Only problem I see with these scramjet powered craft is that you have to be doing Mach 4.5 just to get the engines lit.
That requires an extremely long runway.
Dr. Eugen Sanger was working on the same concept in the 1940s. You are correct. I would like to give you an alternate. Why not a sled to carry the craft to power by conventional rockets then its release it using its own power in space
You use a jet to carry the plane to a high altitude and then drop it. The dive will produce speeds up to 4.5 Mach and then the scramjet engines kick in.