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This question already has an answer here:

I read that this would be the holy grail of jet engines:

Next Gen NB Engines : Contra Rotating Turbofan Fan

Why is this so and what problems are stopping us from doing this?

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marked as duplicate by Jan Hudec, fooot, vasin1987, Ralph J, Carlo Felicione Apr 4 '17 at 18:04

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ How is this different from the question you just asked? Contrarotating turbo fan fan $\endgroup$ – fooot Apr 3 '17 at 19:40
  • $\begingroup$ I was unsure whether there were advantages other than efficiency and why it being geared would make a difference, i thought you would want them to be equal speeds. The question i swhy are there these advantages and what difference being geared would have. @fooot $\endgroup$ – SRawes Apr 3 '17 at 19:52
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what problems are stopping us from doing this?

In some sense, it's business reasons. Developing a brand new jet engine from scratch costs a lot of money. Usually the pricetag runs into the billions. It will often take 10 years from the time you start to develop a new engine until you see a single dime of profit. If you are the board of directors of an engine manufacturer, and you give the greenlight to develop a new product, you might be literally betting the entire company on the success of that new product. So, you have a tendency to be conservative. If you can get a 10% performance improvement from some new never-tried-before fancy contra-rotating fan thingy, or you can get a 5% performance improvement by keeping the same basic tried-and-true design but just tweaking the aerodynamics a bit here and there, you are probably going to go with tried-and-true. Eventually, a limit will be reached where the basic turbofan architecture cannot be tweaked anymore, and then you may see some new thing like this tried, but I don't think we are quite there yet right now.

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