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CBMet t1_j68gsy4 wrote

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

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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