It probably seemed like a good idea at the time, to use Global Hawks owned by NASA and operated under a joint agreement with Northrop Grumman that allows both of them access to the aircraft for their own research purposes.
But for Northrop’s DARPA-funded Autonomous High-altitude Refueling (AHR) project (aka KQ-X), the time needed by NASA to demod and remod the aircraft for hurricane tracking flights that began in September meant the program ended before actual high-altitude refueling could be demonstrated.
“The AHR program schedule was bounded from the start, and the Global Hawk aircraft were committed to NASA atmospheric science missions starting in June,” DARPA program manager Jim McCormick tells Aviation Week.
It didn’t help that there were delays in what was only a two-year program, the probe-equipped tanker flying in January and the drogue-deploying receiver following in February, so it was March before two-ship testing began.
And it didn’t help that NASA, according to Inside Defense, put a halt on flying its aircraft following the June crash of a US Navy Global Hawk after taking off from NAS Patuxent River, Maryland. That meant close-formation flights completed in May became the last to be performed under AHR.
KQ-X tanker (Photo: NASA)
As a result, the ninth flight under AHR, on May 30, became the program’s graduation exercise. During that final flight, the modified Global Hawks flew in close formation, 100ft or less between refueling probe and receiver drogue, for most of a 2.5-hour engagement at 44,800ft. This was the first time the aircraft flew close enough to measure the full aerodynamic and control interactions.
In the absence of actual wet contact and fuel transfer, data from the flights was fed into simulations to verify system safety and performance. “The results of the first close-formation flight were informative and, through analysis, largely met our objectives,” says McCormick. The biggest challenge to be overcome is the lack of aircraft control authority at high altitude, but analysis indicates that 60% of refueling attempts would result in contact, compared with an initial expectation of just 17%, or one in every six.
KQ-X receiver (Photo: NASA)
The hoped-for seven-day flight will not take place, but ground tests of the fuel systems demonstrated Northrop’s “reverse-flow” approach, with the tanker following the receiver, which DARPA says “opens valuable trade space” for future implementations of hose-and-drogue refueling. ”The actual seven-day demo was not funded at the outset of the program, but the demonstration system was designed to be capable of staying up that long,” says McCormick.
Simulation is no substitute for the real thing, but Northrop must now move on to the autonomous air-to-air refueling tests planned for 2014 under the US Navy’s unmanned combat air system (UCAS-D) program. This will involve the X-47B demonstrator flying behind a Boeing 707 tanker – and a different set of challenges.