AFRL Century Series – Chapter 10: SPEED
Air Force pilots routinely fly faster than the speed of sound, but there was a time when it was considered impossible. Then in 1947, the Air Force broke the sound barrier for the first time in manned flight with its experimental Bell X-1 aircraft. Higher altitudes and greater speeds were also achieved with the X-2 research airplane.
The X-15 hypersonic rocket-powered aircraft set speed and altitude records during the 1960s that remain unbroken to this day. It reached the edge of space and its research flights led to technology deployed in the Project Mercury tracking stations, Apollo spacecraft, and the space shuttle.
The X-51 is designed as a missile-size demonstration aircraft to showcase scramjet operation from Mach 4.5 to Mach 6. It flew four missions with the final finding it travelling more than 230 nautical miles reaching a peak speed of Mach 5.1. Overall the more than 9 minutes of data collected from the X-51A program was an unprecedented achievement proving the viability of air-breathing, high-speed scramjet propulsion using hydrocarbon fuel.