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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A ramjet uses the high speed air entering the engine to compress and combust the fuel. A scramjet is a series of explosions that creates a lot of thrust. A turbojet compresses the air with fans not too dissimilar from a turbocharger on your car. Once running it sustains itself.
Jet engines require air to be squeezed tightly before being mixed with fuel, and being ignited and going “boom” and shooting out the back to make the aircraft go.
In a turbojet, there is a fan inside of the engine that spins really fast to squeeze the air.
A ramjet does not have a fan inside of the engine to squeeze the air. Rather, it uses the speed of the aircraft already moving very fast to squeeze the air when it hits the engine.
A scramjet is a type of ramjet, where the air is moving faster than the speed of sound when it is squeezed and mixed with the fuel to go “boom”.
A turbojet uses a turbine to run a compressor that compresses the incoming air before combustion, these can be used for takeoff because they suck in the air they require.
A ramjet relies on the inertia of the air to compress it before combustion, and needs a high velocity stream of air to get started. In a normal ramjet the air slows down to subsonic velocities(relative to the plane) as it is compressed. In a scramjet the air stays supersonic as it travels through the engine, but as the flame cannot propagate upstream, it’s more difficult to keep the engine lit.
Turbojets are the slowest of the group, and scramjets are the fastest. Some engines, like those used in the SR-71, blur the line between turbojet and ramjet.
Turbo jet is what you find on all commercial airliners, they are arguably the most complex engine and are capable of performing from 0mph all the way to just over MK1. By having turbines and mechanically compressing the surrounding air. Ram and scram jets remove mechanical forms of air compression in favour of using the surrounding air pressure (only available at considerably high speeds). You don’t exactly see scramjets too often as they require something like MK5 speeds or higher to even start operating, hence why they require some form of propellant to get them up to speed to begin with, I believe their main application going forward is to be used to drive warheads unmanned compared to the standard intercontinental missile style launches we have (that have very predictable parabolas and are much easier to intercept)
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
All engines need to compress the air before burning the fuel in it to generate power. Turbojets use a mechanical compressor to do that powered by a mechanical turbine after the combustion. The disadvantage of this is a lot of moving parts and complexity, but the advantage is it can create thrust while sitting still, so an airplane can get moving and take off with it.
Ramjets use the velocity of incoming air and some clever geometry to get the compression with no moving parts. This makes them much simpler, but they can only generate thrust at relatively high velocity, so you need some other way to get a vehicle with one going. And due to using clever aerodynamics and geometry, they don’t like supersonic air coming in, so they may need other clever things to slow the air down (which turbojets need as well). Which also means they may stop working at significantly supersonic speeds.
Scramjet means Supersonic combustion ramjet. It’s a ramjet with somewhat different clever geometry meant to do the same thing while the whole air stream flowing through it continues to go at supersonic speeds. Just the thing if you want to generate thrust with an air-breathing engine at higher Mach numbers. But again you’d need some other engine to get the vehicle well into the supersonic speed range before it started working.
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