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Is there any a point where it becomes possible/ideal to run two large front fans (side by side) powered from a 'single' core? Otherwise shoot this to pieces and explain the ways this is a bad idea.

This inspiration for this question is from the design of some rocket motors which utilize a single turbine driving both pumps on a single shaft to feed multiple combustion chambers and nozzles. Example would be the RD-180.

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    $\begingroup$ Partially related, aerodynamic issues with engines side by side $\endgroup$ – fooot Dec 5 '16 at 15:28
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    $\begingroup$ The GE Propfan has two counter-rotating fans, but they're not inside the nacelle like a fanjet $\endgroup$ – TomMcW Dec 5 '16 at 15:39
  • $\begingroup$ @fooot I had considered the points mentioned, however in the case I have presented the following differences are apparent. There aren't two engines, the size of the fans and physical configuration with relation to the proposed core is also open to speculation. $\endgroup$ – jCisco Dec 5 '16 at 16:27
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    $\begingroup$ are you planning on running a gearbox to split the single shaft to the fans? Gearboxes are ... a challenge! Ask any Chinook engineer. $\endgroup$ – Rory Alsop Dec 5 '16 at 18:20
  • $\begingroup$ @RoryAlsop: And yet we have geared turbofans... $\endgroup$ – Sean Mar 9 at 5:29
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Generally, it is more efficient to have one large than two small devices.

The twin fan solution not only needs a gearbox and driveshafts that a single fan solution does without, it also has more intake and nozzle surface area per cross section than the single fan. This will cause more viscous losses and lower efficiency. The only advantage would be if size limitations make a single fan impossible, but in that case it would be more straightforward to have each fan driven by its own core engine.

Famous examples of aircraft which used a single engine and two "fans" would start with the Wright Flyer models, all of which had a single engine drive two propellers via bicycle chains. But the list is short. The list of opposite designs where two engines would drive a single propeller or fan would probably be longer, and my favourite from this list is the LearFan.

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  • $\begingroup$ A splitter gear on the shaft to run a second fan, surely must weigh less than a full engine core, minus the large amounts of ancillary systems required to run a full engine. $\endgroup$ – jCisco Dec 6 '16 at 8:07
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    $\begingroup$ @jCisco: Still, the one large engine is almost as heavy as two small ones. Add gear and driveshafts, and the mass advantage is lost. Also, the systems on the large engine must be duplicated for redundancy while they can be single on the two small engines. $\endgroup$ – Peter Kämpf Dec 6 '16 at 13:17

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