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.
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.
@jwenting correctly reminds me that for completeness a single power plant driving two contra-rotating propellers should also be mentioned. This is indeed the only way on airplanes of one engine driving two propellers, albeit not adjacent, that has seen wider adoption, from the RR Griffon on the Avro Shackleton to the NK-12 of the AN-22, Tu-95 and Tu-114.
Of course, all single-engine helicopters also use this one engine to drive their two propellers …
From what I've read, bigger props (and fans) are more efficient, because they move more air. However, at high subsonic speeds, the tips of props start to run into the sound barrier.
So, that seems to argue for more smaller props/fans, whose ends won't hit the sound barrier.
As others have mentioned, extra crankshafts and gears cost weight. I think I've seen designs using superconducting generators and motors driving multiple fans. However, I can't find it at the moment, and superconductors are probably not ready for prime time...
I don't know if you could bleed air from the edges of one fan, to drive, say, a fan on each side. But if you could, that would move more air. Although it might weigh a lot.
This is all assuming one turbine has enough oomph to drive multiple fans. I don't know if that's the case, or will be any time soon.
Disclaimer: I'm guessing, from limited reading.
Actually, there's one example already: F35B STOVL.
It's not so impossible I guess. When the efficiency and weight penalty is offset by some other requirements, e.g. ground clearance (next gen 737-Max?), we may soon see a dual-fan-single-core layout in the future!
Mechanically it is difficult, although moving more air with lower rpm props has the possibility of improving thrust efficiency per horsepower.
Keep in mind, while this approach certainly benefited the Wrights 30 to 50 mph craft, slower rpm props will more rapidly lose thrust efficiency as forward speed increases due to changes in their relative wind (Angle of Attack). Especially for fixed props, smaller, with higher rpm, gave a greater effective thrusting speed range as aircraft design moved beyond the Wrights. Variable pitch props partially solved this issue, enabling another leap forward to higher speeds, but now power output became more of an issue as 2x speed meant 4x drag leading to:
Multiple engines turning multiple props, which also improved safety in case one engine failed.
Further improvements in power output lead to: more blades on props. Still more power from jet turbines: many bladed fans.
Both with lower thrusting efficiency but much more thrust output and greater speed range than 2 blade props.
So, could a turbo fan have 2 adjacent fans? Increases in reliability and greater power producing efficiency certainly has started a trend towards fewer, larger jet engines with bigger fans. One giant turbine running 2 fans? Possible.
One giant turbine generating electricity for 4 fans? Aha! No mechanical linkages needed!
But, especially for large passenger aircraft, going with less than 2 power plants will be unlikely in the near future, unless it's a zeppelin.
