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Energy from the sun is used to provide clean, renewable power for homes and businesses, but with the development of solar aircraft, the concept of renewable energy is reaching new heights. Solar aircraft have been around since the 1970s, but they have generally flown under the radar. While solar-powered, commercial flights may be a distant dream, the use of the technology for unmanned surveillance, research, and telecommunications aircraft may be close to reality.
Solar Aircraft DesignThe major components of a solar aircraft are not new; solar cells, motors, and batteries have been around for decades, but designing an aircraft and integrating its components for safe, reliable, long-term flight based on power from the sun is state-of-the-art technology.
Solar aircraft do not carry much on board. Power is limited and there is a strong emphasis on getting rid of as much weight as possible to maximize altitude and flight time. Solar aircraft are typically built from light-weight composite materials, such as carbon fiber. Solar cells— typically much thinner and more flexible than those found on ground-based solar panels—are attached to the wings. The aircraft contains sensors that record light, voltage, and wind data, which is communicated to the pilot. Typically, the planes are powered by motors that use rechargeable lithium batteries to provide power at night.
Solar aircraft may or may not contain landing gear. "Some solar airplanes basically drop off the landing gear in flight because you're not going to need them. The plane may land on skids or crash-land. Engineers are getting rid of every bit of weight you can possibly imagine," says John Del Frate, an engineer at NASA's Dryden Flight Research Center. (Nasr 2009)Pioneering Solar FlightsThe first flight of a solar-powered aircraft took place in November 1974. The Sunrise I, developed by Astro Flight, Inc., was constructed almost entirely of wood. This remote-controlled plane had a 32-foot wingspan, and was powered by an array of 4,096 solar cells on its wings. (Boucher 1984)
The Gossamer Penguin, designed and built by AeroEnvironment, Inc., was the first solar plane piloted by a human. It had a 71-foot wingspan and weighed only 68 pounds. The plane’s power source consisted of 28 nickel-cadmium batteries, which were replaced by 3,920 solar cells that produced 540 watts of power during flight. On August 7, 1980, the plane was flown approximately two miles in a public demonstration that lasted 14 minutes. (NASA 2002) The plane's pilot, Janice Brown, was a school teacher and licensed pilot who weighed less than 100 pounds.
After this historic flight, the Gossamer design team built the Solar Challenger, a larger and more powerful solar aircraft. On July 7, 1981, the Challenger became the first manned, solar aircraft to cross the English Channel, averaging 50 miles-per-hour and reaching an altitude of 12,000 feet.
In 1990, the Sunseeker, a solar glider developed by Eric Raymond, became the first manned solar airplane to successfully cross North America. In August of that year, Raymond successfully guided the plane from California to Kitty Hawk, North Carolina with a series of more than 21 flights and 121 hours of total air time.NASA ResearchIn the 1980s the National Aeronautics and Space Administration's (NASA) Environmental Research Aircraft and Sensor Technology (ERAST) program began exploring unmanned, solar-powered flights as part of an effort to develop technologies that would allow long-term, high-altitude aircraft to serve as atmospheric satellites, performing research while serving as communications platforms.
The Helios prototype model, developed in 1999, was the culmination of this research. The Helios had a total wingspan of 247 feet—longer than the wingspan of a Boeing 747—and weighed more than 1,300 pounds. The all-wing aircraft was powered by 14 two-horsepower motors, six lithium battery packs, and more than 62,000 solar cells installed on the wings.
In August 2001, the Helios reached an altitude of 96,863 feet; a world-record altitude for a non-rocket powered aircraft. (Curry 2009) The Helios was lost in June 2003, when it failed in mid-flight and crashed into the Pacific Ocean. An investigation following the incident determined that the crash occurred as a result of an inability to predict the "aircraft's increased sensitivity to atmospheric disturbances such as turbulence." (Noll 2004)Continuing DevelopmentsDespite the setback of the Helios aircraft, research and development in solar-powered flights continues.
In July 2010, the Solar Impulse HB-SIA, a glider-like craft, completed the first night flight of a plane powered exclusively by solar energy; it flew for more than 26 hours. Weighing 3,500 pounds (about as much as a mid-sized car) the plane is powered by 12,000 solar cells, which are built into its 193-foot wingspan. Over the next two years, the developers of the Solar Impulse hope to achieve solar-powered flights lasting multiple days and a flight around the world. This goal came closer to reality on June 5, 2012 when the Solar Impulse, finishing a 1,554-mile trek from Switzerland to Morocco, became the first solar airplane to complete an intercontinental flight.
The Defense Advanced Research Projects Agency’s (DARPA) Vulture program has the goal of developing unmanned aircraft that can stay aloft for five years. The Zephyr, a solar-powered aircraft, has received funding under this program. In July of 2010, the Zephyr set a new world record, flying non-stop for more than 14 days without refueling. Another aircraft in the program is the Odysseus, which uses a modular shuttling design. This includes three unmanned vehicles that can dock and separate in the air; simplifying take-off and landing procedures.
With continuing advances in lighter energy-storage technology and more efficient solar arrays, the future looks bright for solar flight in surveillance, communications, and other applications.ReferencesBoucher, Robert J. History of Solar Flight. AIAA Paper 84-1429, Presented at the AIAA/SAE/ASME 20th Joint Propulsion Conference. Cincinnati, OH. June 1984.
Curry, Marty. Helios Prototype: The Forerunner of 21st Century Solar Powered Atmospheric Satellites. NASA Dryden Flight Research Center. December 1, 2009.
Nasr, Susan L. "How Solar Aircraft Work." July 7, 2009. HowStuffWorks.com. http://science.howstuffworks.com/environmental/green-tech/sustainable/solar-aircraft.htm April 20, 2012.
Noll, Thomas. Investigation of the Helios Prototype Aircraft Mishap. National Aeronautics and Space Administration. January 2004.