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greatvaluemeeseeks t1_j681ftj wrote

A turbojet is a type of jet engine that uses a compressor wheel connected to a turbine wheel by a shaft with a combustion chamber in the middle. Jet fuel is burned and spins the turbine which in turn spins the compressor wheel which sucks in more air into the combustion chamber.

A ramjet is an engine that forces air down an intake, through the aircraft's forward movement through the air. The air being forced down the intake compresses by way of the aircraft's speed, then fuel is injected and ignited and exits through the exhaust creating thrust. It's essentially a turbojet without the turbine or compressor; but you need to be moving first before it can work. Instead of the compressor compressing the air, the aircraft's forward momentum compresses it.

A scramjet is a ramjet, but airflow through the engine is supersonic; whereas it airflow slows down in a ramjet.

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n4rf t1_j682qut wrote

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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steruY t1_j68m7w7 wrote

What's an afterburner?

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koolaidman89 t1_j68upm9 wrote

That’s where fuel is dumped straight into the jet exhaust where it burns with oxygen that wasn’t consumed in the core of the engine. It functions like a rocket engine to generate additional thrust. Great for a burst of power but it consumes fuel very quickly and inefficiently.

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Dysan27 t1_j68ybmp wrote

and can only be used in short bursts.

The only engine ever designed to run on continuous after burner was the Pratt & Whitney J58 on the SR-71 afterburner. Though the J58 acts more as a ramjet with a turbojet stuck in the middle.

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CBMet t1_j68gsy4 wrote

Are there any pros/cons for each? Is one the future and one the "old fashioned" type? What would make aircraft designers pick one over the others?

Thank you in advance!

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Sand_Trout t1_j68j36e wrote

Turbojets can work for takeoff and low speed, where ramjets and scramjets will not. However, they have more moving parts and are therefore heavier per thrust.

Ramjets cannot operate with superaonic airflow through the engine, and thus must slow down the intake air highspeeds, reducing supersonic efficiency.

Scramjets are very efficient at supersonic speeds but very inefficient at low speeds.

Due to initial speed requirements, ramjets and scramjets are reserved for niche high-speed applications.

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CBMet t1_j68t1v8 wrote

Thank you! That's really interesting!

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noopenusernames t1_j69635y wrote

Do you happen to remember the name of the type of engine that they experimented with in, I think, the 80’s, maybe early 90’s? It was kind of like a turboprop, but the it looked more like a jet engine. The defining feature was that the “propeller” blades were short and stubby and mounted on what would look like the exhaust cone of a turbine engine, and there were a lot more of of these stubby blades than you’d see on a turboprop. It basically looked like if you took one of the compressor stages off a turbine engine and rotated it inside out so the blades all stuck outward from a central ring, and then slid that ring up onto the exhaust cone of a turbine engine.

I’ve been trying to remember the name of this thing for a long time but have had a dammed hard time finding it. Apparently they were supposed to have the efficiency of something between a turboprop and a turbine, and so airlines really wanted them, but no one pursued them because they thought the general public would think they are “scary-looking” and wouldn’t want to fly on them

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Sand_Trout t1_j696rjc wrote

It sounds like you're thinking of the propfan

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noopenusernames t1_j6bueks wrote

My fucking hero. Thank you. I tried finding this for so long and for some reason my Google skills were failing me

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SierraTango501 t1_j698u6k wrote

Also, ramjets and scramjets don't work at the speeds that commercial airliners fly at, and turbojets are hugely fuel inefficient.

The engines powering commercial airliners are turbofans, similar in construction to a turbojet, but with a large diameter intake fan, that bypasses a lot of cold air past the compression/ignition stage and mixes it with the exhaust air to generate thrust without burning up a ton of fuel a minute.

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