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Wouldnt the scramjet engines be a pretty big breakthrough?

Scramjet still needs air though, doesn't it? Thus it has to fly lower, and maybe can't get as fast as a rocket engine.

Scramjet planes are faster than other non-rocket planes I would imagine?

The SR-71 cruised at Mach 3.2. Scramjets have been demonstrated to do Mach >5, and theoretically might be capable of something between Mach 17 to Mach 25(!!!)

Scramjets do need air, although something to consider is that the faster you go, the more air you get. So at high altitudes with very thin air, you can go faster to get more air. In fact you have to go faster (true airspeed) in thin air anyways even if your brought along liquid oxygen for your engine, because as the air thins your stall speed will go up.

The whole thing is a complicated system of equations. Engine capabilities and/or critical mach number (for planes designed for subsonic flight, like the U-2) of the airframe set an upper limit on how fast you can go. Air density decreases with altitude, increasing the stall speed (according to true airspeed, not indicated airspeed.) But for a given true airspeed, with increased altitude the temperature of the air drops, consequently dropping the speed of sound, which reduces the mach number. In subsonic planes as altitude goes up the stall speed and critical mach number eventually pass each other, beyond which your plane can no longer fly. The point of that intersection is known as the 'coffin corner'. In planes designed for supersonic flight passing the critical mach number isn't a problem, but eventually you'll reach a point where the drag on the aircraft is so great that the engines simply don't have enough power to overcome it. Increasing altitude reduces drag as the air gets thinner, but that only works for so long. Air speed inside the engines needs to be considered too. The airspeed past the turbine blades needs to be such that you won't get compressor stall. When you discard the turbine requirement, that's where things start to get interesting. A ramjet has no turbines, it compresses the air using the intake geometry. At full speed the SR-71 was actually producing most of it's thrust with a ramjet effect, most of the air going into the engines was not going through the turbines. The difference between a ramjet and a scramjet is that in a ramjet, the air inside the engine becomes subsonic again for the burn, but in a scramjet the air inside the engine remains supersonic. Scramjets could go really fast.

If we put a cockpit and a pilot in it the X43A would have broken the crewed record at Mach 9.6 back in '04 but there was no need because we have computer controls now for these test flights and there's not a huge number of uses for having pilots in these insanely fast planes partially because they're just so insanely fast.

I think the hypersonic planes will all be drone munition delivery (or just munitions themselves) for a long time. Maybe some day they'll carry people but there's no supersonic aviation much less hypersonic and even then the piloting at speed will probably be all automated.

Yes, scramjets are faster than other non-rocket planes. They have a lower bounds of approx. Mach 3, but then go up from there.

The big challenge is basically making sure that you mix your fuel with the air, combust it fully, and extract the energy before it flies out the back of the engine. Scramjets are the engines capable of this at the highest speeds.

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