Small unmanned aircraft systems (UAS) have become an increasingly important intelligence, surveillance and reconnaissance tool for the military and their use will only continue to expand. While vehicles that are small enough to be carried provide soldiers in the field with capabilities such as over-the-next-hill imagery or the short-term monitoring of convoys, much research is going into finding ways to extend mission duration through better battery systems. DARPA researchers say they have overcome this limitation with the development of a compact solid oxide fuel cell (SOFC) fueled by propane that quadruples the endurance of Lockheed Martin's Stalker small UAS.

The new fuel cell formed the basis of the hybrid power source for the Stalker XE, (eXtreme Endurance), a variant of the Lockheed Martin-developed Stalker UAS that boasts a wingspan of 120-in (305 cm) and is powered solely by a battery-driven electric motor that gives it a flight time of around two hours. By combining a high density fuel cell system with a conventional lithium polymer battery to handle peak power requirements, the Stalker XE is able to quadruple the endurance of the baseline Stalker version to provide a flight time of over eight hours.

By using propane, which is a very high energy density hydrocarbon fuel, rather than a relatively low energy density battery, the Stalker XE is able to achieve the extended flight time without impacting its mobility or payload capabilities. It is launched by a 22-pound bungee-launch system and operates without the large footprint and high cost associated with current tactical UAS platforms.

"A small unmanned aircraft system with long-endurance capability could give the military the ability to do with a small craft what has previously been doable only with larger airframes. This has potential for tremendous cost savings - we can maintain and even improve on our UAS capabilities with a much smaller footprint and lower operating cost," said Brian Holloway, DARPA program manager.

DARPA says it is the ruggedization of the SOFC power source and its integration into a fieldable platform that is the most notable achievement of the Stalker XE demonstration. Rigorous flight-testing saw the small UAS performing back-to-back flights on a single airframe and single fuel cell with turnaround times of less than 30 minutes. During these tests, the aircraft performed at altitudes of over 15,000 feet, and encountered wind gusts of 46 mph (74 km/h) and sustained winds of up to 28 mph (45 km/h).

"Stalker XE is a great example of how TAP (DARPA's Tactical Advanced Power program) may help reduce logistical burdens, enhance mission capability and fundamentally change how the U.S. military uses power and energy," said Holloway.

Weighing in at 14 pounds (6.3 kg), the Stalker XE is comparable to the AeroVironment Puma, which weighs 13 pounds (5.9 kg), has a wingspan of 102 inches (259 cm) and is also powered by a fuel cell battery hybrid energy system. The Puma set a long-duration flight record of over nine hours in 2008, which was triple the endurance of the aircraft when running solely on battery power.