the X-24C aimed to build on the advancements of the X-15 and X-24B programs, with plans to reach speeds of Mach 8 using a scramjet engine. Had the program continued, the U.S. might have been far ...

The first scramjet, an airbreathing jet engine capable of pushing an aircraft beyond Mach 5, was successfully flown in the early 1990s. But while pretty much any other technology you could imagine ...