DARPA RQ-4A Air-to-Air Refueling Program Analysis

Air-to-air refueling (AAR) has been commonplace in manned aviation for more than six decades.  Today, remotely piloted vehicles fly hundreds, and sometimes thousands of miles from their launch bases to for intelligence, surveillance, and reconnaissance missions.  The inability to refuel in-flight limits time on station and increases the number of aircraft required to surveil a given target location.  DARPA's KQ-X program, initiated in 2010, was designed to prove the technologies required to refuel UAVs in-flight.

DARPA has released videos of the close-proximity test flight of two modified RQ-4A Global Hawk unmanned aerial vehicles.  The final test flight last May demonstrated the ability of close formation flight for most of a 2.5-hour engagement at 44,800 feet.  Although the UAVs did not actually pass fuel, post-flight analysis indicated that 60% of  attempts would achieve contact between the
"Navy style" refueling probe and drogue. 

Perspective from receiving aircraft.

Perspective from drogue-equipped aircraft.

This article in Air Force magazine provides an interesting background on the history and technical aspects of the program.  The Navy's Unmanned Combat Air System (UCAS) Aircraft Carrier Demonstration (UCAS-D) program has utilized a surrogate Boeing F/A-18D equipped with X-47B avionics to test AAR technologies.  NAVAIR plans to perform an actual air-to-air refueling with an X-47B in 2014.  The follow-on UCLASS program will likely be the first system to use this technology operationally.