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Is it possible for a turbofan jet - like on a 747 - to run without the fan, if for example you just want the thrust from the internal jet to use to heat up water, not to use as thrust?

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    $\begingroup$ Surname checks out 👌 $\endgroup$ – ymb1 Jan 28 at 3:09
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    $\begingroup$ Sounds about like an APU... electricity + bleed air, but no thrust. $\endgroup$ – Ralph J Jan 28 at 3:16
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    $\begingroup$ Tangential: companies like General Electric use/modify the gas generator core of aviation turbine engines for use as terrestrial or marine power generators. An example would be the LM2500, which was derived from the CF6 turbofan. $\endgroup$ – aerobot Jan 28 at 3:33
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    $\begingroup$ If you just took away the fan, there'd still be thrust (from the core), and you'd have a turbojet. $\endgroup$ – Sean Jan 28 at 4:21
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    $\begingroup$ So... hotel mode for turbofan. $\endgroup$ – Antzi Jan 28 at 6:26
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No, the fan is directly connected to the outer compressor and turbine (N1). There is no direct connection to the inner compressor and turbine (N2), which could run independently, but there is no switch to somehow stop the N1 rotation in a 747 (and I am not sure, if it would run properly anyway).

Some turboprops (e.g. ATR-72) can run an engine without the propeller (Hotel Mode, see e.g. this question), which is connected to the engine via a gearbox. While there are also geared turbofans, you cannot completely disconnect the gearbox from the core engine.

As the other answers have said, it is however possible to build a jet engine without a fan in the first place. The Boeing 747 even has one in the tail: the Auxiliary Power Unit (APU), which will provide electricity and bleed air while the main engines are not running.

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    $\begingroup$ The only correct answer so far - but it would be useful to mention that this operating mode of turboprops is known as "hotel mode". And it's loud and inefficient compared to an APU. $\endgroup$ – pericynthion Jan 28 at 16:20
  • $\begingroup$ @pericynthion Thanks, I added a link to the relevant question. $\endgroup$ – Bianfable Jan 29 at 7:54
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Yes, it is possible, in fact it is pretty common.

It is called a turboshaft engine or gas turbine. You can find examples of their use in:

APUs

An acronym for Auxiliary Power Units, these are probably the closest to what you have in mind, this is a jet engine, usually quite small, that is used to provide onboard power and sometimes hydraulic pressure. You can often find these located in the tail cone of modern airliners, with the exhaust at the cusp of the cone.

Helicopter engines

Most helicopters worldwide are powered by this form of jet engine, optimized for mechanical output, since helicopters generally do not benefit from exhaust jets. Otherwise it is very much a jet engine that drives a helicopter rotor (via multiple gearboxes to reduce rpm) instead of a fan.

Tank engines

Just like the helicopter version, although with different optimizations. You can find these in the M1 Abrams and the T-80, among others.

Electrical power plants

The generation process at most modern gas-fired electrical plants involves a gas turbine that works like an upscaled turboshaft, although usually in combination with other systems.

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  • $\begingroup$ Would the downvoters explain their reasoning? If it is because the OP´s question can be interpreted literally as taking an existing turbofan and using it in hotel mode, then I have to point out the OP also asks about using "thrust from the internal jet to use to heat up water", so I feel the reinterpretation is a bit too liberal. $\endgroup$ – AEhere Jan 29 at 20:32
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Yes. A turbofan without the fan would be called a turboshaft engine.

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