The X-15 was a rocket-powered aircraft operated by the United States Air Force (USAF) and the National Aeronautics and Space Administration (NASA) during the early 1960’s. The X-15 set speed and altitude records reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. Another version of the X-15 still holds the official world record for the highest speed ever reached by a manned aircraft. Its maximum speed was 4,520 miles per hour (7,274 km/h) or Mach 6.72 meaning it went 6.72 times the speed of sound.
A larger version of the X-15 was considered for the first United States (US) orbital flight. I sometimes wonder if NASA had gone in this direction how much different the space program might have gone. X-15 flights started in 1959 some 22 years before the first Space Shuttle mission.
The National Air and Space Museum has the historic X-15 #1 on display which was first piloted by Neil Armstrong who later became the first Man to walk on the Moon.