Ultra-long endurance and ultra-fast deployment are the targets of new US programmes to demonstrate technology for future autonomous aircraft

Whether it is an aircraft that will stay aloft for its entire five-year life, or be rushed by rocket to fill a surveillance gap half the world away, the US Defense Advanced Research Projects Agency wants to push the boundaries of unmanned aviation to extremes.

Exploring the outer limits of technology is DARPA's charter, and the agency has its successes and failures, but the stated goals of its latest UAV programmes are raising eyebrows.

"We want to completely change the paradigm of how we think of aircraft," says Daniel Newman, manager of the Vulture programme to demonstrate an unmanned aircraft capable of staying aloft for five years. "Aviation has a perfect record - we've never left one up there. We will attempt to break that record."

High Altitude UAV 
© NASA   
NASA work on high-altitude umanned aircraft with endurance measured in months laid the groundwork for DARPA's bold goal of five years aloft

Record breaking has played a key part in advancing aviation, from Bleriot's 1909 Channel crossing and Lindberg's 1927 transatlantic flight to Voyager's 1986 unrefuelled circumnavigation, but designing an aircraft that can be launched then not touched for five years seems a stretch, even for DARPA.

Northrop Grumman's Global Hawk UAV can stay aloft for 40h Aerovironment's hydrogen-fuelled Global Observer aims for a week and there are designs with endurance measured in months. "What would it take to force people to break with the way they operate aircraft, so they no longer operate it as an aircraft?" says DARPA's Wade Pulliam. "It's more than a month - five years gets you there."

The ability to "close the business case" on an aircraft that could be launched and then not maintained or brought home for five years would be a "paradigm shift", says Newman. "We would no longer define an aircraft by the launch, recover, maintain, launch cycle."

But the challenges of an "infinite endurance" aircraft are enormous, and include increasing system reliability, closing the energy cycle, maximising aero-structural effiency and preventing materials degrading over their long exposure to stratospheric flight.

To enable a paradigm shift, DARPA has set the goals for Vulture as five years on station with a 450kg (1,000lb) payload, 5kW of onboard power and sufficient loiter speed to stay on station for 99% of the time against winds encountered at 60,000-90,000ft altitude,

Essentially, the Vulture is an aircraft that operates like a satellite, but is not regulated by orbital mechanics. "It could be positioned over the battle, at 65,000ft versus 260 miles," says Pulliam. Operating as a pseudo-satellite in the stratosphere and not low Earth orbit would provide a 65dB improvement in communications capability, he says, and significantly increase onboard sensor resolution.

There are three architecture options, says Newman: a single ultra-reliable system equivalent to a satellite a modular vehicle where pieces can fly home to be repaired and replaced and an aircraft that can be serviced and replenished while remaining on station.

Energy cycle solutions

There are also three energy cycle solutions, one of which - nuclear - is barred from consideration. One option is to refuel the aircraft in flight. This favours fuel cells, says Newman, because they are more efficient and reliable than internal-combustion and gas-turbine engines. The other option is solar energy, either solar thermal, which is unproven, or solar electric, which is a proven technology.

But reliability is the key consideration. The Vulture reliability goal is 200 times that for the Global Observer UAV. "We have to design for inherent reliability," says Newman. "Dollars per pound is not the metric - this may not be the lightest aircraft. Outside of satellites, no one thinks about five-year reliability."

The Vulture is expected to draw heavily on spacecraft experience, pushing the reliability of components, designing in redundancy and graceful degradation where systems can fail, reducing parts count and derating components to increase their lifetimes.

There will be differences because of the environment an aircraft operates in compared with a satellite. "The day/night cycle is longer for an aircraft, so it gets a really cold soak then gets really warm," says Pulliam. There will be less radiation than in space, but higher ultraviolet than on the ground, with implications for how materials degrade over time.

Aurora MarsFlyer 
© NASA   

A drop test in 2002 under NASA's Mars Flyer programme demonstrated that a folding-airframe UAV could be deployed at high altitude

Infinite endurance and ultimate expendability will also have implications for how an operational aircraft is designed, manufactured and tested. Because of each air vehicle's persistence, relatively few are expected to be built, and opportunities to test the system to failure will be limited. "We need to develop design tools that ensure the first aircraft is as good as the 100th," says Pulliam. Such tools could have wider application in aerospace.

DARPA is close to awarding contracts for the initial concept definition phase of the Vulture programme. "We have at least one successful offeror," Newman told an IDGA unmanned air vehicles conference in Washington DC at the end of February.

During the 12 months of Phase 1, contractors will define the objective system and design both full-scale and subscale demonstrators. Phase 2, scheduled to run until mid-2012, will involve fabricating and testing the subscale demonstrator, which will be capable of flying for three months.

"That will not take us to a technology readiness level [TRL] of 6," says Newman. TRL 6 is required before a programme can enter development. "Only the full-scale demonstrator will get us to TRL 6," he adds. DARPA plans to have a service partner on board before Phase 3, which would involve building and flying a full-scale aircraft capable of staying aloft for 12 months. "Hopefully, we will leave one up there," he jokes.

The Vulture's longevity offers one way of maintaining a global UAV presence without foreign bases or maintenance depots. Although slow, aircraft could be pre-deployed on station ready to respond more quickly. At the other end of DARPA's extremes is Rapid Eye - a UAV that could be rocket-deployed to anywhere in the world within hours.

UAV in a rocket

"Rather than staying on station for five years, Rapid Eye is about how quickly can we get an aircraft there," says Pulliam. "Something has occurred, satellites are not in position and it will take days to get aircraft there. So we put a UAV into a rocket and launch it suborbitally to get anywhere in 1-2h and stay there for a significant period."

Challenges include packaging an aircraft with high aspect-ratio wing into a small volume atop the solid rocket. Some way is needed to slow the payload high in the atmosphere, so the UAV can be deployed at its operating altitude of 65,000ft. The propulsion system must start and operate in the oxygen-starved environment at high altitude, because a glider would not provide the 7-15h endurance sought, says Pulliam.

The Rapid Eye programme is expected to get under way soon as a companion to the Vulture. Whether the choice is fast or slow, DARPA's new programmes aim to expand the design space for unmanned aircraft to unprecedented extremes.


Source: Flight International