The X-15 was also a milestone in the development of flight control systems -- in some phases of flight, the aircraft was too unstable to be easily controlled by

The X-15 was also a milestone in the development of flight control systems -- in some phases of flight, the aircraft was too unstable to be easily controlled by a human pilot, and on most flights, it had machinery massaging the pilot's control inputs. "Digital Apollo", by David Mindell, has a chapter on this, before it gets into the space program proper.