The US Air Force wants to continue exploring opportunities for very high altitude unmanned air systems, but major technical challenges need to be overcome before the concept of an operational system becomes feasible, according to Lt Gen Don Hoffman, military deputy in the office of assistant secretary of the Air Force for acquisition.
The Northrop Grumman RQ-4 Global Hawk is likely to remain the highest altitude endurance aircraft fielded by the service for the foreseeable future, he argues, with any options for near space aircraft requiring significant gains in either comparative cost or mission effectiveness.
The USAF has previously explored a number of concepts for very high altitude and near space aircraft, including the use of unmanned airships. However, Hoffman says Global Hawk is likely to remain a more viable solution, particularly if its endurance can be further extended through the use of in-flight refuelling.
Speaking at the Association for Unmanned Vehicle Systems International's Unmanned Systems North America convention in Washington in early August, Hoffman said "we are kind of capped now with Global Hawk as the high end where you can operate an unmanned air vehicles until you get up to space and then you can have orbital dynamics apply".
There are multiple attractions for a class of air vehicles that can operate at very high altitudes, he said. "One is that you can get a good look from up there you have great line of sight and radio communication, and if you can get long endurance up there whether it is through a big lifting device or some sort of balloon device, it is very attractive."
Such a class of air vehicle, however, "has to buy its own way in. It isn't just the novelty of being up there. It has got to survive up there and the environment is probably the toughest survival challenge it could have. Can it survive gusting wind and so forth? And it has got to have a payload and most of the options that I have seen so far. The payloads are very marginal compared with a Global Hawk. You have got to displace either mission effectiveness or cost with a Global Hawk or its class of aircraft right now."
Extending the endurance of Global Hawk by fitting it with a probe and drogue in-flight refuelling system and modifying some air vehicles of the type to act as tankers have been previously flagged Hoffman says, but more innovative approaches need to be considered.
He suggests it may be possible to refuel the type using existing air force tankers modified to support a reversal of standard probe and drogue concepts: "A unique concept may be that the probe comes out of the back of the Global Hawk and it descends down to an altitude where a [Lockheed Martin] C-130 with a probe can now spear the basket and push gas the other way into the Global Hawk.
"So the Global Hawk is just a stable platform and the manned aircraft comes in and takes all the variables in the equation and pushes gas the other way. I think we need to be creative about how we do air-to-air refuelling in the future."
Achieving extended mission endurance at altitude would provide the USAF with opportunities to achieve mission growth for UAVs in a variety of areas Hoffman says, including communications relay roles. "If you could park a UAV up there for a day or a week that would be great. By the way, while he is up there, why not make him a network node and give him servers and routers and storage and allow him to be your reach back."
That concept has been operationally proven with Global Hawk and Northrop Grumman's Advanced Information Architecture, Hoffman says, and "there is a lot of potential there".
Hoffman also reiterated USAF interest in the use of endurance UAVs as a satellite gap filler in the event of loss of space assets during a conflict. "We are very concerned about the security of our satellite systems. The recent Chinese ASAT test shows us we are vulnerable, especially in low-Earth orbit."
Source: FlightGlobal.com
