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I recently have been looking at different types of jet engines and came to this thread wherein they talk about how most of the thrust from airline turbofan engines (high bypass) comes from the bypass air and not the combusted air coming out of the internals of the engine.

So my question is, has there ever been a sort of fan engine used that only uses bypass air and the fan is run by a separate engine? Would this even be viable?

Edit: Basically a turbofan that just uses the power of the increasing speed of the air through the smaller space, no combustion (other than the standard piston engine/electric motor/whatever). image of what I mean

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    $\begingroup$ I'm not sure what you're looking for. It sounds like you are just describing a turbofan. $\endgroup$ – Thomas Mar 5 '18 at 18:05
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    $\begingroup$ @Thomas except that it would be hard to have a turbofan without a combustion chamber like the title says. Closest would be a turboprop where the exhaust creates very little thrust. $\endgroup$ – fooot Mar 5 '18 at 18:13
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    $\begingroup$ I'd suggest much more practical questions would be "can you remove the combustion chamber from a jet engine" or "What's the limit to how high of a bypass ratio you can get?" $\endgroup$ – Cody P Mar 5 '18 at 18:47
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    $\begingroup$ It's not really bypass air if it isn't going into the chamber though. $\endgroup$ – Sebo Mar 5 '18 at 18:51
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    $\begingroup$ I think you're talking about a perpetual motion machine which...I don't believe is physically possible. You can't generate enough force from the incoming air in order to have enough force to force the air in. You'd be going against the laws of physics at that point. Somewhere in there something external has to be adding power to the system to make it work. Engines, technically, only convert energy, they don't create it (which, you know, is impossible.) $\endgroup$ – Jay Carr Mar 5 '18 at 22:21
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Well what you are describing is a ducted fan. They are common on RC aircraft with the fan driven by either a gasoline engine or electric motor.

On large scale applications like an airliner, you have to find a source of power for the fan that is 1) compact 2) light weight 3) delivers a very high specific power and 4) draws energy from a reservoir with a high energy density, all while remaining 5) extremely reliable and 6) safe. For these kinds of applications, you really can’t currently do better for this than use a kerosene fueled gas turbine engine to drive the fan. And that's all a high bypass turbofan really is - just a gas turbine powered ducted fan.

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    $\begingroup$ Thanks for the detailed explanation. That last sentence is a great way of summing it up for me and the engineering behind the turbofan engine makes so much sense now. Power the fan with the air that comes in, duh. So simple (conceptually) yet so smart. Thanks again $\endgroup$ – TrapAlcubierreDrive Mar 5 '18 at 19:28
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    $\begingroup$ @TrapAlcubierreDrive. No, you cannot "power the fan with the air that comes in". You need to supply additional energy to gain thrust, because the incoming air needs to be accelerated to cause an increase in momentum, which gives you thrust. You need a motor of some sort. Electric, diesel, gasoline, kerosene. Pick any one. $\endgroup$ – Penguin Mar 7 '18 at 8:29
  • $\begingroup$ @Penguin: Well, it is technically possible to power a jet engine from a power source that doesn't require siphoning off any of the intake air (such as an electric, nuclear, or nuclear-electric motor), which might have been what TrapAlcubierreDrive was thinking of... $\endgroup$ – Sean Oct 3 '18 at 21:29
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The UL-39 ALBI is a prototype ultralight that uses a ducted fan driven by a reciprocating motorcycle engine. The idea is that for the levels of power needed by such a small aircraft, piston engines have better thrust-to-weight and thrust-to-cost ratios.

You can also find piston-driven ducted propellers on some airships, mainly so they can vector the thrust for steering.

There is also the case of the F-35 STOVL. Here, the lift fan is driven by a jet engine not in the flow path; this is so you don't need a dedicated engine for that fan.

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  • $\begingroup$ That's it! The ducted fan. It seems really obvious when I don't talk about jet engines in the same sentence. Sorry for anyone that got really confused by my wording, I'm new to this stuff. $\endgroup$ – TrapAlcubierreDrive Mar 5 '18 at 19:20

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