Now that I am back from my prolonged vacation after piling it higher and deeper, I can contribute again.
Couple of odd theoretical bits.
1. The theoretical maximum speed of a reaction based propulsion system is a direct function of its Isp (specific impulse). A perfect gamma ray/antimatter engine has an Isp that gives it a maximum velocity of the speed of light.
2. One of the many, but highly accepted definitions of hypersonic region is where the difference between the actual Mach number and M=infinity is sufficiently negligible, i.e, around 10%. Conversely there is a "hyposonic" region, i.e., where the Mach number is sufficiently close to zero that incompressible assumptions can be used, below Mach 0.3.
As to the ability of a propulsion system to work, it isn't disassociation of the incoming fluid, that is actually a plus since separating O2 into 2-O requires a fair amount of energy, that kills you it is the lack of recombination after the fact. If you only have OH
after combusting CH4 and H2O you are missing out on a tremendous amount of the energy extraction.
As an aside those flame trails you see behind afterburing jets are OH
to H20 combinations that are lost energy. You can solve this problem by increasing combustion pressure, but this adds to the NOX problem. The biggest problem with Scramjets is combustion length, of which a sufficient length is necessary to allow for recombination. This is, more than anything else, what defines the operational speed range. Hydrogen requires a significantly shorter combustion length than hydrocarbons, and produces about twice the Isp.
+ O2 reaction is quite complex with several stages especially on the recombination side.
If you want to do your own Isp calculations I recommend looking up the heats of disassociation and recombination in the JANAF tables. Hill and Peterson's jet propulsion book is a good reference for the actual equations.
Your friendly gas-dynamicist: