Two B-52s served as NASA motherships for the X-15 programme. The rocket plane hit Mach 6.7 and 354,200 feet. Its data helped build Mercury, Apollo, and the Space Shuttle.

NASA strapped a rocket plane to a B-52's wing and it flew to the edge of space

The North American X-15, a U.S. Air Force and NASA rocket plane, reached record speeds of Mach 6.7 and altitudes above 67 miles. With 199 flights and astronaut pilots, it bridged earth flight and space travel. 

This fighter jet flies so fast, people call it a rocket with wings, reaching a top speed of Mach 6.72

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NASA strapped a rocket plane to a B-52's wing and it flew to the edge of space

This fighter jet flies at Mach 6.72, so fast it's called a rocket with wings