The biggest change was in the plane's power source. The original HALSOL was battery powered. (Only a few solar cells had been carried on the HALSOL to test the effect of wing flexing.) The modified aircraft would carry some two hundred square feet of new lighter-weight solar cells. They would cover about one-third of the wing and provide about 3.8 kilowatts of power. This was enough to fly on solar power alone after 9:30 A.M. In practice, however, the aircraft would fly on dual solar-battery power.[576]
The modification work was completed in the late summer of 1993 — a full decade after the HALSOL's last flight. Because of the extensive modifications, a new name was given to the aircraft. It was now called 'Pathfinder.'
It is important to note that, up to this time, there had been no hint that there was a Black Gossamer-type UAV or that it had been flown a decade before. The HALSOL-Pathfinder finally came out of the Black with an October 1, 1993, air force press release, which announced the aircraft's existence and described its history. It stated that the Pathfinder would make a series of low-altitude test flights at Edwards Air Force Base in October- November 1993. It was explained that the project had been declassified to allow use of commercially available technology and open discussion of technical ideas. At the same time, a photo of the original HALSOL was released.[577]
The Pathfinder's first flight was made on October 20, 1993. It lasted forty-one minutes and involved six trips around a 1.2-mile racetrack course on the Edwards lake bed. The top speed was sixteen knots, and the plane's altitude was limited to 200 feet. The Pathfinder was a majestic sight as it flew slowly above the tan lake bed like some huge transparent bird. As it flew, the wing tips arched up, forming a U-shape. The propellers made a humming noise as they spun. At least 60 percent of the Pathfinder's power was provided by the solar cells. The Pathfinder was controlled from the ground by Ray Morgan, who originally conceived of the project.[578]
The Pathfinder's public debut came on November 23, 1993, at the Edwards Air Force Base Air Show. Six weeks before, even its existence had been a secret; now, more than 200,000 people saw it on display. Never before had a Black airplane's first public showing come so soon after its existence had been revealed. The Pathfinder was in a roped-off area of the main hangar, and there were armed guards nearby. This was not due to any secrecy about the plane — people were kept back because the light in the hangar was enough to start the motors, and a spinning propeller might injure a spectator. The guards were to protect the other plane in the enclosure, a B-2 Spirit.[579]
The October-November flight-test series pushed the Pathfinder to the edge of its low-altitude flight envelope and measured the stability and performance of the modified aircraft. The data collected was used to develop autopilot software to allow the aircraft to operate independently of constant ground control.[580]
Following the first flight tests, the Pathfinder was returned to AeroVironment for modifications that would enable it to undertake high-altitude flights. Control of the program was also transferred from BMDO to NASA due to budget reasons. Rather than detecting Scud launches, it would be used for atmospheric research.[581]
While at AeroVironment, about 70 percent of the wing surface was covered with solar cells. (For aerodynamic reasons, the wing's leading edge cannot be covered.) These cells were lighter than those used for the first flights.
The high-altitude flights were seen as critical to demonstrate the feasibility of the Pathfinder concept. It was planned that the tests would run from late August through mid-October 1994 at Edwards.[582] The addition of the solar cells and the structural modifications took longer than expected, however, and by the time the Pathfinder was ready, it was late in the flight season. With the sun lower in the sky, it was felt it would be better to wait for more favorable conditions.[583]
The high-altitude tests were rescheduled for April-July 1995. As the Pathfinder lacks an energy-storage system, the flights would be made during daylight hours. The plane would take off at dawn and climb all day.
Peak altitude would be between 60,000 and 65,000 feet. The Pathfinder's flight characteristics at high altitudes are very different than the heavier and faster-flying U-2s. At altitude, the U-2s' minimum and maximum speeds, limited by wing flutter, provide only a small margin for flight. With the lighter and slower Pathfinder, both speeds are considerably slower and therefore much less susceptible to structural failure. This is important, as tight limits would be very demanding for the automatic control system.
The estimated cruising speed at altitude would be around 100 knots. [584] At sunset, electrical power would be lost from the solar cells, and the Pathfinder would start down. Due to the plane's high glide ratio, it would not land until about 2:00 A.M.
Following the high-altitude tests, long-duration flights could be attempted. The Pathfinder could take off from Alaska and fly over the North Pole for weeks at a time to measure ozone levels. During the summer, there is nearly continuous sunlight over the pole, so the Pathfinder would not need any energy storage system. The continuous flight time could be 2,000 to 3,000 hours, or 80 to 120 days aloft.[585] The Pathfinder's solar array would be equipped with double-sided cells. These would use the light reflected from clouds, the ice cap, and the atmosphere to produce power during the evening hours, when the sun is low. The plane would also have a set of small computers, gyroscopes, and a four-antenna satellite navigation system to allow automatic flight.[586]
The HALSOL-Pathfinder was designed to act as a technology demonstrator for the eternal aircraft, which would be a much larger and heavier aircraft. Currently called 'Helios' or solar rechargeable aircraft (SRA), it would have a wingspan of 200 feet, made up of five, 40-foot-long segments.
There would be four landing gear gondolas. The solar array would produce 21.6 kilowatts, powering the eight motors — the same motors as were used on Pathfinder. The Helios-SRA would weigh about 1,100 to 1,200 pounds, and have a payload of 150 to 200 pounds. Its operational ceiling is planned to be about 60,000 to 65,000 feet; top speed would be around 100 knots.
Unlike the HALSOL-Pathfinder, the Helios-SRA would be optimized for high-altitude cruise, rather than climb.[587]
To allow around-the-clock flights, the Helios-SRA required an energy-storage system — a proton-exchange membrane fuel cell. Excess electrical current from the solar array would electrolyze water into hydrogen and
In the post-Cold War political environment, attention is shifting to dual-use technology — military systems that can also have civilian applications.
Pathfinder and Helios-SRA are prime examples. The ability to reach high altitudes, then stay there for months at a time, opens numerous possibilities in the area of atmospheric research. Sporadic, one-shot measurements of atmospheric conditions cannot give the needed baseline data to detect changes. To distinguish normal atmospheric variations from changes caused by natural and man-made sources, it is necessary to take measurements throughout the day-night cycle.
The Pathfinder, for example, could be flown continuously over the Yukon Valley in Alaska during the summer. It could monitor the flow of air pollution from Russia as well as the effect of local volcanoes. The data would be provided at very low cost, compared to satellites. Another advantage is that the Pathfinder's electric motors give off no exhaust emissions.
It could carry sensitive chemical detectors without their measurements being contaminated by the plane itself.
Other research projects would utilize the Helios and its ability to fly continuously through the night. One possibility would be storm tracking.
Major storms develop in the subtropical areas of the Atlantic and Pacific, then move toward North America
