At 4,520 mph, Mach 6.7, Maj. William J. Knight pushed the rebuilt X-15A-2 faster than any other flight in the X-15 program. He flew it on Oct. 3, 1967, with the airplane wearing its full ablative coating and carrying external propellant tanks, and the speed he reached still stands as the program’s highest.
The flight was the high-water mark of a research airplane that had already been rebuilt once, lengthened, and coated for exactly this kind of sustained hypersonic run. It also showed how close the design was running to its thermal limits.
The Air Force Test Center History Office’s account of the flight records that “Major William J. Knight flew the modified X-15A-2 to a speed of Mach 6.7 (4,520 mph).” The history office states that the aircraft “was flown with its full ablative coating and external fuel tanks,” and that the flight “marked the highest speed achieved in the X-15 program and remains the highest speed ever reached by a manned airplane.”
Three airplanes, 199 flights
The X-15 was a joint effort. The National Museum of the U.S. Air Force’s X-15A-2 fact sheet describes it as a U.S. Air Force, Navy and NASA project and notes that four of its 12 pilots were Air Force officers. “Three X-15s were built, and they made 199 flights between 1959-1968,” the museum states.
NASA’s X-15 program reference page dates those 199 flights from June 8, 1959, to Oct. 24, 1968, and says the program’s missions produced more than 765 research reports on hypersonic aerodynamics, the behavior of materials, and pilot performance. The rocket engine behind all of it was the Reaction Motors XLR-99, rated at more than 50,000 pounds of thrust, per the Air Force museum.
The program’s altitude mark belongs to a different flight. NASA’s reference page records it as an altitude of 354,200 feet, flown on Aug. 22, 1963, with NASA pilot Joseph Walker at the controls.
How the second X-15 became the X-15A-2
The record airplane began life as the program’s second X-15. After a 1962 accident in which pilot Jack McKay was injured, North American Aviation repaired the damaged aircraft and redesignated it the X-15A-2, according to a NASA history article by John Uri published Oct. 13, 2023. NASA’s reference page says the modified airplane was delivered to the agency in February 1964.
The changes were aimed squarely at speed. NASA’s history article says engineers extended the fuselage by more than two feet and added two external fuel tanks; the Air Force museum describes the lengthening as about 18 inches and the tanks as “large orange and white propellant tanks.” NASA’s reference page says the external tanks held liquid ammonia and liquid oxygen and provided an additional 60 seconds of engine burn.
Heat was the other problem. To protect the skin during sustained flight at Mach 6, engineers coated the X-15A-2 with an ablative material. The Air Force Test Center’s history note defines ablation as “the removal of material from the surface of an object by vaporization, chipping, or other erosive processes,” so the coating protects by wearing away. NASA’s reference page notes the airplane was then covered with a white sealant coat.
What Mach 6.7 did to the airframe
The record flight tested that protection past its margin. The Air Force Test Center history office records that a dummy ramjet mounted on the lower ventral stub stabilizer fell away during the flight because of severe heat damage to the fairing.
NASA’s history article describes the heating in more detail: hypersonic shock waves burned through the ablative material and exposed the airplane’s skin to 2,400 degrees, twice its design limit. NASA’s account calls Knight’s Mach 6.70 an unofficial speed record, while the Air Force Test Center describes it as the highest speed ever reached by a manned airplane.
The program flew its last mission in 1968. The Air Force museum’s fact sheet says it was delivered to the museum in 1969, and NASA’s history article states that it went on display at the Air Force Museum in Dayton, Ohio, where it still resides. The first X-15 went to the Smithsonian Institution in Washington in June 1969, per the same NASA article.
Top image: The North American X-15A-2 at the National Museum of the U.S. Air Force. Image Credit: Bubba73 — CC BY-SA 3.0 / Wikimedia Commons
This article was produced with the assistance of AI and reviewed by Combat Rated editors prior to publication.