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A question that I have been wondering about lately is whether it would be possible to power the fan in a modern hi-bypass turbofan with an electric motor and achieve performance comparable to the kerosene-fueled engine. Or is the 10% of air that flows through the combustion chamber crucial for the operation of the system?

(I wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity)

Edit: I have realized that the "electric" part of the question is really not the core of the question. What I am really interested in is whether it would be feasible to power a plane using only the fan and not the exhaust jet with a performance similar to a current jet (which would make the engine similar to a turboprop with many blades and a cover around it)

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    $\begingroup$ What would power the electric motor? If you wish to ignore the weight of batteries you would need an engine driven generator... do you think this would be somehow more efficient than driving the fan directly? $\endgroup$ – Michael Hall Dec 28 '20 at 16:08
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    $\begingroup$ I don't understand how you can "ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity" ... if the plan won't fly because of the weight of the engines to provide the power, or because the number of batteries carried would cause the weight to be too much, then it's not a plane, it's a brick that can't take off. $\endgroup$ – CGCampbell Dec 29 '20 at 13:47
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    $\begingroup$ Why do you think that "the 10% of air that flows through the combustion chamber" could/would not still flow through? $\endgroup$ – Mike Brockington Dec 29 '20 at 15:26
  • $\begingroup$ @erikkallen maybe a better question (or a follow up for this one) would be which thrust apparatus would be most suitable for electric airplanes: ducted fan, propeller, unducted fan etc. Electric motors have some characteristics that differ from turbines and piston engines, and taking these characteristics into account might tip the scales to a surprising direction. The application is of course one variable: what kind of missions would the plane need to fly etc. $\endgroup$ – Jpe61 Dec 29 '20 at 19:25
  • $\begingroup$ See my comment on the latest answer with the embedded video. It is not clear at all what you are actuall asking, and your edit only makes it worse. Voting to close until you can clarify what you really mean. $\endgroup$ – Michael Hall Dec 29 '20 at 23:41
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Taking into account your wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity, the answer is yes.

You would, however, need to compensate the missing jet engine thrust by increasing the fan thrust.

Otherwise the answer is no.

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    $\begingroup$ Should someone feel irritated by this answer, you're wellcome! Throwing reality out the window does not spawn very good deliberations 🙃 $\endgroup$ – Jpe61 Dec 28 '20 at 17:04
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    $\begingroup$ Nobody should be irritated by truth. Sometimes these random and theoretical musings just need a blunt debunking. $\endgroup$ – Michael Hall Dec 28 '20 at 18:17
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    $\begingroup$ Any irritation should be directed at the question -- your answer is spot-on. "If we ignore all the reasons that this doesn't work & isn't a very good idea, can you in theory do this & make it work?" Sure, ignore enough reality & all kinds of silly things become "possible." I think it's entirely fair game to include reminders of how much has to be ignored, with the answer to such a question. +1 $\endgroup$ – Ralph J Dec 28 '20 at 18:47
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    $\begingroup$ @Criggie of course the big difference between the battery-powered electric turbo-pumps on a rocket engine, and electric turbo-pumps for a turbine engine is that one flies for five minutes, and the other flies for five hours. That makes all the difference. $\endgroup$ – RonJohn Dec 29 '20 at 0:23
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    $\begingroup$ @Criggie "or reputation or advertising" as in "Look how green we are!" $\endgroup$ – RonJohn Dec 29 '20 at 0:39
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"using only the fan and not the exhaust jet" does not make sense.

This is because the exhaust jet powers the fan. That's what the turbo in "hi-bypass turbofan" means: the exhaust jet spins turbine blades attached to a center shaft, which thus spins turbine blades in the front of the engine. Those front turbine blades then compress air into the combustion chamber.

Something must power the fan. If you don't "steal" some thrust gas to do it, then some other energy source must, for hours and hours and hours. Therefore, somewhere in the limited volume and weight requirements of the plane, you must also account for an energy source and motor.

Bottom Line:

Stealing some thrust gas is the most compact and economical method of spinning the fan.

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would [it] be possible to power the fan in a modern hi-bypass turbofan with an electric motor?

Yes, but it would not be the most efficient way to convert electric energy into thrust. The superior way would be to attach a large, slowly spinning propeller to that engine. Since electric airplanes have a poor power-to-weight ratio once their range becomes practical, the available power should be used most efficiently. Jet engines only come into their own at flight speeds above Mach 0.7 when propellers run into increasingly unsurmountable compressibility problems.

and achieve performance comparable to the kerosene-fueled engine?

Not quite because the fan-motor combination is likely heavier than the jet engine. Small electric motors can weigh as little as 0.1 kg/kW but larger ones struggle to reach even half of that. Siemens demonstrated a GA-sized engine with 5 kW/kg, and scaling this up to airliner power levels would most likely result in no more than 2-3 kW/kg. Even with 95% efficiency, 5% of power will be turned into heat, creating cooling problems for large electric motors.

is the 10% of air that flows through the combustion chamber crucial for the operation of the system?

Yes, it is. Its mixing with fuel of high specific energy and combustion provides the mechanical energy to drive the fan. Its much higher exit speed at the nozzle also lets those 10% contribute a disproportionally higher thrust.

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I believe this has actually been done; please see this video and decide if this meets your theoretical question setting:

This Airbus custom demonstration aircraft flew from England to France using two battery-powered electric ducted fan engines, which could be considered similar to or the same as electrically-powered high-bypass (indeed, 100% bypass) turbofan engines.

At 20:55 the pilot stops the fans prior to getting on the runway and you can see via the front that there are fan blades and a small central core section inside the nacelles. At 21:05 there's a good look from straight aft of the same.

Takeoff is at 24:30 and performance may or may not match your definition of "comparable to a kerosene-fueled engine", but this is reality, not theory, so they were obligated to consider battery and motor limitations.

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  • $\begingroup$ This answer just triggered me to downvote the question because it is now completely unclear to me whether the OP is simply asking whether electric powered flight was possible, (which I presumed was common knowledge...) or asking about some hybrid combination of thrust from an electrically driven fan augmented by a gas turbine. The answer is a good one, if the question is about purely electric ducted fans... $\endgroup$ – Michael Hall Dec 29 '20 at 23:32
  • $\begingroup$ I saw the word "theoretical" in the question, and how existing answers all seemed to address the limitations of generating the power to drive the fans in the first place, and I tried instead to address the unanswered aspect of the question: simply whether, in theory, any kind of all-electric-driven (not-really-)turbofan could function as well as current kerosene examples. The answer is obvious (yes, push air, make plane go) but the questioner also asked about the jet exhaust contributory aspect, which I did not directly answer. But I gave a practical, extant example, with video. $\endgroup$ – X Goodrich Dec 30 '20 at 2:42
  • $\begingroup$ I'd say a Large Fan moved by an Electric Engine in a Turbofan won't need batteries at all, Hybrid ICE-electric systems may need just an small battery as buffer or emergency safety backup, if the weight of generator needed to supply power to electric engine, it can be turned by the power section shaft of turbine, adds no weight to te current reduction gears or specific turbine to move the fan, and overall efficiency is good, it can be considered, but this needs calculations and wind tunnel/ engine testing. Blessings + $\endgroup$ – Urquiola Dec 30 '20 at 12:54
  • $\begingroup$ Original question said to ignore issues of battery / power source, so all discussion of efficiency, weight, capacity etc. are IMO not germane to answering the original question. $\endgroup$ – X Goodrich Dec 30 '20 at 19:43

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