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n4rf t1_j682qut wrote

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

Important distinction too; a turbojet isn't the engine on an airliner, those are turbofans.

They're called the because the big turbine blade at the front produces most of the THRUST. A turbojet is referred to as a "low bypass" engine versus a turbofan being a "high bypass" engine. Bypass just refers to the fact that air is being diverted around the core of the engine.

A high bypass engine like an airliner is using the "fan" like a propeller to push air back and around the main engine, this is why you see a big turbine blade right a large circular duct directing the air behind it.

Low bypass engines are what you'd expect in fighters, where you see all the thrust exiting a cone in the back.

Edit: corrected from lift to thrust

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biggsteve81 t1_j68if9c wrote

True turbojet engines don't have any bypass ratio at all. Even a low-bypass jet engine is still a turbofan engine, not a true turbojet.

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n4rf t1_j68mou7 wrote

Fair point. they usually do have bleed or bypass channels but that's a technicality.

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Dysan27 t1_j68y2r0 wrote

Those are ususaly to prevent damage due to too high pressure/temperature.

In a turbo fan there is air that is deliberately bypassed with the intention of adding to thrust.

One way to look at turbofans is that they are turbojets with an additional fan on the front to accelerate more air. So the point of the turbojet is now not to accelerate the air, but to power the fan that accelerates the air.

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ImReverse_Giraffe t1_j68ur3u wrote

Thrust not lift. Lift is made by the wings.

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noopenusernames t1_j694h7c wrote

The blades do behave like a wing, except in a horizontal direction instead of a vertical direction. I’ve heard people accidentally say ‘lift’ when they mean ‘thrust’ many times, but everyone in the industry knows what they mean just because of the design.

But you are technically correct, which is the best kind of correct!

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n4rf t1_j698rjd wrote

Yep! And I've corrected it. Thanks everyone

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Albs610 t1_j691wba wrote

Just to clarify a typo for others. The large blade in the front isn't a turbine blade it's a fan blade. That's why they are called turbofans.

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noopenusernames t1_j694zl2 wrote

It’s been a while since I’ve studied this. How do they get the ramjets into a forward motion to make them work in the first place.

Also, for scramjets, what kind of changes in engine behavior result from the air being supersonic? Does the air even spend enough time in the engine to burn long enough to put any useful energy into the system? Or is it still burning on its way out (while exiting the exhaust section), kind of creating an explosion just behind the engine that pushes the engine forward?

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milky-mandolin t1_j6adh3e wrote

You typically need a turbojet or turbofan engine on an aircraft to get that initial forward momentum before you can use a ram/scramjet. They both have a minimum speed required to operate which is achieved with a more typically jet engine.

All jet engines (as far as I know) require compressed air to create thrust. Think like the otto cycle of a four stroke engine, there is a compression stage where the air fuel mixture is compressed. Jet engines also require this compression, you'll see other comments mentioning "compressor stages" on turbojets - this is for the air compression.

The advantage of a scramjet is that supersonic air is already compressed, and therefore required no moving parts and can operate at higher speeds.

I am possibly wrong about some of this, I'm only an aero student sorry!

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JakeMeOff11 t1_j6agk1q wrote

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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