The SR-71 is probably too small to carry a space-faring rocket, but the concept is reasonable: http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_air_l

The SR-71 is probably too small to carry a space-faring rocket, but the concept is reasonable: http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_air_launch http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_... This splits combined-cycle propulsion into multiple vehicles: air-breathing propulsion for the aircraft, chemical rocket for the spacecraft, thereby improving the efficiency of the whole launch.