The XLR-99 Pioneer rocket motor, manufactured by the Reaction Motors Division of the Thiokol Chemical Corporation, was the most powerful rocket engine ever installed on a piloted aircraft and was the most powerful man-rated (directly throttleable) rocket engine. This engine powered the fastest, highest flying aircraft ever built, the North American X-15. More than just a record setter, the X-15 performed scientific research which provided invaluable data toward the design of the Space Shuttle and advanced hypersonic flight.

Rated at 50,000 lb thrust, the engine had a single thrust chamber, fuel injector, gas generator, two-stage ignitor, turbopump, and variable controls. The fuels were anhydrous ammonia and liquid oxygen. This engine, Serial Number 108, powered a number of notable X-15 missions. For example, it was used on: NASA's first XLR99 mission (Flight No. 34); William Knight's (Flight No. 151), for which he became known as the "fastest man on earth" and; an "Astronaut Wings Flight" (Flight No. 153), a flight over 200,000 feet in which Joseph Engle earned his astronaut wings (even though the astronaut's vehicle was an aircraft).