"...truly an outstanding reference book on this North American design." -- Large Scale Planes
Developed in the mid-1950s by the US military and operated by NASA between 1959 and 1968, the X-15s were the first rocket-powered hypersonic research aircraft. They made 199 flights and reached speeds in excess of 4,000 mph and altitudes above 270,000 ft, considered to be beyond the atmosphere and into the vacuum of space.
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