The Proteus approaches the Global Hawk in a flight test on January 21.

The phrase "fill 'er up" is being redefined for the age of robotic aircraft.

Northrop Grumman said yesterday that in a flight test earlier this year, it took a big step closer to an eventual autonomous aerial refueling between unmanned aerial vehicles as part of the $33 million DARPA KQ-X program.

In the "risk reduction flight test," which took place January 21, a Global Hawk UAV from NASA played the role of the aerial tanker, and Northrop Grumman's Proteus test aircraft--a manned UAV surrogate, we should point out--was the one in search of the refueling boom. Among the matters being evaluated were wake turbulence between the two aircraft, which at their closest were just 45 feet apart, along with engine performance and flight control responsiveness in the stratosphere.

The interaction took place at 45,000 feet in what Northrop Grumman called a "landmark flight." The Global Hawk has a ceiling of about 65,000 feet and can reach speeds approaching 340 knots.

What didn't take place was any actual refueling. This was strictly a dry run.

"Demonstrating close formation flight of two high-altitude aircraft, whether manned or unmanned, is a notable accomplishment," said Geoffrey Sommer, KQ-X program manager in Northrop Grumman Aerospace Systems, in a statement.

The January flight was a prelude to an actual autonomous aerial refueling involving two Global Hawks, a DARPA KQ-X event that's scheduled for spring 2012. Successful refueling would allow for flights lasting up to one week. According to Northrop Grumman, the 44-foot-long, 13-ton NASA Global Hawk has a maximum endurance of 31 hours.

The broader context here is that the defense sector has its sights fixed on high-altitude, long-endurance (HALE) operations for unmanned aerial vehicles, often using more exotic energy sources, for missions such as surveillance and communications. Boeing, for instance, is working on a UAV called the Phantom Eye that is intended to use hydrogen fuel to eventually fly at 65,000 feet for up to four days. In January, Aerovironment's Global Observer made the first-ever hydrogen-powered UAV flight--just a few hours at low altitude, but the aim is to get it to fly for up to a week at a time at 55,000 to 65,000 feet.

Solar power also gets a nod in UAVs from the likes of Qinetiq, whose Zephyr last July stayed aloft for 14 days nonstop, and Boeing, whose SolarEagle has a short-term goal of flying continuously for 30 days and an extremely ambitious goal of flying without a break for five years.