92 As discussed in our cover feature on page 24, the world of UAVs is converging around fuel-based powertrains. Countless organisations have outgrown their DJIs, Yuneecs and other semidisposable ‘starter’ vehicles, and there is a now huge demand for uncrewed assets that can work longer, travel further and carry more weight than is possible using battery power alone. Although battery technology continues to advance, the rate at which it is doing so has levelled out in recent years, particularly with respect to specific energy. Hydrocarbon fuels remain far superior in this respect, having around 60-80 times the specific energy of batteries, depending on the type of fuel. That makes their use pretty much unavoidable for powertrains in most high-end UAVs, which unlike autonomous cars and boats are fighting a constant battle against gravity to maintain endurance at altitude. However, using fuels comes with the caveat of emissions and inferior energy efficiency compared to battery-electric designs, whether a conventional or hybridised fuel powertrain is used. The drive for greater endurance and performance is therefore hampered by emissions regulations, particularly for UAVs performing aerial logistics over cities or mapping around airports for instance. On top of that, the fact that IC engine powertrains inherently have more moving parts than battery-electric configurations means there are more potential points of failure, making their safety a paramount concern for those charged with the wellbeing of civilians below. Aviation authorities’ regulations for UAVs are however still being finalised IC engines remain preferred for some types of UAVs but they face efficiency and emissions challenges – which fuel system suppliers are meeting. Rory Jackson reports Burning issues Fuel bladders made from flexible materials can give considerable stretching, compression and weight saving over rigid fuel tanks (Courtesy of Fuel Safe Systems) June/July 2023 | Uncrewed Systems Technology
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