1. Scramjet's don't suffer all that much from having extra air move though the engine because they don't slow that air down. 2. Scramjet's have already been de

1. Scramjet's don't suffer all that much from having extra air move though the engine because they don't slow that air down. 2. Scramjet's have already been demonstrated at over Mach 9.5 and are still more efficient than rockets at those speeds. (aka more thrust per unit fuel.) 3. Scramjet's are actually fairly simple designs compared to a traditional get engines. (Modeling and testing them however is much harder.) Granted, there are plenty of downsides which is why we have not built anything like that. But, the issues are far more in line with heat dissipation and drag vs. limits on the basic physics. Still the main limitation seems to be the far lower thrust-to-weight ratio.