom is under human control whether the receiver is manned or unmanned. The drogue is not, and this can produce unexpected results.
In pursuit of an autonomous aerial refueling capability for its proposed N-UCAS naval unmanned combat air system, the Navy has requested information on a concept for an actively stabilized drogue for use with the A42R-1 aerial refueling pod carried by F/A-18E/Fs. The service plans a flight demonstration in FY2012, initially with a manned F/A-18 receiver, but ultimately with the X-47B UCAS-D demonstrator.
The Navy wants an active stabilization system that will keep drogue motion to within 10cm, at frequencies of 0.7Hz or less, while refueling at altitudes from 1,500-30,000ft. The system must keep the drogue at a predetermined relative position, compensating for turbulence, tanker motion and receiver forebody effects. It should also provide drogue position data to the tanker for relay to the receiver.
Several ideas have been floated for drogue stabilization. A few years back Smiths - now GE Aviation Systems - patented a passive technique that gyroscopically stabilizes the drogue by using an air turbine to spin a rotating mass. Active stabilization ideas range from aerodynamic control surfaces to thrust vectoring on the drogue. DARPA and NASA have already demonstrated hands-off probe-to-drogue contact, but active stabilisation would make it easier and safer.