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22 and future waveforms. And importantly, we can still use and interact with all existing telecoms company infrastructures, so adopting our platform won’t come with any significant new ground infrastructure costs or similar burdens for such companies. “When you get a handle on just how complicated it is to design, build and run ground RF equipment on a large scale, you start to understand why telecoms companies are investing so much money in alternative solutions with greater flexibility and modularity, and far lower entry costs – like UAV-mounted relays. “Those companies can also do a lot at their end, on the ground, to handle interference and other issues, which makes it much easier to roll out these kinds of systems. We’ve been helped greatly on that side by input from Deutsche Telekom, our main investor.” The antenna system comprises more than just RF innovations. Bean and his team have also conducted extensive r&d into thermal management technologies in order to provide a comprehensive stratospheric cooling system. “Making sure we can cool the array in the stratosphere is really important. Obviously there isn’t much air that far up, so we need to get rid of the heat in a lightweight way,” he says. “We’ve spent a lot of time and effort optimising that cooling system to work in low stratospheric pressures and temperatures.” Liquid hydrogen As Stratospheric Platforms plans to fly long missions across northern Europe and North America, the company immediately realised that its UAV would need to carry or source a lot of power. “We looked at solar, but there just isn’t enough sunlight in the latitudes where we’ll be flying to keep our payload in the air,” Bean explains. “That’s why we went for liquid hydrogen – it’s the only way we can supply enough power to carry the payload, keep it running, and still get enough propulsion for the flight patterns we’re looking at. “Hydrogen contains about three times the specific energy of avgas, and unlike avgas or biofuels it doesn’t pollute the atmosphere. It does take up more space though, which is why the body of the aircraft has to be designed around the fuel tanks. “But in our use-case, that doesn’t actually mean we’re sacrificing aerodynamics for energy density in any way, because we’ll be carrying that massive, wide antenna array. “That payload means the HAP already has a minimum width to its fuselage, so we designed tanks front and rear of February/March 2022 | Unmanned Systems Technology Stratospheric Platforms has made extensive use of TWI’s extreme temperature cycling test rigs in developing its liquid hydrogen solutions (Courtesy of TWI) The first HAP prototype is expected to be completed and in flight tests by mid-2023, with certification and commercial service ready for 2025 (Courtesy of Stratospheric Platforms) Hydrogen power systems for mobility applications are very well placed now – if we’d tried to do this 10 years ago it would have been impossible