> However, these engines, which are still in their testing phase and yet to have any real-world implications, have a major flaw which stops them exceeding Mach

> However, these engines, which are still in their testing phase and yet to have any real-world implications, have a major flaw which stops them exceeding Mach 7. https://en.wikipedia.org/wiki/NASA_X-43 https://en.wikipedia.org/wiki/NASA_X-43 : Achieved Mach 9.6, 2004