I remember scramjets.

They're still working on them, and apparently they're getting better at it. The latest test was another sucessfull splashdown after this time reaching a speed of mach 5.5. Yeah, I can't even imagine a missile traveling at mach 5.5. Just for comparison that thing will go 12.5 miles in the time I (once) could run 100m. Yowza.
New Scientist- scramjet