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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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Moor3z t1_j682ae3 wrote

>What is the difference between turbojet, ramjet, and scramjet?

Turbojet is a type of jet engine that uses a turbine to compress incoming air before it enters the combustion chamber, where it is mixed with fuel and ignited to produce thrust.

Ramjet is a type of jet engine that uses the forward motion of the aircraft to compress incoming air before it enters the combustion chamber, where it is mixed with fuel and ignited to produce thrust.

Scramjet (supersonic combustion ramjet) is a type of jet engine that uses supersonic airflow to compress incoming air before it enters the combustion chamber, where it is mixed with fuel and ignited to produce thrust. The main difference between the three is the method of compression used to compress the incoming air before it enters the combustion chamber.

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wingsofirisheagle413 t1_j6891ov wrote

Just a small clarification because it made my slightly hungover head hurt more, going to have a tough time using a turbine to compress air. Typically we refer to turbines as extracting energy from the airflow, which in the turbojet or turbofan case is used to spin the fan/compressor.

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r3dl3g t1_j68l1k5 wrote

So, all air-breathing heat engines (from internal combustion engines in your car to supersonic aircraft engines) require the air to be compressed above atmospheric conditions before you add fuel and combust it. The more you can compress the air before igniting it, the easier it is to extract the energy from the reaction and use it for work or thrust.

For engines designed for really really high speeds this gets tricky as the physics behind how air flows get really wonky when you get up to the speed of sound or faster, which can make it really really difficult to compress the air (and, more importantly, make it really difficult to make a single kind of engine that can compress both subsonic and supersonic airflows).

Turbojets are just turbine engines, where the airflow is run through a series of compressor blades on the inlet of the engine. These are outwardly similar to the engines on most jet aircraft, although modern engines are turbofans, which bypass a significant portion of the air around the compressor for efficiency reasons (instead of in a turbojet where everything goes into the compressor).

Ramjets have no compressor blades, and instead are ducted in a special way to use the aircraft's forward momentum to compress the air as it enters the engine ducts. Scramjets are the same concept, but are used for even higher speed airflows (ramjets compress air to subsonic velocities inside the engine before combustion, scramjets allow it to remain supersonic). However, this poses a problem at low speeds, because without the forward motion of the aircraft, the ramjets don't work.

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Seraph062 t1_j6a26em wrote

> So, all air-breathing heat engines (from internal combustion engines in your car to supersonic aircraft engines) require the air to be compressed above atmospheric conditions before you add fuel and combust it.

You can design an engine off the Lenoir cycle (i.e. a pulsejet) that doesn't require compression.

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Devil_May_Kare t1_j69iwzd wrote

In all three of these types of jet engines, air from the front enters, is compressed, and is mixed with fuel. Then the fuel is burned and the resulting hot gas leaves out the back of the engine. Because the fuel heats up the air, it's hotter when it leaves than when it enters, so you get more energy from letting it expand than you spent on compressing it.

In a turbojet, a fan at the front pulls in and compresses the air, and an inverted fan called a turbine captures some energy from the departing air to keep the fan spinning. In a ramjet or scramjet, the engine draws in air by flying forward into it ("ramming" into the air), and compresses the air by having internal geometry that air has to increase in pressure to pass by.

The difference between a ramjet and a scramjet is that in a ramjet, the compressing geometry slows the compressed air to below the speed of sound before the added fuel starts burning, whereas in a scramjet the combustion happens in air that's moving faster than sound. "Scramjet" is short for "supersonic combustion ramjet."

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