The X-15 is a strong contender for the title of “Everyone’s Favorite X-plane.” Conceived in the 1952-1954 period, before Sputnik (October 1957) and the birth of NASA (October 1958), the North American Aviation-built rocket plane was intended to pioneer the technologies and techniques of hypersonic flight - that is, of flight faster than Mach 4 (four times the speed of sound).

Between 1959 and 1968, three X-15 rocket planes, two modified B-52 bombers, and a dozen pilots took part in 199 joint U.S. Air Force/NASA X-15 research missions. Before the start of each mission, an X-15 was mounted on a pylon attached to the wing of a B-52 carrier aircraft at Edwards Air Force Base, California. Wearing a silver pressure suit, the rocket plane’s single pilot boarded the X-15 as it hung from the pylon, then the B-52 taxied and took off from a runway.

Early X-15 missions were “captive” flights, meaning that the rocket plane stayed attached to the B-52, or gliding flights, meaning that it carried no propellants. Early powered flights used stand-in rocket engines taken from earlier X-planes. By late 1960, however, the X-15′s throttleable 600,000-horsepower XLR99 rocket engine was ready. The engine was designed to burn the nine tons of anhydrous ammonia fuel and liquid oxygen oxidizer the X-15 carried in about 90 seconds at full throttle.

Most missions followed two basic profiles. “Speed” missions saw the rocket plane level off at about 101,000 feet and push for ever-higher Mach numbers. The X-15 reached its top speed - Mach 6.72, or about 4520 miles per hour - during its 188th flight (3 October 1967) with Air Force Major William “Pete” Knight at the controls.

More: The X-15 Rocket Plane: Implications for Reusable Booster Schedule & Cost (1966)