Northrop Grumman has turned the concept of aerial refueling completely on its head for the KQ-X UAV-to-UAV autonomous high-altitude aerial refueling demonstration.
In time-tested air-to-air refueling practice, the tanker deploys its hose and drogue (or boom) and flies a racetrack pattern, providing a stable target for the receiver, which maneuvers behind the tanker to connect with, or "plug" into, the refueling basket (or boom). Fuel then flows downhill.
Northrop does it exactly the other way round. In the KQ-X demonstration, planned for 2012, one of NASA's Global Hawks will fly a racetrack and deploy a hose and drogue - but as the receiver. The second NASA Global Hawk - the tanker - will then maneuver into position behind the receiver and plug its refueling boom into the basket. Fuel will flow uphill.Proteus (left) simulates refueling Global Hawk. (Photo: NASA Dyden)
All of this is taking place at 45,000ft, between aircraft that are not designed to maneuver at such altitudes. Well above the normal altitudes for aerial refuelling, 45.000ft has been picked because it's close to the Global Hawk's cruise altitude and well above atmospheric turbulence that would make connecting difficult.
Northrop is pursuing air-to-air refueling as a low cost and low risk way to extend the endurance of high-altitude, long-endurance (HALE) unmanned aircraft - lower cost and risk than a hydrogen- or solar-powered UAVs, that is. ANd the company argues that conventional aerial refueling would require every UAV to be equipped for and capable of maneuvering behind the tanker.
But with its "reverse-flow refueling" concept, only a small number of UAVs need to be permanently modified to rendezvous and maneuver as tankers - the bulk of the fleet can be equipped as "passive" receivers simply by fitting a buddy hose-and-drogue refueling pod as required.
Tests are getting under way with a January close-proximity flight in which Scaled Composites' manned Proteus, simulating the Global Hawk tanker, flew to within 40ft of a NASA Global Hawk, simulating the receiver, at 45,000ft. The test was intended to measure the wake effect on the aircraft, check for engine plume ingestion, and test the autonomous "break-away" maneuvers required if the tanker misses the basket or has to break off refueling.
All of which reminds me this is not the first time
the direction of air-to-air refueling has been reversed. In the 1930s, Royal Air Force Sqn Ldr Richard Atcherly developed the "looped hose" method. The receiving aircraft trailed a long line with a grapnel at the end. The tanker trailed a weighted line and approached the receiver from behind and one side, the crossed over to the other side so the lines touched. The receiver then hauled in the lines and the hose from the tanker.
Ultimately this was refined by Alan Cobham into the hose-and-drogue method used today - and the company he formed is providing the refueling system to Northrop for KQ-X.