Two Global Hawk unmanned aircraft have flown in close formation at distances as close as 30 feet (9 m) for the first time. The series of flights took place between January 11 and May 30 this year and marked a major milestone on the way to demonstrating the first autonomous aerial refueling between two unmanned, high-altitude aircraft as part of DARPA’s Autonomous High-Altitude Refueling (AHR) program.

The flights were conducted by a team from Northrop Grumman, DARPA and NASA Dryden Flight Research Center and involved two modified NASA Global Hawk UAVs. The trailing aircraft was configured as a tanker aircraft, while the lead aircraft was configured as a receiver and extended and retracted its aerial refueling hose several times during the flights. Fuel systems were also fully integrated into the UAVs and ground tested.

During the ninth and final test flight lasting over 2.5 hours, the two aircraft rendezvoused at 44,800 feet and spent the majority of the time flying autonomously within one wingspan (100 ft/30 m) of each other. This was designed to demonstrate that High Altitude Long Endurance (HALE) aircraft can safely and autonomously operate under the conditions of in-flight refueling.

DARPA says that analysis of the flight data carried out over the last few months indicates that 60 percent of refueling attempts would achieve positive contact. This surpasses the initial expectations of 17 percent (or one in six attempts) held at the start of the program.

The US$33 million AHR program is a follow-on to the 2006 DARPA Autonomous Aerial Refueling Demonstration (AARD) that saw a specially configured NASA F/A-18 serving as a surrogate unmanned aircraft to autonomously refuel via a probe and drogue from a 707 tanker.

The aim of the AHR program is to bring the same aerial refueling capabilities enjoyed by current manned military aircraft to UAVs and extend the flight endurance of the Global Hawk to up to a week.

As part of an effort to extend the endurance of future carrier-based unmanned systems, Northrop Grumman is also developing autonomous aerial refueling (AAR) technology that is scheduled to be demonstrated in 2014 using the U.S. Navy’s X-47B unmanned demonstrator aircraft.

Video of the two Global Hawk UAVs flying in close formation appears below.

Source: Northrop Grumman, DARPA