For hypersonic it's been all about computer simulations. The costs are just too high. The tests for such aircraft are very expensive and typically result in catastrophic failure (complete loss of the system). Though they learn a lot about what works and doesn't in each test. A major driver for better CFD has been to better simulate hypersonic speeds and reduce the number of real test flights needed.
Someone in the FEA/CFD/Simulation industry upon seeing the SpaceX video on their capability indicated that no commercial product he was aware of could match what they were doing. Firewall won't let me confirm this link is correct:
To be fair, it is a VERY hard problem. 'Most' airplanes are designed in a laminar flow situation. Meaning that turbulence is just left out. This makes the calculations much easier, as you can set boundary conditions. In super-sonic conditions, the air is no-longer in-compressible, so the boundary conditions change and you have to add more variables to the equations. In hyper-sonic flow, the air itself is now a variable, as the molecules are split and recombine along the wing in ways we have not yet even empirically measured. This ads a complexity to the equations that is very large and the 'educated guess' of those chemistries on the wing becomes very important.
...as the molecules are split and recombine along the wing in ways we have not yet even empirically measured.
I'm sure one can speculate. In the video they do talk about all the possible intermediate species, so one might have to add air molecules (which can break apart and add to the reactants) to the mix. But yeah, lots of unknowns. Doing experiments and comparing to the simulations should help figure it out. The problem has traditionally been an inability to do the simulations, but that's possible now.
https://www.google.com/search?q=hypersonic%20flight%20test%2...