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So, I don't understand much about engineering in general, but since this is a discussion forum, I like to think that this is just a thought exercise (because it is probably useless).

So, the idea would be to use an electric compressor that would pump pressurised air into a combustion chamber without the low pressure fan, and the combustion would happen with traditional jet fuel or other propellants (like pressurised methane?).

The idea is that the pressurised fuel would turn a small electric generator after exiting the fuel tanks, but I doubt that it would generate enough energy to drive the compressor.

Well, it would probably be terrible in subsonic or even supersonic speeds, but maybe it could be also a ramjet/scramjet hybrid (of course, with all the extra equipment it would require to do these functions).

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    $\begingroup$ "since this is a discussion forum"... no this is not. From What types of questions should I avoid asking?: "Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much". $\endgroup$
    – mins
    Commented Dec 18, 2021 at 22:31

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With ideas like this it is always helpful to look at where the energy is coming from. Specifically the compression stage adds energy to the air and that energy needs to come from somewhere. In a conventional jet engine that comes from the turbine stage. The exact number varies but 30% of total output power seems a reasonable starting point.

So to a first order approximation, to run the four electrically compressed engines for a 747 you need enough electricity to drive 30% of 65 Megawatts or 19.5MW of electric power. Lithium batteries contain about 250 Watt hours per Kg so one hour of flight will need at least 78000 kg of batteries. For comparison Jet fuel contains around 12 Kilo Watt Hours per kg and as a bonus once you burn it your aircraft is lighter so even allowing for only 30% efficiency turning that energy into compressor power an hour of flight comes out as needing 4800kg of jet fuel*.

So there are seemingly few uses for electrically driving the compressor stage of a jet engine other than when starting it up, and even there it is apparent why most large aircraft use compressed air from the APU to start the main engines rather than electricity.

*Power number source appears to be for takeoff so cruise flight would be 30% or so lower which does not meaningfully change the outcome.

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