This is for a sci-fi story I'm writing, so the answer doesn't have to be perfect, just close as possible.

Can the lift fans on a F-35 be driven by a turboshaft instead of the jet engine If yes, which one? The jet engine that drives the lift fans in a F-35 is called Pratt & Whitney F135 engine. The engine dumps 20,000 shaft horsepower onto shaft that spins the fans. But i need a way to do this with a turbine, not jet engine with thrust coming from the rear.

I'm assuming that it takes a certain amount of shp (shaft horsepower) to spin the lift fans at the correct rpm. The fictional vtol is to have 4 lift fans driven by four turbine with little heat signature to avoid infrared.

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    $\begingroup$ Although I'd say this is off topic, the answer is yes, of course you can. Turbine engines are used in a variety of applications; you ought to look into a turboprop, which is basically what you describe in a different configuration. $\endgroup$ – Sanchises Dec 15 '15 at 7:43
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    $\begingroup$ worldbuilding.SE is probably a better place for this kind of semi-fictional question $\endgroup$ – Pondlife Dec 16 '15 at 17:52
  • $\begingroup$ Can someone explain how this is off-topic? The question is about an aircraft power system which would seem to fit here. $\endgroup$ – fooot Dec 17 '15 at 16:10
  • $\begingroup$ You could power it with a separate engine, but that would cost added weight and space. If it was beneficial, they would've done it. To get rid of the heat signature, one could mix the exhaust with ambient air and have it exit at the top the the center of your vessel. That would mask the heat from the ground quite effectively. Another option if it's science fiction is to have it being all electric with hardly any heat production at all $\endgroup$ – Chris V Dec 21 '15 at 23:05
  1. There is no difference between turbofan, turboprop and turboshaft engine except for what is mounted on the shaft. There are many cores that are used in all of turbofan, turboprop and turboshaft engines. So yes, they can be driven by turboshaft. In a sense, the engine that drives them qualifies as turboshaft simply because it drives them.

  2. That said, the lift fan of F-35 is absurdly inefficient. The thing is that lift is equal to mass flow through the fan/propeller times change of speed of the air, but the power required is equal to mass flow times square of change of speed, plus drag that grows with RPM.

    Now the fan is small, so it has small mass flow and so it needs to accelerate the air to great speed and to do that it also needs high rpm, both greatly increasing the power required to spin it.

    F-35 can live with that, because it only spends very short time in hover and a lot of time at high speed, so it can trade efficiency at hover for low drag at speed. But anything that should work like a helicopter should better have a large rotor like a helicopter, because it takes much less power to spin it (and thus has much lower fuel consumption).

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  • $\begingroup$ Except that I've never heard one refer to a turbo fan as a turbo shaft engine. I think it's a grey area $\endgroup$ – Chris V Dec 21 '15 at 23:02
  • $\begingroup$ @ChrisV. so far every engine had either fan or shaft, so it was called turbofan in the former and turboshaft in the later case. Some cores were already used in both configurations, but the PW F135 in F-35B is unique in that it has both fan and shaft at the same time. $\endgroup$ – Jan Hudec Dec 22 '15 at 6:14
  • $\begingroup$ What is probably another distinguishing factor between a turboshaft and turbofan is that with a turboshaft the aim is horsepower alone. With a turbofan, the core engine still produced thrust even though these days it's usually less than 20-30%. You are right the F135 is unique in that respect $\endgroup$ – Chris V Dec 25 '15 at 0:24
  • $\begingroup$ @ChrisV, indeed. And F135 is a low-bypass turbofan, so it probably generates around half of thrust from the core (I can't quickly find bypass ratio though). $\endgroup$ – Jan Hudec Dec 25 '15 at 10:22
  • $\begingroup$ I'd be surprised if it was more than 30% . I believe the F16 has about 10% bypass $\endgroup$ – Chris V Dec 25 '15 at 10:24

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