How Many X-15s Were Built?

The North American X-15 experimental rocket-powered aircraft were used for high-speed and high-altitude research flights in the 1950s and 1960s. But how many were made?

Tobias Holm

29 Jun 2023

Few aircraft have captured the imagination, like the North American X-15. A marvel of engineering and a testament to human ingenuity, the X-15 was a groundbreaking experimental aircraft that pushed the boundaries of flight.

But just how many of these incredible machines were built? Let's take a look at the amazing aircraft's history.

About the North American X-15

The iconic North American X-15 was a rocket-powered aircraft capable of hypersonic flight, built by North American Aviation in the 1950s.

During the 1960s, NASA and the United States Air Force used the X-15 for research, and the aircraft and pilots returned valuable data for future aircraft and spacecraft designs. During these years, the X-15 also set numerous speed and altitude records, and the plane reached the edge of space.

On 3 October 1967, pilot William J. "Pete" Knight took the X-15 on its 188th flight and set the world speed record when he reached Mach 6.7 (4,520 mph / 7,274 km/h) at an altitude of 102,100 feet (19.34 miles / 31,120 m). This record for the highest speed achieved by a powered and crewed aircraft still stands today.

The X-15's first flight happened on 8 June 1959 and was an unpowered glide flight piloted by Scott Crossfield, who also piloted the first powered X-15 flight in September same year.

Among the 12 X-15 pilots was Neil Armstrong, who later became the first man on the moon.

Three X-15s were built in total — A Boeing B-52 Stratofortress flies over an X-15 as the crew secures the craft after a flight. Modified B-52s were used to launch the X-15.

How Many X-15s Were Built?

The North American X-15 was an experimental aircraft, and only three X-15s were built.

These three rocket aircraft flew a total of 199 test flights. The final flight happened on 24 October 1968.

In 1954, Walter Dornberger made a concept study for a hypersonic research aircraft for the National Advisory Committee for Aeronautics (NACA), the predecessor of NASA. This design study later became the X-15.

Two major contractors built the X-15. North American Aviation developed and constructed the airframe, and Reaction Motors was contracted to build the X-15's rocket engine.

A full-scale mockup of the X-15 was completed in November 1956 for NACA and the Air Force to inspect. Construction of the three X-15s was scheduled to take about two years after design approval. On 15 October 1958, the first X-15 rolled out of the factory.

The X-15 was unconventional in several ways. It was designed to be carried under the wing of a modified Boeing B-52 and launched from there after being released. This would happen at an altitude of about 45.000 feet. North American Aviation made the aircraft's skin out of a special heat-resistant nickel alloy called Inconel-X to resist the high amount of heat generated from the air's friction at hypersonic speeds. The rocket engine only operated for a relatively short time but still managed to push the X-15 to hypersonic speeds and high altitudes.

The X-15 performed high speed flight research — A mockup of the North American X-15 at Edwards Air Force Base. Photo: Wikimedia.

The X-15 program ended on 20 December 1968 when the 200th flight was canceled due to bad weather. The year before, the program's only fatal accident occurred when pilot Michael J. Adams was killed on Flight 191 on 15 November 1967. His X-15 entered a high-speed spin while descending, and the g-forces broke apart the airframe.

Today the X-15s are at museums around the US. Plane number one is at the Washington, DC's Smithsonian Air and Space Museum. The second plane is in Dayton, Ohio, at the National Museum of the US Air Force. The wreckage of X-15 number three was buried at Edwards Air Force Base.

Conclusion

The hypersonic North American X-15 rocket aircraft was an experimental aircraft built for research into hypersonic flight. Its 12 brave pilots took the groundbreaking aircraft to record-setting speeds and altitudes.

Three North American X-15 aircraft were built in total. What started as a design study for the predecessor to NASA became an iconic aircraft capable of speeds never seen before. Or since.