Jet engine design?

Why don't jet engines (or a type of engine like it) have the exhaust from the jet engine spinning a turbine on the outside of another turbine to produce thrust, I would think if you had this system you could make it so the outer turbine had different gearing from the inner turbine to create more rpm meaning

Why don't jet engines (or a type of engine like it) have the exhaust from the jet engine spinning a turbine on the outside of another turbine to produce thrust, I would think if you had this system you could make it so the outer turbine had different gearing from the inner turbine to create more rpm meaning more thrust from the inner turbine

Other answer:

Liam:
In a funny kind of way, you are describing what we already have, the Turbo-Fan. This is the kind of engine fitted to every airliner and indeed, the core ''jet'' while producing thrust with its exhaust, also drives a turbine which drives the big fan in front of the engine.

This does move a truly huge volume of air backwards and, in doing so, produces a huge ''equal and opposite'' reaction pushing the aircraft forward. So yes, Rolls-Royce, GE, Pratt & Whitney, Williams and all of the other engine makers have thought of this one!

Vincent G:
Like wow… You have just invented the double spool jet engine, which in on every turbofan engine since the 1950's, except for the triple spool engines.

Call us when you invent the light bulb, that is another neat thing we're waiting for.

tavares:
they already do

(you think jet engine manufacturers are as stupid as you?)

JC:
Unnecessary. 70% of the centrifugal torque energy generated by the turbine is already transferred back to the compressors through a transfer shaft. It is technically a self sufficient piece of engineering. Very little energy goes into waste.
Skipper747:
This all is achieved by compressor stage fans, rather than "turbine fans" –
Aft fans, were at one time designed by GE – the CF-700 and the CJ-805-23 –
Most of thrust in modern jet engines is produced by fan rather than the core of the engine –
Steve:
The only thing in the pipeline that remotely resembles your rambling is the P&W geared turbofan, where the fan section (normally driven directly by the low pressure spool) is instead driven through a 3:1 reducing gearbox which results in the fan operating at a much more optimum speed.
Mincing Meat:
I'm not sure I understand.
The exhaust going out of the back of the jet engine – yes you could put an extra fan there to extract power to drive another fan to generate forward thrust. But the act of putting the extra fan there will create some drag which will pull the plane backwards, so cancelling out the newly generated forward thrust.
charles:
I think Mr Whittle must have missed that.
Hannah:
they already do

(you think jet engine manufacturers are as stupid as you?)

Gray:
I think Mr Whittle must have missed that.

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