Bell Aircraft X-1ABell X 1A airplane model. Ordered by the Air Force on 2 April 1948, the X 1A (serial number 48 1384) was intended to investigate aerodynamic phenomena at speeds greater than Mach 2 (681 m s, 2,451 km h) and altitudes greater than 90,000 ft (27 km), specifically emphasizing dynamic stability and air loads. Longer and heavier than the original X 1, with a stepped canopy for better vision, the X 1A was powered by the same Reaction Motors XLR 11 rocket