Design of Long-Endurance Unmanned Airplanes Incorporating Solar and Fuel Cell Propulsion

Preliminary performance analysis and conceptual design are described for a class of unmanned airplanes possessing multi-day endurance capability. A mixed-mode electric power system incorporates solar cells for daytime energy production and a non-regenerative H2-O2 fuel cell to supply energy for night flight. The power system provides energy for all onboard systems, including propulsion, payload, and avionics. Excess solar energy is available during significant portions of the day, and may be used for climbing, maneuvering, or payload functions. By jettisoning fuel cell reactant product (water) during flight, vehicle endurance may be increased under certain conditions. Empirical structure sizing algorithms are combined with low-Reynolds number aerodynamics algorithms to estimate airplane size and geometry to meet prescribed mission requirements. Initial calculations for summertime, high-altitude flight (above 40,000 ft (12 km)) at moderate latitude (31 degrees N) indicate that mission endurance of several days may be possible for configurations having wing loadings on the order of 0.9 to 1.3 lb/ft squared. These aircraft tend to be somewhat smaller than solar-powered aircraft previously conceived for multi-month endurance utilizing regenerative fuel cell systems for night flight.