Small unmanned aircraft systems (UAS) provide valuable intelligence, surveillance and reconnaissance (ISR) capabilities for units at the infantry company level and below, allowing over-the-next-hill imagery or short-term monitoring of convoys as an example. State-of-the-art battery power for these small UASs, however, has limited the duration of missions to about two hours.

“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, Defense Advanced Research Projects Agency (DARPA) programme manager.

DARPA’s Tactical Advanced Power (TAP) programme has addressed the power limitation by developing a compact solid oxide fuel cell (SOFC) fuelled by propane, a very high energy density hydrocarbon fuel. DARPA researchers have also developed the Stalker XE, a small UAS powered by this fuel cell to provide extended mission endurance for more than eight hours with the reliability and ruggedness required to perform realworld missions. The Stalker XE has demonstrated an improvement of more than four times the endurance of existing state-ofthe-art small unmanned aircraft systems.

The fuel cell was the basis for the hybrid power source on the Stalker XE, in which the high energy density fuel cell system was combined with a conventional lithium polymer battery to handle peak power requirements. Its high energy density hydrocarbon fuel to handle energy storage rather than a relatively low energy density battery. The Stalker XE enables persistent surveillance operations for small units. As a 22-pound bungee-launched system, it operates without the large footprint and high cost of current tactical UAS platforms.

The enabling technology is the compact SOFC developed by DARPA for portable power applications, but the notable achievement of the Stalker XE demonstration is ruggedisation of the advanced SOFC power source and integration into a fieldable platform. Stalker XE was subjected to rigorous flight-testing, where it was required to perform 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 encountered wind gusts of 74 kph and sustained winds as high as 45 kph. The system also performed at altitudes of greater than 15,000 feet.

The TAP portfolio programme develops advanced portable power and energy in program efforts ranging from fundamental materials and chemistry to systems engineering of mature portable power systems. The fuel cell power source in the Stalker XE was developed and matured under DARPA’s palm power and robust portable power sources programmes. Under the TAP programme, the fuel cell system was ruggedised for integration into a fieldable small UAS.

“Stalker XE is a great example of how TAP may help reduce logistical burdens, enhance mission capability and fundamentally change how the US military uses power and energy,” said Holloway.