You know, maybe I should put this on a log scale. Choice of axis transformation says a lot. Is our implicit model of growth additive, multiplicative, or something elseitive?
The turbojet engine is absolutely amazing and there is nothing on the horizon that is at all close to replacing it. Chemical energy is so much lighter than electrical and nuclear energy that it seems like there’s no better source of energy in the air (though perhaps you could beam energy to a plane with microwave lasers from the ground? hmm). And given that you use chemical energy in a thermodynamic engine, it’s hard to think of anything that can do better than the turbojet. I would not be at all surprised if humans were still using turbojet jet engines in 10,000 years, just as they will still probably be using the wheel. (Caveat: ramjets and scramjets and turbofans and pulse jets and project orion and yada yada.)
Technically, the fastest manned atmospheric vehicle was the Apollo Command Module, which hit about Mach 30 when re-entering the atmosphere. There is also speculation that faster planes exist, but have not yet been declassified.
Wikipedia has a great series on aircraft propulsion.
More analysis coming soon. (Fine, I’ll cop. We both know that we both know that that sentence is a lie. More analysis coming sometime, if ever. But probably not soon.)