Interestingly, right now in Oxford UK is being tested a precooler for the sabre jet engine designed to power skylon spaceplanes to Mach 6 in air breathing mode (before going to orbit once out of the atmosphere with stored oxidiser).
This precooler is a crucial and fascinating bit of technology, cooling the incoming air down by 1000Kelvin in 100ms without itself frosting up, so it can be compressed and burnt. It is designed to solve exactly the problem outlined in the reddit comment. There was a fascinating BBC article about this recently:
"Why would you cool down to later compress (and heat up again)?"
Because the efficiency of any combustion engine depends on the temperature differential between the input and the output. It cannot produce more energy than that differential (second law of thermodynamics) so cooling the input allows the engine to produce more energy, assuming the output remains the same.
At Mach 6 it's just too hot and energetic to compress and stabley combust, as I understand. Recall that stagnation temperature is a function of the square of velocity. The reddit article says the compressor 'puts heat back in' but that's a byproduct rather than the aim. The aim of the compressor in this engine is to get enough moles of oxygen in the combustion chamber with enough moles of hydrogen to combust and produce enough thrust. You couldn't compress the air sufficiently to squeeze it into the combustion chamber if you didn't cool it a bit first.
This is my understanding based on reading what's publically available on the net anyway.
They had to develope a special fuel for the SR 71. One with very high flash point. I imagine that the temperatures at mach 6 are too high for normal fuels too.
Yep. Skunk works describes how they pumped it around the cockpit to help cool the aviation electronics and pilot. Just to add to the scariness of flying at mach 3+, you're also surrounded by jet fuel.
They had to develope a special fuel for the SR 71. One with very high flash point. I guess that the temperatures at mach 6 are too high for normal fuels too.
This precooler is a crucial and fascinating bit of technology, cooling the incoming air down by 1000Kelvin in 100ms without itself frosting up, so it can be compressed and burnt. It is designed to solve exactly the problem outlined in the reddit comment. There was a fascinating BBC article about this recently:
http://www.bbc.co.uk/news/science-environment-17864782
Looks like a startup rather than 'dinospace' too, for what that's worth round here.