I have been researching hypersonic aircraft and scramjet engines after running a few calculations looking at ramjet/rocket hybrids for making single stage to orbit vehicles. After doing the analysis and reading a little bit of the development history, I am still a little baffled as to what stopped the development of the engines to get deployed on aircraft and rocket.
What is keeping the engine cool enough to not melt or fly apart at design speeds? Was it making sure the inlet compressed the air quickly enough without slowing the flow with a normal shock? Was it making sure the structure of the engine was light enough to still produce enough thrust to fly the aircraft?
If these were the problems, why were they hard to solve? Was it that we did not know how to get enough cooling for low pressure drop? Or know how to locate shockwaves in the intake because the ratio of specific heats changes?