NASA Pathfinder

Background

The Pathfinder originated from a secret 1983 project designated 'High Altitude Solar' (HALSOL), funded by an unspecified US government agency. The HALSOL prototype flew nine times at Groom Lake, Nevada, using radio control and battery power without solar cells.

After being stored for ten years, it was briefly reactivated in 1993 by the Ballistic Missile Defense Organization with small solar arrays, flying five low-altitude checkout flights at NASA Dryden. In 1994, the aircraft was transferred to NASA's ERAST program and renamed Pathfinder, reflecting its role as a pathfinder for future solar-powered UAVs. On October 21, 1995, the aircraft was severely damaged in a hangar accident but was subsequently rebuilt.

The Pathfinder's wing span was 98.4 ft, and it was powered by eight electric motors (later reduced to six). Two underwing pods housed landing gear, batteries, a triple-redundant instrumentation system, and dual-redundant flight control computers. Solar cells eventually covered the entire upper wing surface, providing power for motors, avionics, and communications, with a backup battery system enabling limited flight after dark. Pitch control used tiny elevators on the trailing edge, while turn and yaw control was achieved by varying motor speeds on the outboard wing sections.

On September 11, 1995, Pathfinder set an unofficial altitude record for solar-powered aircraft of 50,000 ft during a 12-hour flight from NASA Dryden. The National Aeronautic Association recognized the flight as 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 on Kauai, Hawaii. On July 7, 1997, it raised the altitude record for solar-powered and propeller-driven aircraft to 71,530 ft. During those flights, it carried lightweight imaging instruments to study terrestrial and coastal ecosystems.