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I assume turbofan fan rpm could be increased only when:

  • Using most modern jet engines (Airbus A380 and the Boeing B747-800)
  • Using a magnetic gearbox which can handle more torque dor the same weight
  • Fan and compressor blades are reinforced
  • Design muffles and silences the fan sonic boom
  • Engine is designed to prevent a flame out.

Assuming the problems above are solved, and one was able to redesign the fan and compressor blades to be efficient at 10 times their current rpm, without changing the power rating:

  • Would the engine have enough torque and power to spin the fan at cruise altitude?

  • What would be the effect on thrust?

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An initial thought is that at 10X the RPM, it is likely that the blades would be well into Mach numbers, which would have a negative effect on power requirements (that is substantially more torque in particular) to spin the fan, and more importantly the fan would not be effective at those speeds. There would likely be blade damage as well from the loading.

However this is a rather hypothetical question, where if an engine were developed to do this, there would be other trades to be made in the design of an engine. Changing just one part of a system such as a jet engine, usually does not favorably alter the overall performance.

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  • $\begingroup$ Most high-bypass turbo fans have the tip speed just slightly supersonic at top speed. Maybe mach 1.1, maybe a little higher. At 10x speed, the blades would be Mach 11! $\endgroup$
    – Daniel K
    Oct 18, 2017 at 22:43
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    $\begingroup$ Not to mention that the pitch of the blades would be far too much, even if they didn't melt, which they would. $\endgroup$
    – mongo
    Oct 18, 2017 at 23:12
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Turbofans don't need to spin faster, they need to spin slower to limit fan tip speed:

  • Compressibility drag is not a good item to spend fuel on.
  • Blade tension is a quadratic function of rotational speed. At 10x spinning speed, blade tension is 100 times higher. The blade root would have to be 100 times thicker at current technoloy.

Magnetic gears are made of permanent magnets, heavy clumps of magnetic iron: not a good combination with spinning 10x as fast. Mechanical gearboxes are used to gear down the propellers in turboprop engines, and geared turbofans use mechanical planetary gearing to reduce fan blade speed.

Perhaps magnetic gearboxes will find their way into turbine aero engines in the future, when the technology is fully mature, but for now mechanical gearing does the job. They reduce prop & fan speed though, not upgear them 10 times.

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