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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?

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    $\begingroup$ Has a scramjet run for more than a few seconds? $\endgroup$ – zeta-band Feb 28 '18 at 0:09
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    $\begingroup$ Why do you think development has stopped? $\endgroup$ – jamesqf Feb 28 '18 at 5:15
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    $\begingroup$ I meant historically. I know the effort hasn’t stopped, but was asking why we don’t have an engine ready to put onto an aircraft right now. $\endgroup$ – user11377 Feb 28 '18 at 15:50
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I'm don't research hypersonic flight, but I bet the primary barriers of hypersonic flight are less technological and more political/economical. After all, we pay money and conduct research in order to solve technological problems.

You would think that developing an engine is primarily a task for aerodynamicists since they can design better intakes, compressors, nozzles, etc. However, materials science will typically limit what an engine manufacturer can actually build as you mentioned in your question. Let's imagine aerodynamicists design can design a scramjet that can reliably run for a thousand cycles, but the engine is useless if the aircraft's skin can't handle the high speeds.

More importantly, who's willing to pay for super-inefficient scramjets? Commercial airliners (consumers) weren't even willing to pay for the supersonic Concorde, so you can easily discount them from buying scramjet-powered aircraft. That leaves the military. In the 60s, the military was willing to pay for speed (see SR-71, XB-70), but then stealth became more attractive than speed and super/hyper-sonic research fell by the wayside.

Now the US, China, and Russia are again funding hypersonics research in part because stealth is losing its advantage. Moreover, some startups (Aerion?) are exploring supersonic business jets for the really rich folks, so you're starting to see their engineers exploring hitherto exotic supersonic tech. However, hypersonic flight will still be beyond the means of an average consumer.

TL;DR: Hypersonic research probably stopped less because of tech problems and more because stealth offered more bang for the military's buck. Now the pendulum is swinging the other way. Military supersonic technology is also making its way into the commercial sector for the uber rich.

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    $\begingroup$ I'd suggest that fuel consumption is another reason why scramjets wouldn't have much practical use in transportation. As far as I've found, the longest scramjet test flight to date is about 200 seconds. That might be sufficient for a space launch, or a high-speed drone weapon, but for a practical transport the fuel load would seem prohibitive. $\endgroup$ – jamesqf Mar 1 '18 at 19:43

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