During the 1960s and early 1970s, a number of man-powered aircraft tried for the Kremer Prize, but all were doomed to failure. They were of conventional design, fitted with chain-driven propellers. Principally made of wood, they were too heavy and incapable of turning.
One man realized what was needed. Dr. Paul B. MacCready of AeroVironment Incorporated understood that neither a conventional design nor conventional thinking would work. The Kremer Prize would take an innovative aircraft, designed with no preconceived notions.
The result was the Gossamer Condor. It had a long, tapered wing. Like the Wright Flier, it had a canard and wing warping to provide control. The pilot rode in a gondola suspended below the wing. The Gossamer Condor was designed to fly at extremely low airspeeds. Dozens of times the plane crashed, once as it was approaching the second pylon and victory. Finally, on August 23, 1977, Bryan Allen peddled the Gossamer Condor around the course. Man had flown.[562]
With the Kremer Prize won, other frontiers beckoned. In 1980, AeroVironment was sponsored by DuPont to develop a solar-powered aircraft able to fly from Paris, France, to England. The first test aircraft, the Gossamer Penguin, was so fragile and unstable that it could be flown only in the early morning when the air was calm. Based on the results of these tests, the Solar Challenger was designed. This was a much smaller and more rugged aircraft. It could reach altitudes of over 12,000 feet. Power came from 16,128 solar cells, which covered the entire upper surface of the wing and stabilizer. Despite the large surface area, the array provided a maximum of only 2,600 watts — enough to power two hair driers. The Solar Challenger had a wingspan of 47 feet, but weighed just over 200 pounds. In July 1981, the Solar Challenger successfully completed a 163-mile flight from Paris to RAF Mansion, England, averaging a speed of about forty miles per hour at just over 11,000 feet.
With the success of the Solar Challenger, AeroVironment began studying the possibility of a high-altitude, solar-powered UAV. If provided with some type of energy storage system, it could fly "eternally" and remain at altitudes above 65,000 feet. Flight duration would ultimately be limited only by such factors as mechanical systems wear.[563]
The design of a solar-powered Gossamer-type aircraft flying at these altitudes faced a number of problems. One peculiarity was engine performance. At takeoff, the Solar Challenger could barely climb; at 11,000 feet, however, it could equal a Cessna light plane. A piston engine loses power as it climbs, due to the thinner air. In contrast, a solar-powered aircraft's climb performance increases as the solar cells "see" a brighter sun, and the colder air improves efficiency. Above 30,000 feet, however, problems start to appear. The air temperature becomes constant, so there is not a continued improvement in efficiency. At the same time, the plane faces an increasing power requirement. Because of the thinner air, the plane must fly faster for its wings to generate the same amount of lift.
Another demand was the need to store power. More than half the current generated by the solar cells would have to be stored in some type of battery to keep the plane aloft during darkness. The "eternal" solar-powered aircraft would need twice the collection area, with no increase in weight, over the Solar Challenger.[564]
The design would have to be extraordinarily light — one-half pound per square foot of wing area, the same weight as foam art board. In contrast, an eagle has a wing loading of four to six pounds per square foot of wing. The eternal aircraft would need a wing loading half that of the Solar Challenger (including the pilot). The Gossamer Penguin's structure was light enough, but was too fragile. Despite these problems, theoretical calculations convinced Ray Morgan, vice president at AeroVironment's Design Development Center, that such an airplane could be built.[565] This also opened Black possibilities.
By 1983, AeroVironment was able to attract government sponsorship from a "classified customer" to build a proof-of-concept test aircraft.
(Among the possible "customers" that have been suggested are the National Reconnaissance Office, the CIA, and the Naval Research Laboratory.) It was called "HALSOL," for high altitude solar. Unlike the other Gossamer-type aircraft, this was to be a Black airplane in the classic sense. The HALSOL was developed and flown in secret. Its existence was not to be revealed for another decade.[566]
The HALSOL design was a pure flying wing, with no rudders or canard.
It had a span of 98.4 feet. From front to back, the wing was eight feet wide; it was made of five, 20-foot-long segments joined together. The main wing spar was made of thin wall carbon fiber tubes. Attached to this were ribs of Styrofoam reinforced with Kevlar and spruce. The wing was covered with Mylar plastic. Despite what one might think, the wing was far from being weak. It was stressed for a +5/-3g load factor (greater than the U-2). The center segment had two gondolas that enclosed the landing gear.
The aircraft was powered by eight electric motors — two mounted on the center of the wing, two on each inboard wing segment, and one on each outer wing segment. Spreading them out along the full span of the wing distributed the load. The HALSOL propellers had a variable pitch to match the available load on the power source, in order to permit the maximum efficiency. For the test flights, they were powered by silver-zinc batteries.
The HALSOL was controlled by radio from the ground. To pitch up or down, an elevator on the center wing section's trailing edge was used. To make a turn, the outermost motor on one side was sped up, while the opposite motors were slowed down.
HALSOL could hardly be called a high-performance aircraft. It flew at twelve knots and had a never-exceed speed at low altitude of twenty-seven or twenty-eight knots. Above this speed, it would go into a nose-down tuck.
It was estimated a climb to 70,000 feet would take about six hours. Because it was a test vehicle, the HALSOL was designed to climb, rather than to remain at high altitudes.[567]
Total gross weight of the aircraft was about 410 pounds, with a payload of about 40 pounds.[568] The efforts to control weight led to creative thinking and some unusual solutions. The front wheel assembly on the two gondolas used dual baby-buggy wheels, while the main landing gear assembly had a sixteen-inch bicycle wheel.[569]
The HALSOL made its first flight in June of 1983.[570] Over the next two months, a total of nine flights were completed at Groom Lake. These lasted for thirty to sixty minutes and reached an altitude of 8,000 feet. Although the aircraft was proven to be aerodynamically and structurally sound, studies indicated that 1983-vintage solar cell technology was not efficient enough to permit very long, high-altitude flight. In particular, the solar technology lacked sufficient "power-density," the energy available per pound of the components. The HALSOL program was discontinued, and the aircraft was placed in storage.[571]
AeroVironment remained active in solar-energy research. In 1987 (four years after the HALSOL project ended), General Motors (GM) selected AeroVironment to develop the Sun Raycer car, which won. the first trans-Australia race for solar-powered vehicles. The following year, GM selected AeroVironment to develop the Impact, a battery-powered commuter car suitable for mass production. Both these electric car projects would have a major effect on the discontinued HALSOL project: by the end of the 1980s, lightweight solar cells, electric motors, and power-storage technology had advanced to the point that the original HALSOL concept became practical.[572]
563
AeroVironment Inc., "Backgrounder — Pathfinder Solar-Electrical Aircraft" (Simi Valley, Calif.: AeroVironment Inc., 1993).
566
Michael A. Dornheim, "Raptor/Pathfinder to Test High-Efficiency Propulsion,"
567
Dornheim, "Raptor/Pathfinder to Test High-Efficiency Propulsion," 60. The overall program to develop UAVs to carry our ballistic missile detection-interception systems was called "RAPTOR." This stood for "Responsive Aircraft Program for Theater OpeRations."
568
Ballistic Missile Defense Organization, " 'Pathfinder' Solar-Electric Unmanned Air Vehicle Data Sheet" (Washington, D.C.: BMDO, 1993).
569
Author's examination of the HALSOL-Pathfinder aircraft at the 1993 Edwards AFB Air Show.
570
Letter: Ballistic Missile Defense Organization to Curtis Peebles, reference FOIA Request 93-F-2470, February 16, 1994.
571
Brown, "The Eternal Airplane," 75; and David A. Fulghum, "Solar-Powered UAV to Fly at Edwards,"