Introduction to Space Exploration - The X Series

Even before World War II ended the United States began developing rocket-powered planes. In 1943 the NACA initiated the research program in conjunction with the Air Force and Navy. Because the planes were experimental, they were given the name X-aircraft. In 1944 a company called Bell Aircraft began work on the X-1. At first it was called the XS-1, with the "S" standing for supersonic. Later the "S" was dropped from the name.

On October 14, 1947, a young Air Force captain named Charles (Chuck) Yeager (1923–) flew the X-1 at the speed of sound, which is known as Mach 1. The X-1 was only the first of many high-performance planes tested in the program. Eventually X planes flew at hypersonic speeds, that is, speeds greater than Mach 5. The X-15 was a rocket-fueled plane tested during the late 1950s and early 1960s. It was taken up to an altitude of approximately 45,000 feet by a carrier plane, a B-52 aircraft, and released. A rocket engine was then fired to propel the X-15 to incredible speeds and heights. An X-15 flew at Mach 6.04, the fastest suborbital speed ever reached, on November 9, 1961. On August 22, 1963, an X-15 soared across the boundary into space to an altitude of sixty-seven miles. This record would remain unbroken for more than four decades.

The X-series were high-speed, high-altitude planes unlike any ever built before. Most of them were tested over desolate desert areas near Muroc, California. Daring young test pilots flew the X-series planes. Unfortunately this was a very dangerous profession. Numerous pilots were killed or seriously injured while testing X-series planes. The pilots who survived became the first men considered for the nation's astronaut program.