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44 G oogle’s acquisition of Titan Aerospace and its ‘atmospheric satellite’ has placed more emphasis on the power requirements of autonomous vehicles. The prospect of using such solar- powered craft instead of a satellite to host a wireless service to deliver the internet to PCs and phones in areas without data coverage presents some very real engineering challenges. An example of the challenges comes from the 2.5 m wingspan Green Falcon UAV developed at the Queensland University of Technology in Australia. This is powered by 28 monocrystalline solar cells generating just 500 mW, but that is enough to power its electric engine, onboard cameras and sensors to track the progress of bushfires. Larger, High Altitude Long Endurance (HALE) UAVs can use the increased surface area of their wings to generate more power from solar cells to run their propulsion and payload systems – which, when the payload is a cellular base station, is a serious challenge that is still in the research lab. Titan, based in New Mexico, has developed the Solara 50, which is 15.5 m (54 ft) long with a wingspan of 50 m and carries a payload of 32 kg (70 lb). It is powered by 3000 solar cells across the upper wing, elevator and horizontal stabiliser to provide up to 7 kW, storing any excess in lithium-ion batteries in the wings. Lithium-sulphur rechargeable batteries provide an energy density of 400 Wh/kg while normal lithium-polymer batteries provide 200 Wh/kg The combination of the solar cells and the lithium-sulphur batteries provides enough power to keep the UAV in the air for months, say the designers, at a cruising altitude of 20 km (65,000 ft) as an airborne communications station, and eventually up to five years flying at 65 mph. This speed is determined by the average wind speed of just 15 mph at that altitude, coupled with the lower air density, which means the craft has to travel three times faster than closer to the ground to maintain its lift. These capabilities appealed to Google, which bought the company in April 2014 to be part of its Google X technology r&d lab. Solara could potentially replace the balloon-based systems that Google has been testing out, but the power generated from the cells is the crucial engineering constraint. The Solara 50 uses a third-party fixed- wing autopilot that has been integrated with a GPS navigation system to enable it to fly, take off and land autonomously, and alongside various onboard sensors in the payload it also has high-speed radio links for transmitting telemetry data Aiming for the UAVs look set to take on a key role as communications base stations – once various engineering challenges have been overcome. Nick Flaherty reports November 2014 | Unmanned Systems Technology