The successful transcontinental flight of the Solar Impulse has drawn renewed interest in the use of renewable fuels for aviation. As we progress through this blog, we will trace the development of solar fuel technology, as well as explore its potential.
The first successful flight of a solar- powered aircraft was performed by the Sunrise, a small radio control aircraft weighing 27 lbs. with a 32 in. wingspan. The Sunrise was powered by 1,000 solar fuel cells located in both wings, producing approximately 450 watts of power, which gave the aircraft a surface ceiling of 20,000 ft. The Sunrise was built by Astro Flight, formed in 1969 to build a radio controlled sailplane for use in AMA certified competitions. The funding for the flight of the Sunrise in 1974 was the result of a government contract with Lockheed to begin research on solar powered aircraft, subcontracting the project to Astro. In 1979, DuPont sponsored a project led by Dr. Paul MacCready to develop a solar- powered plane capable of carrying a human. This effort resulted in an aircraft called the Gossamer Penguin, which first flew the following year. The Penguin used a 600 watt solar panel similar to the Sunrise, along with a production version Cobalt 40 motor. This aircraft could achieve passenger flights of short distances, which gave DuPont an incentive to build a solar plane capable of crossing the English Channel. It took three months to construct the Solar Challenger out of 16,128 cells. These cells yielded 2,500 watts at sea level with approximately 4,000 watts at cruise altitude. The Solar Challenger successfully crossed the English Channel in 1981.
While the early flights of solar aircraft were successful, interest declined during the 1980s. In 1993, a project known as Pathfinder was restored for the purpose detecting ballistic missile launches . The new project known as HALSOL (High Altitude SOLar aircraft) had both high- altitude and long- endurance capabilities. However, after several test flights in 1993 and 1994 the project was cancelled. That same year NASA established the Environmental Research Aircraft and Sensor Technology (ERAST) program, in which additional solar cells were added, covering most of the upper wing surface. After a series of tests were conducted in 1995, Pathfinder achieved an altitude of 50,500 ft. – a record for a solar-powered aircraft. In 1997, Pathfinder was tested at the U.S. Navy Ballistic Missile Test Center in Barking Sands, Hawaii. This site was selected due to its ideal climatic conditions from which to test the solar panels. After additional modifications, the Pathfinder was able to set two new world altitude records at 71,530 ft. and 80,201 ft. for both solar-powered and propeller-driven aircraft.
While the Solar Impulse is not the first plane to fly under solar power, nor the first aircraft of its kind to cross the United States, it represents a quantum leap over previous designs. The plane already holds three records for manned solar flight, altitude (30,300 ft.), length (26 hrs.) and distance (693 miles). The plane is capable of overnight flight with solar charged batteries. The carbon-fiber construction of the aircraft adds to its efficiency. Although the Impulse’s wings are as long as an Airbus A340, the weight of the aircraft is only 3,527 lbs. – less than the average sports car. With both improvements in batteries and solar panels over the last ten years, we may soon reach a point when solar-powered passenger service becomes a reality.