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Solar Aircraft

The first thing to understand about solar aircraft is that they are really electric aircraft that have their electricity delivered through photovoltaic cells. In the world of aircraft, there are electric motors and internal combustion engines (ICEs). ICEs can power either jet propulsion systems or propellers. Electric motors currently only power propellers.

A number of alternative electric aircraft propulsion systems are in research and development stages throughout the world. In one case, regenerative charging using vertical updrafts have been proposed for electric aircraft that are in flight. These updrafts can provide 10 times the amount of energy as solar at altitude, making solar necessary only for takeoff and landing. At this time, all solar aircraft are considered experimental.

AstroFlight Sunrise

The AstroFlight Sunrise was the first aircraft to fly on solar power on November 4, 1974 in the United States. It was built by Roland Boucher who was an engineer for Hughes Aircraft in 1970. The plane used “off-the-shelf” commercial solar panels that were 10% efficient.

The plane was unmanned and operated at an altitude of 22,250 m (73,000 feet). During night, the aircraft slowly descended from 22,500 m to 3,000 m as it did not carry any batteries. This allowed the plane to remain in flight 24 hours a day, descending at night and ascending again at dawn. It had a 10 m wingspan and weighed only 10 kg. It was constructed mostly of spruce, balsa, and maple woods. It ran on a single Samarium-Cobalt magnet electric motor that provided 0.45 kW (0.6 hp) of energy and was capable of speeds of only 22 km/h (14 mph).

Solar Riser

After the AstroFlight Sunrise can manned solar flight in 1979. The system used a Hughes 500 helicopter battery that powered an electric motor made by Bosch. 1.5 hours of solar charging through its 350 watts of solar cells allowed the engine to operate for a maximum of 5 minutes (usually 3 minutes). This was enough time to reach a gliding altitude.

The Solar Riser was capable of speeds up to 32 km/h (20 mph) and had a maximum ceiling of only 12 m (40 feet). So, while it wasn’t glamorous or practical, it was the first manned solar flight during its inaugural 0.8 km (0.5 mile) flight.

Solar One

The Solar Riser was followed by the British, mid-wing aircraft called the Solar One. Though originally designed for human-powered flight, it was converted to solar when peddling proved an impossibly heavy way of powering the aircraft. Its first flight was at Lasham Airfield in the United Kingdom where it flew at 78 km/h at a height of 24 m (80 feet).

The plane carried a single passenger and was powered by 4 Bosch permanent magnet motors. It required 750 solar cells to charge the battery prior to flight. The solar cells were not numerous enough to support flight on their own.

Solar Challenger

The Solar Challenger was built by the U.S. company called AeroVironment in 1981. Unlike the Solar One, this plane was powered entirely by solar cells capable of sustaining flight and had no batteries at all. Its first successful flight was 262 km (163 miles) across the English Channel from Pontoise – Cormeilles Aerodrome in France to Manston Royal Air Force Base in England. The light took 5 hours 23 minutes.

To sustain flight, the Solar Challenger required over 16,000 solar cells for its 2 samarium-cobalt electric motors. Each motor produced 2.2 kW (3 hp). Exotic materials like Kevalar, Teflon, and Mylar were used in the construction of the body of the aircraft, giving it high strength to weight ratios. The materials were all supplied by DuPont. Its top speed was 64 km/h (40 mph) and its maximum altitude was 4,270 m (14,000 feet).

Solair 1 and Solair II

Both of these aircraft were developed by Gunther Rochelt in Germany. The Solair 1 used 2499 solar cells to produce a maximum of 2.2 kW (3 hp) of power. The plane flew using solar power and thermals for 5 hours 41 minutes in 1983. The Solair II was halted in developed after Rochelt died in September 1998.


In 1994, the Environmental Research and Sensor Technology (ERAST) program was begun at NASA to develop an unmanned aerial vehicle (UAV) for long-duration scientific studies at altitudes greater than 60,000 feet. The aircraft was to be slow-flying and inexpensive (at least by NASA standards). Four aircraft were born from the project, two internal combustion models and two solar-powered electric models.

The internal combustion models were called the ALTUS and ALTUS II. Both were civilian variants of the MQ-1 Predator drone. The ALTUS II set records for flight above 55,000 feet by an UAV by flying for over 3 hours at that altitude. Its longest flight was more than 24 hours straight.


Pathfinder was building in 1993 based on earlier work on a plane called HALSOL 10 years earlier. Though HALSOL was capable of battery-powered flight, neither fuel cells nor solar panels were advanced enough at the time to provide prolonged power to the aircraft.

In 1993, the HALSOL was used for a brief mission that included the addition of a limited number of solar arrays. After the mission, the aircraft was transferred to the ERAST program where it was renamed Pathfinder and additional solar arrays were added. It became the test plane for solar-powered unmanned flight at NASA and flew to altitudes as high as 24,000 m (80,000 feet). Top speed was 40 km/h (25 mph).

Pathfinder was powered by 8 electric motors in its original configuration. The number was reduced to 6 at a later date. Solar cells covered the entire 30 m (98.4 feet) span of the wings, providing power for daytime flights by producing 7,500 watts at 14% efficiency conversion. Back-up battery power provided 3 to 5 hours of night-time flight.

In 1998, Pathfinder was modified to increase wingspan and was subsequently dubbed the Pathfinder-Plus. Overall wingspan on the Plus reached 37 m (121 feet). Newer, more efficient (19%) solar panels were also added that increased power generation to 12,500 watts. The number of motors was increased to once again be eight. The Plus reached a record altitude of 24,445 m (80,201 feet).


Using the Pathfinder and Pathfinder-Plus as starting points, NASA aimed to produce a solar UAV that could reach an altitude of 30,000 m (100,000 feet). The first prototype was called the Centurion, which proved the functionality and aerodynamics of the design using battery power.

Following the Centurion, the Helios prototype was launched. The Helios had an additional wing section to bring the total wingspan to 75 m (247 feet). This made plane wider than the C-5 military transport at 68 m (22 feet) and the Boeing 747 at 66 m (195 feet). The total area on the wings is 186.3 square meters (1,976 square feet).

The Helios reached a top height of 29,524 m (96,863 feet) on August 13, 2001, breaking the record of 85,000 feet set in 1976 by the SR-71. Two years later on June 26, the Helios broke apart during a test flight off the Island of Kauai. The alterations to the wingspan were at least partly to blame. The Helios was never repaired because the ERAST project was canceled in 2003.


The Sunseeker became the first manned solar aircraft to cross the United States in 1990. It used a solar powered battery to drive a small propeller and took advantage of soaring and thermals when possible. In 2002 it was upgraded and in 2009 became the first solar plane to cross the Alps. It is the only manned solar aircraft in operation at this date.

Solar Impulse

The Solar Impulse is an ambitious project started by the Swiss Ecole Polytechnique Federale de Lausanne with the goal of circling the Earth on solar power only. In 2010, the Solar Impulse flew for 26 hours to demonstrate its diurnal (day-night) capabilities. The goal is to circumnavigate in 20 – 25 days. The flight is planned for 2013.
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