Northrop Grumman has revealed an internal goal to perform a seven-day, non-stop flight with its RQ-4 Global Hawk after completing an autonomous, high-altitude refuelling (AHR) demonstration in the second quarter of next year.

The proposed endurance flight is aimed at proving that adding in-flight refuelling to a currently 30h-endurance RQ-4 will beat a new class of potential rivals powered by liquid hydrogen fuel, such as the AeroVironment Global Observer and Boeing Phantom Ray.

“That is the goal,” says Geoff Sommers, Northrop’s programme manager for the AHR demonstration. “One of the strategic imperatives here is to demonstrate a reliably low-risk and low-cost approach to extending the range of a high-altitude [unmanned air vehicle].”

Sommers describes propulsion systems which rely on advanced fuels as inherently risky compared to achieving ultra-endurance by using autonomous, high-altitude refuelling technology. “We see this as an alternative that builds upon existing technologies,” he says.

The seven-day flight by the Global Hawk is not yet funded under the US Defense Advanced Research Projects Agency’s $33 million AHR programme, which was awarded to Northrop last July. However, the aircraft will be capable of performing the sortie at the end of the funded programme, awaiting only DARPA’s go-ahead and funding to launch the flight demonstration, Sommers says.

For the AHR demonstration, Northrop is modifying two NASA-owned Global Hawks to serve as a tanker and receiver, respectively. Aircraft AV-1 will gain a refuelling probe, while AV-6 will be equipped with a hose reel in its payload bay.

The two aircraft will rendevous with a Sierra Nevada guidance system using differential GPS and an optical tracker to guide the probe into the drogue. The concept uses a reverse flow refuelling system, where the AV-1 tanker will push fuel into AV-6 flying up to 15m (49.2ft) in front. Using this method requires Northrop to modify only a few tankers to manoeuvre in close formation with another unmanned system, Sommers says.

Northrop plans to complete a series of autonomous dry engagements by the end of this year using the AV-1 and AV-6 Global Hawks. During wet engagement flights next year, AV-1 will offload 1,360kg (3,000lb) of fuel, then wait for 3h before supplying another 1,360kg, Sommers says.

 Global Hawk Proteus - NASA
© NASA Dryden Flight Research Center

On 21 January, Northrop completed a flight simulating refuelling distances between the unmanned AV-6 Global Hawk receiver and the manned Scaled Composites Proteus operating as a tanker.

Source: Flight International