A mission for such an eternal aircraft had also appeared. An aircraft like the HALSOL could be used to detect missile launches, such as the Scud ballistic missiles Iraq fired against Israel and Saudi Arabia during the Gulf War. This was seen as a preview of future regional conflicts. Scud missiles had been exported and were in production throughout the Third World.
Both Iran and North Korea were active in this area, as well as having ongoing chemical, biological, and nuclear-weapons programs.[573]
With the technology now available and a military need, the HALSOL was taken out of storage in early 1992. Under the direction of the Ballistic Missile Defense Organization (BMDO), the successor to the Strategic Defense Initiative, AeroVironment began a modification program. The basic airframe was retained, with the addition of new systems. One example was the motor-propeller system. The original variable pitch props were replaced with fixed props, with an electronic "peak-power tracker." Removing the original propeller system reduced the number of parts and increased reliability.[574] The original rare-earth DC motors were replaced with brushless AC motors, which also improved reliability and efficiency. The motors also had new custom-designed inverters to improve efficiency. Rows of cooling fins were added behind the propellers to radiate heat. Keeping the motor's temperature within limits while flying at high altitude was a problem due to the thin air. The complete motor and propeller assembly weighed only thirteen pounds.
In addition, the control surfaces were modified. The original HALSOL had only one elevator powered by four servos. In the new version, twenty-six elevators ran the full span of the wing's trailing edge.[575]
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]
573
Ballistic Missile Defense Organization, "Pathfinder Questions and Answers" (Washington, D.C.: BMDO, 1993).
577
United States Air Force Press Release "BMDO to Display HALE" (Washington, D.C.: BMDO, October 1, 1993); and "Pathfinder Questions and Answers."
578
Ballistic Missile Defense Organization, "Solar-Powered Pathfinder Soars on First Test Flight" (Washington, D.C.: BMDO, 1993); and NASA videotape of first Pathfinder flight, 1993 Edwards AFB Air Show.
580
Fulghum, "Solar-Powered UAV to Fly at Edwards," 27; and Brown, "The Eternal Airplane," 100.
581
Interview with Lt. Col. Dale Tietz, director of the Raptor/Talon program, April 5, 1994.
583
David A. Fulghum, "Scud-Killing UAVs Now Science Tool,"