The NASA Pathfinder, NASA Pathfinder Plus, NASA Centurion and NASA Helios Prototype were an evolutionary series of solar- and fuel-cell-system-powered unmanned aerial vehicles. AeroVironment, Inc. developed the vehicles under NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) program. They were built to develop the technologies that would allow long-term, high-altitude aircraft to serve as “atmospheric satellites”, to perform atmospheric research tasks as well as serve as communications platforms.
AeroVironment initiated its development of full-scale solar-powered aircraft with the Gossamer Penguin and Solar Challenger vehicles in the late 1970s and early 1980s, following the pioneering work of Robert Boucher, who built the first solar-powered flying models in 1974. Under ERAST, AeroVironment built four generations of long endurance unmanned aerial vehicles (UAVs), the first of which was the Pathfinder.
In 1983, AeroVironment obtained funding from an unspecified US government agency to secretly investigate a UAV concept designated “High Altitude Solar” or HALSOL. The HALSOL prototype first flew in June 1983. Nine HALSOL flights took place in the summer of 1983 at Groom Lake in Nevada. The flights were conducted using radio control and battery power, as the aircraft had not been fitted with solar cells. HALSOL’s aerodynamics were validated, but the investigation led to the conclusion that neither photovoltaic cell nor energy storage technology were mature enough to make the idea practical for the time being, and so HALSOL was put into storage.
In 1993, after ten years in storage, the aircraft was brought back to flight status for a brief mission by the Ballistic Missile Defense Organization (BMDO). With the addition of small solar arrays, five low-altitude checkout flights were flown under the BMDO program at NASA Dryden in the fall of 1993 and early 1994 on a combination of solar and battery power.
In 1994 the aircraft transferred to the NASA ERAST Program to develop science platform aircraft technology. It was renamed “Pathfinder” because it was “literally the pathfinder for a future fleet of solar-powered aircraft that could stay airborne for weeks or months on scientific sampling and imaging missions”. A series of flights were planned to demonstrate that an extremely light and fragile aircraft structure with a very high aspect ratio (the ratio between the wingspan and the wing chord) can successfully take-off and land from an airport and can be flown to extremely high altitudes (between 50,000 feet (15,000 m) and 80,000 feet (24,000 m)) propelled by the power of the sun. In addition, the ERAST Project also wanted to determine the feasibility of such a UAV for carrying instruments used in a variety of scientific studies.
On October 21, 1995, the aircraft’s fragility was aptly demonstrated when it was severely damaged in a hangar accident, but was subsequently rebuilt.
Pathfinder was powered by eight electric motors — later reduced to six — which were first powered by batteries. It had a wing span of 98.4 feet (30.0 m). Two underwing pods contain the landing gear, batteries, instrumentation system, and flight control computer. By the time the aircraft was adopted into the ERAST project in late 1993, solar cells were being added, eventually covering the entire upper surface of the wing. The solar arrays provide power for the aircraft’s electric motors, avionics, communications and other electronic systems. Pathfinder also had a backup battery system that can provide power for between two and five hours to allow limited-duration flight after dark.
Pathfinder flies at an airspeed of only 15 miles per hour (24 km/h) to 25 miles per hour (40 km/h). Although pitch control is maintained by the use of tiny elevators on the trailing edge of the wing, turns and yaw control are accomplished by slowing down or speeding up the motors on the outboard sections of the wing.
Flight Testing and Records
Major science activities of Pathfinder missions have included detection of forest nutrient status, forest regrowth after damage caused by Hurricane Iniki in 1992, sediment/algal concentrations in coastal waters and assessment of coral reef health. Science activities are coordinated by the NASA Ames Research Center and include researchers at the University of Hawaii and the University of California. Pathfinder flight tested two ERAST-developed scientific instruments, a high spectral resolution Digital Array Scanned Interferometer (DASI) and a high spatial resolution Airborne Real-Time Imaging System (ARTIS), both developed at Ames. These flights were conducted at altitudes between 22,000 feet (6,700 m) and 49,000 feet (15,000 m) in 1997.
On September 11, 1995, Pathfinder set an unofficial altitude record for solar powered aircraft of 50,000 feet (15,000 m) during a 12 hour flight from NASA Dryden. This and subsequent records claimed by NASA for Pathfinder remain unofficial, as they were not validated by the FAI, the internationally-recognized aviation world record sanctioning body. The National Aeronautic Association presented the NASA-industry ERAST team with an award for one of the “10 Most Memorable Record Flights” of 1995.
After further modifications, the aircraft was moved to the U.S. Navy’s Pacific Missile Range Facility (PMRF) on the Hawaiian island of Kauai. On one of seven flights there in the spring and summer of 1997, Pathfinder raised the altitude record for solar-powered aircraft — as well as propeller-driven aircraft — to 71,530 feet (21,800 m) on July 7, 1997. During those flights, Pathfinder carried two lightweight imaging instruments to learn more about the island’s terrestrial and coastal ecosystems, demonstrating the potential of such aircraft as platforms for scientific research.
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