A second one was lost in 1967, just minutes after its launch. The craft had taken a video of the horizon at the edge of space and began its descent to the world below. As the craft descended, it entered a hypersonic spin. Even though its pilot, Michael J. Adams, was able to recover the plane at 36,000 feet, it then went into an inverted dive at Mach 4.7. The plane broke up under the stress and Adams was killed.

Pilots who flew the X-15 to its highest altitudes were eventually given astronaut wings by the US Air Force, considering the craft broke the USAF threshold for the edge of space at 50 miles above the surface of the earth. The craft would also make faster and faster hypersonic flights until October 3, 1967, when William J. “Pete” Knight took the craft to its maximum speed of 4,520 miles per hour.

Aside from these two achievements, the X-15 also had a number of notable firsts, including being the first restartable, throttle-controlled and man-rated rocket engine. It also tested the first spaceflight stellar navigation system and advanced pressure suits. The X-15 program was a direct ancestor of the modern Space Shuttle program, and without it, many notable achievements would not have happened.

Image Credits: NASA