Solar Impulse: A Sun-Powered Plane to Fly Through the Night

The first milestone in manned solar aviation was realized in 1981 when the late Dr. Paul MacCready’s Solar Challenger successfully crossed the English Channel running only on energy from the sun. More recently, interest in unmanned solar aircraft was fueled by the realization that high-altitude sun-powered airplanes could stay aloft for months and perform the same functions as communications safelights at a fraction of the cost. To meet this mission, NASA made dramatic headway in the unmanned solar aviation arena with the Pathfinder Aircraft, co-designed by Dr. MacCready’s company, AeroVironment.

Nonetheless, no aviation technology is considered truly mature until it has passed the ultimate test: manned circumnavigation of the Earth. The first aerial circumnavigation occurred in 1924, ushering in the age of propeller-driven aircraft. In 1957, jet aircraft came to dominate aviation as three B-52s circumnavigated the globe; with four mid-air refuelings they accomplished the feat without landing. The first helicopter circumnavigation was achieved in 1982, and the first balloon circumnavigation occurred in 1999.

As reported in Mechanical Engineering Magazine’s “The Sun Also Raises,” the deck is stacked against manned circumnavigation by a solar airplane. Since keeping pace with the sun is impossible, the greatest challenge is storing enough energy during the day to continue flight through the night. Until an all-night manned solar airplane flight is demonstrated, circumnavigation (and true maturity of this aviation technology) will remain elusive. The major impediment is keeping aloft systems to collect and store energy – these components represent substantial parasitic weight that does not encumber a conventional aircraft. Either batteries or some version of AeroVironment’s fuel cell system for solar-powered airplane will be needed, but at the moment, these technologies are too heavy to remain airborne overnight in a manned aircraft. As reported in the recent Design News article, “Composites Develop Dramatically with Solar Impulse”, significant advances in composite material technology are being employed to close the weight gap.

To develop technologies needed to meet the challenges of manned solar aviation, two aerospace entrepreneurs, Bertrand Piccard and André Borschberg, founded a company called Solar Impulse. Their goal is to circumnavigate the Earth in a solar powered airplane by 2011. Trip planning has already begun. The company’s Web site contains an Indiana-Jones-style animation showing the proposed circumnavigation path of the Solar Impulse airplane.

This year, Solar Impulse plans to build a 60-meter-wingspan prototype aircraft to demonstrate key technologies. While this plane will not be capable of an around-the-world flight, it will demonstrate the feasibility of an all-night manned flight by a solar airplane, the key enabling feat for the 2011 circumnavigation attempt.

If Solar Impulse is successful, the final aviation frontier will be circumnavigation of the globe in an autogyro.