A NASA series of experimental aircraft and spaceplanes that acted as prototypes for operational vehicles. The series started in 1946, and the X-1 flew 49 missions before breaking the sound barrier on 14 October 1947. In 1962, the X-15 was the first plane to reach space, and was flown at a record atmospheric speed of Mach 6.7 in 1967. The X-15 served as an important test bed for Apollo project and space shuttle technology. The X-23 and X-24 acted as space shuttle aerodynamic prototypes in 1969–75. The X-38 was a flight demonstrator model for the International Space Station's crew return vehicle, built in 1997. Most of the later X-series planes are for aeronautical use.