After the war, Yeager became a test pilot and flew many types of aircraft, including experimental rocket-powered aircraft for the National Advisory Committee for Aeronautics (NACA). Through the NACA program, he became the first human to officially break the sound barrier on October 14, 1947, when he flew the experimental Bell X-1 at Mach 1 at an altitude of , for which he won both the Collier and Mackay trophies in 1948. He then went on to break several other speed and altitude records in the following years.