U.S. Department of Energy

    Unmanned Airplane Flies Two Days on Solar and Battery Power

    June 29, 2005

    AC Propulsion announced early in June that it successfully flew an unmanned aircraft for more than two days using only solar energy. Dubbed the "SoLong," the craft stores solar energy in a lithium-ion battery pack during the day to keep it flying at night. The company incorporated the lightweight batteries into an energy-efficient craft made of composite materials, weighing only 28 pounds with a wingspan of slightly more than 15 feet. Along its wing are 76 Sunpower solar cells that could produce 225 watts of power, while the craft required only 95 watts for level flight. According to the company, a critical factor in the SoLong's success was its high-efficiency electric motor, driven by the company's patented power controller. The radio-controlled craft featured 23 channels of telemetry, navigation data from a global positioning system, and even a live video downlink. AC Propulsion claims that the SoLong could have remained flying indefinitely. See the press release, a detailed report on the craft, and a detailed description of the electric drive system on the AC Propulsion Web site.

    Illustration of a shiny silver airplane with long wings that carry two propellers and a teardrop-shaped cockpit,
from which a slender tube extends back to a tail assembly.

    The latest conceptual illustration of the Solar Impulse. Artist: Claudio Leonardi

    The news must be encouraging for Bertrand Piccard's much more ambitious Solar Impulse project, which aims to design and build a solar-powered airplane that will circumnavigate the world. First announced in 2003, the project is taking shape, as Solvay, Altran, and Dassault Aviation have signed on as financial and technical partners. The European Space Agency is also continuing to provide its expertise, which led to a model unveiled in mid-June. The craft will have a 262-foot wingspan and will weigh only two tons, carrying a single pilot. Piccard hopes to build the craft by 2007 and begin test flights the following year. The actual circumnavigation in 2010 will involve four- to five-day flights with stops on each continent to change pilots. See the press release, schedule, and flight plan on the Solar Impulse Web site.

    Of course, any attempt to cross new boundaries carries with it the risk of failure, as the leaders of the Planetary Society's solar sail project found out last week. The Cosmos 1 spacecraft was meant to test the concept of using thin films of reflective material to capture the solar wind as a means of propulsion in space. Unfortunately, according to the Russian space agency, the solar sail's launch vehicle failed, and the rocket and its spacecraft crashed into the Barents Sea. See the update from the Planetary Society.