Helios Solar Powered UAV
Helios is one of several remotely-piloted aircraft-also known as uninhabited aerial vehicles or UAV's-being developed as technology demonstrators by several small airframe manufacturers under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project.
Developed by AeroVironment, Inc., of Monrovia, Calif., the unique craft is intended to demonstrate two key missions: the ability to reach and sustain horizontal flight at 100,000 feet altitude on a single-day flight, and to maintain flight above 50,000 feet altitude for at least four days, both on electrical power derived from non-polluting solar energy. During later flights, AeroVironment's flight test team will evaluate new motor-control software which may allow the pitch of the aircraft-the nose-up or nose-down attitude in relation to the horizon-to be controlled entirely by the motors. If successful, productions versions of the Helios could eliminate the elevators on the wing's trailing edge now used for pitch control, saving weight and increasing the area of the wing available for installation of solar cells. The craft has a wingspan of 247 feet. The solar cells generate about 40 kilowatts of power -- about the amount used each day by four to six homes -- to drive 14 propellers on the craft.
Development of a regenerative hydrogen-oxygen energy storage system which would make the multi-day continuous flight possible is progressing at AeroVironment. The system uses excess power generated by the solar arrays during the daytime to run an electrolyzer that separates water into its component parts, hydrogen and oxygen, which are then stored under pressure in specially-designed tanks. At night, the hydrogen and oxygen are recombined by the fuel cells, producing electricity as a by-product to power Helios' motors and systems.
A similar craft to the Helios could be used in exploratory missions on Mars. This is because Helios is designed to fly at high altitudes on Earth which resemble the atmosphere near the Martian surface.
Below is a diagram of such an undertaking: