Three aircraft were modified from existing Lockheed F-104A airframes, and served with the Aerospace Research Pilots School between 1963 and 1971, the modifications included a small supplementary rocket engine and a reaction control system for flight in the upper atmosphere. During the test program, the maximum altitude reached was more than 120,000 feet (36,600 m). One of the aircraft was destroyed in an accident while being flown by Chuck Yeager.