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JakeMeOff11 t1_j6agk1q wrote

Reply to comment by noopenusernames in ELI5: What is the difference between turbojet, ramjet, and scramjet? by Global_Maize_8944

You don’t typically use them on something that’s starting from a stop. I think ramjets are common on missiles. I think there are ramjet/scramjet planes which would also use a turbojet engine to get the plane off the ground before switching to the ramjet engine.

I’m pretty sure scramjet engines will have a shockwave inside the engine which will change the properties of the airflow through it. It’s been many a year since I studied propulsion and compressible fluid dynamics so I’m probably misremembering a fair amount of this but after the shockwave the air will flow slower through the engine. I think its temperature and pressure increases across a shockwave while velocity of the air decreases.

The thrust from a jet will always come from pretty much throwing the air out of the nozzle. You only have explosions outside of the engine being used for propulsion in a very specific kind of rocket engine.

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noopenusernames t1_j6c1cbx wrote

What about the SR-71? I’m not too familiar but that plane did not have an alternate engine to get it airborne. Is that why the nose cone shifted, to make the engine behave more like a scramjet as opposed to a ramjet during certain phases of flight? Or was that more just to guide air into the intake better at higher speeds?

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JakeMeOff11 t1_j6c1wnz wrote

Looks like the SR-71 ran on two turbojet engines. The article states that the engines used some sort of compressor bleed to increase power for the afterburners at speeds greater than Mach 2, which kind of made it seem like it was a sort of “turbo-ramjet” engine, which I don’t think is actually a thing, but it was just a turbojet engine.

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