I’m an aviation nut but for the love of me I can’t find an explanation that makes sense to me
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Basic premise here is that you take air in, compress it, add fuel, combust that to make the air even hotter, and let it expand out the back to push you along. [Here’s a good image summary of the three](https://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Turbo_ram_scramjet_comparative_diagram.svg/838px-Turbo_ram_scramjet_comparative_diagram.svg.png)
In a turbojet you use turbines at the front to take subsonic air, compress it down, then run it through the combuster, where it runs through another turbine that steals some energy to power the compressor at the front and the rest of the exhaust goes out the back to provide some thrust. A lot of turbojets include a big fan at the front also powered by the rear turbine to accelerate a lot more air by just a bit to provide a lot more thrust(see modern commercial airliners)
Ramjets get the compression through ramming the air into a small spot. Air hits the large area of the opening at the front then slides down the outside of the cone into a smaller and smaller area making it hotter and dense but also slower, then fuel is applied, and air shoots out the back super fast. No turbines here, just ramming.
Scramjets are supersonic ramjets. They work off the same cone squeezing air premise but the cone is shaped so it won’t slow the air below supersonic speeds which means the forces and flows inside a scramjet are all supersonic and thus very different than inside a subsonic ramjet.
They each function well at different speeds so they get used for different purposes. A scramjet just won’t work on a subsonic commercial airliner, it’d never get the compression it needs. Similarly a turbojet would struggle on a hypersonic cruise missile as the intake temperatures would start getting crazy
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