Still the fastest airplane ever flown, the North American X-15 earned its title 40 years ago, when on October 3, 1967 Air Force Major William “Pete” Knight flew the rocket-powered aircraft to 4,520 mph, Mach 6.72. It was built to find out how aircraft structures, materials, and control surfaces would perform at hypersonic speeds and very high altitudes. In 199 research flights, the X-15 provided that information and more. The program has been acknowledged as the most successful flight research program in history, and it helped make human spaceflight possible.

X-15 flights were short; each lasted about ten minutes. So that the rocketplane could use all its fuel for acceleration, it was carried beneath the wing of a NASA B-52 to 45,000 feet, where it was dropped. NASA test pilot Milt Thompson remembered the experience in his book At the Edge of Space: “[The launch] was a surprise no matter how many times I went through it. It felt as if the X-15 exploded off the hooks.…

“The pilot did not have much time to waste after launch. He either had to get the engine lit or abort the flight and make a landing at the launch lake. The problem was that he was losing altitude rapidly (about 12,000 feet per minute) while waiting for the engine to light.”

Joe Walker reached 354,200 feet, an altitude not exceeded until 2004, when Brian Binnie reached 367,442 feet in SpaceShipOne. The pilots would aim for an altitude but were rarely able to predict exactly how high they would end up. The X-15 climbed at 4,000 feet per second, so if the pilot was even one second late (or early) in shutting down the engine, he would miss the mark by 4,000 feet.

The powered portion of an X-15 flight lasted approximately 85 seconds. Each mission ended in an unpowered landing after a steep descent. The pilots flew a low-lift glide that helped develop the energy management techniques space shuttle pilots would use many years later. Only one X-15 pilot did not make it back for a landing. Air Force Major Michael Adams was killed in 1967 when the aircraft broke apart in a hypersonic spin shortly after reentry. The program ended the following year.