Pathfinder-First-Generation ERAST Solar Program Test Vehicle (1994-1997)

The first-generation ERAST solar-test program HALE vehicle was the Pathfinder, which was designed and built by AeroVironment. AeroVironment’s earlier solar aircraft projects included the building of the piloted Gossamer Albatross and a scaled-down version known as the Gossamer Penguin. This experience assisted the company in building the Pathfinder UAV. In addition to Ray Morgan, who was vice president of AeroVironment, the company team included a number of experienced engineers and technicians, including William Parks, who was the com­pany’s chief engineer for the Centurion and Helios Prototype UAVs, and Robert Curtin and Kirk Flittie, who both served as project and later as program managers. The program brought honors for Bob Curtin and Ray Morgan, who both received the Aviation Week Laurel Award in 1996.[1536]

Pathfinder, which initially was battery-powered, was a remotely piloted flying wing that demonstrated a number of technologies, includ­ing lightweight composite structures, low wing loading flying-wing configuration, redundant and fault tolerant flight control systems,

high-efficiency electric motors, thermal control systems for high-alti­tude flight, and a high specific power solar array. The remotely piloted Pathfinder had 6 electric motors that each weighed 13 pounds and con­sisted of a fixed-pitch 79-inch propeller and a solid-state motor with inter­nal power electronics, nacelle, and cooling fins. Differential power to two wingtip motors on either side was used for lateral control. Wing dihedral (upsweep) provided roll stability, and 26 elevator control surfaces were attached to the wing’s trailing edge for pitch control. Pathfinder’s solar array generated approximately 8,000 watts near solar noon. The solar UAV could obtain an airspeed of between 15 and 25 mph and a cruising speed of between 17 to 20 mph. The vehicle had a length of 12 feet, a wingspan of 98.4 feet, a wing chord (front to rear distance) of 8 feet, a gross weight of approximately 560 pounds, a payload capacity of up to 100 pounds, a wing aspect ratio (the ratio between the wingspan and the wing chord) of 12 to 1, and a power-off glide ratio of 18 to 1. Pathfinder had a maxi­mum bank rate of 5 degrees and a maximum turn rate of 3 degrees per second at sea level and 1.7 degrees at 60,000 feet.[1537]

To gain some introductory understanding and experience with the challenges and nuances of solar cell operation, prior to the official start of the ERAST program and the transfer of Pathfinder from BMDO, the project team had arranged for some solar cell flight tests on local sorties over the Edwards dry lake. Pathfinder itself was not equipped with solar arrays on early test flights because the ERAST alliance did not want to risk any damage to the expensive solar cell arrays until Pathfinder’s fly­ing capabilities could be tested. Pathfinder’s gross weight of 560 pounds produced a very low wing-loading load distribution of less than 0.64 pounds per square foot that significantly increased sensitivity to winds during takeoff and landing. This necessitated special training for the ground controllers, especially during takeoff and landing of the UAV. Pathfinder’s first foray to high altitude took it to 50,500 feet and proved immensely productive. "We learned tons from that flight,” John Del Frate recalled afterward, noting, "There were a lot of naysayers that were qui­eted after that flight.”[1538] Unfortunately, afterward the vehicle dramati­cally demonstrated its sensitivity to wind, being seriously damaged in its hangar when ground crews opened both hangar doors, thus creat­ing a draft that blew Pathfinder into a jet that was in the same hangar.

After a number of developmental flights and further modifications at Dryden, Pathfinder was transported to the Island of Kauai, which offered a more favorable wind environment and a greater operational area with less competing air traffic. From testing at Edwards Air Force Base, the NASA-industry alliance learned that weather factors—including wind, turbulence, cloud cover, humidity, temperature, and pressure—were crit­ical in attempting to fly the wing-loaded Pathfinder at high altitudes. In addition, the team noted that the UAV s would probably not be flying in the same conditions as found in standard atmosphere reference tables because testing indicated a surprising variance in actual temperature in comparison with the tables. The team also noted that the higher a solar UAV flies, the greater the downwind drift distance if activation of a flight termination system (FTS) is required.[1539] These factors required careful study of historical weather patterns to determine the optimum

site to attempt to set a world record UAV altitude flight. Accordingly, NASA selected the Navy’s Pacific Missile Range Facility in Hawaii as the location to test the high-altitude capabilities of the solar UAVs.[1540]

In Hawaii, Pathfinder was flown for seven additional flights, one of which in 1997 set a world record of over 71,500 feet for a high-altitude flight by a propeller-driven aircraft. This broke a 1995 Pathfinder test alti­tude flight record of 50,500 feet, which had earned NASA recognition as 1 of the 10 most memorable record flights of 1995. Pathfinder test flights also demonstrated solar-powered HALE vehicles’ potential as platforms for environmental monitoring and technical demonstration missions by gaining additional information relating to the Island of Kauai’s terrestrial and coastal ecosystems. These science missions, which employed specially build lightweight sensor systems (see below), included detection of for­est nutrient status, forest regrowth after damage from Hurricane Iniki in 1992, sediment concentrations in coastal waters, and assessment of coral reef conditions. Experience from Pathfinder test flights, in combination with other UAV testing, also yielded a number of lessons regarding hard­ware reliability, including the following recommended procedures: (1) testing the airframe structure as much as possible before flight, particu­larly the composite airframe joint bondings; (2) testing the vehicle’s sys­tems in an altitude chamber because of the extreme cold and low-pressure conditions encountered by high-altitude science aircraft; (3) recognizing that UAVs, like aircraft, have a tendency to gain weight; (4) maintaining strict configuration control; and (5) ensuring that a redundant system is functional before switching from the primary system.[1541]