I worked with a guy that did a finite analysis program for a scramjet engine. It’s capable of MACH 15 I think. It doesn’t even work until MACH 7. Of course it has to go very high for those speeds.
http://www.nasa.gov/missions/research/f_scramjets.html
“It doesn’t even work until MACH 7.”
Some of them work at slower speeds, i.e., Mach 5. The ones they tested many years ago were more in this range.
Mach 15 might be theoretically possible but all of the problems at Mach 5 are even more challenging, especially the heat issue. I can’t imagine anyone spending the money and time required to make it work, and I doubt the Russians and Indians will, either.