Unmanned Systems Technology 036

93 Hydrogen powertrain A hydrogen powertrain was selected over a battery pack or small engine for several reasons, but a key factor was that the USASOC specified the need for quiet operation at low altitudes. Even so, Zepher performed calculations and simulations of IC engine configurations – particularly series- hybrid powertrains – but found that the propulsion mass fraction and overall system efficiency became problematic for the size of aircraft they needed to build. Also, the use of a series-hybrid arrangement would entail running the engine every 20 minutes or so to recharge the Z1’s batteries, at which point the noise produced would be much more than the USASOC wanted. “That could lead to situations where you’re in the air collecting surveillance or intelligence imagery, but then you’re forced to disengage, or worse, power-up the engine, revealing your position to the people below,” Coatney says. “To fly far above the ground where no- one can hear you, you’d need one of the most expensive IR camera models within our 5 kg weight limit. You’d prefer to use a more cost-effective one, and fly low under the cover of darkness. You can’t risk your powertrain giving you away at that point, and hydrogen fuel cells run much more quietly.” Maximising the Z1’s propulsion system’s energy-to-mass ratio was another key factor. The specific energy of even the best lithium batteries remains the lowest of any type of commercially available energy storage system, while a hydrogen fuel cell system can achieve an overall specific energy about three times greater than the best batteries. Developing the UAV around a hydrogen powertrain therefore quickly became the most promising option for the Z1. Zepher Flight Labs investigated a number of hydrogen fuel cells that had reached scale production and were viable for the SWaP constraints of the Z1, before determining that the 800 W fuel cell from Intelligent Energy (as featured in UST 24, February/March 2019) was the most suitable for its power requirements. Zepher Flight Labs Z1 | Digest Unmanned Systems Technology | February/March 2021