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Why are contra-rotating jet engines so rare? Such a jet engine would have contra-rotating turbines, or contra-rotating compressors, or both.

In fact I cannot think of even one aircraft that has this. If some prop-engines have contra-rotators, I don't see why some jets don't have it either.

It seems to me the contra-rotating parts would be more efficient in extracting power or compressing air. There are already contra-rotating turboprobs (Tu-95), and turboprops are very similar to jet engines, so I think it is not a question of technical knowledge. Another benefit would be reduced or eliminated gyroscopic forces hampering yaw or pitch torque, which I will ask another question about.

BTW, I am considering axial-flow jet engines only. I totally forgot there used to be centrifugally compressed jet engines, which have all but disappeared.

EDIT: The question is about jets (either turbofans or turbojets), but I mentioned the Tu-95 and its turboprop engines because turboprop spools are very similar to jet engines. However, I thought those Tu-95 engines had contra-rotating everything, but it turned out only the propellers were contra-rotating, not the turbines. Nevertheless, if there was a turboprop with contra-rotating spools, I would very much like to hear about it here because I think it would have great bearing on the question of contra-rotating jet engines and their efficiency.

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  • $\begingroup$ The P&W F119 (F-22's engine) has contra-rotating turbines and compressors. (That is, rotors: you can't have one without the other :) They claim it allows to make the engine shorter (by removing stator stages between the compressors and turbines) and improve efficiency. But I don't know how it actually fares against increased wear and mechanical losses. I guess it must be more reasonable for the top-performing and/or special military engines such as F119 or Pegasus than for longer-living civil engines. $\endgroup$ – Zeus Sep 19 '16 at 4:27
  • $\begingroup$ @Zeus The wiki page for the F119 (en.wikipedia.org/wiki/Pratt_%26_Whitney_F119) describes it as a twin spool engine. Nowhere does it mention contra-rotating anything. Are you sure it's contra-rotating and do you have a source? $\endgroup$ – DrZ214 Sep 19 '16 at 8:18
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    $\begingroup$ For example, right here: pw.utc.com/F119_Engine . By the way, it's direct derivative, F135 for F-35, is similar in this regard. $\endgroup$ – Zeus Sep 19 '16 at 12:10
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If the low-speed and high-speed spool rotate in the same direction, the relative speed between them is low. Contra-rotating means you need to double the individual speeds to arrive at the relative speed, and this will increase bearing losses greatly. Both friction and wear will go up with little benefit in regular operation. A notable exception is the Rolls-Royce Pegasus engine for the Harrier jump jet.

Stators between the rotating stages ensure that air flow inside the engine is mainly in axial direction, so aerodynamically a different rotation direction between two spools will make little difference. However, contra-rotation will indeed make the stator between the last stage of the high- and the first stage of the low-pressure turbine obsolete. The video linked by @Moo cites a 10% reduction in part count as the advantage. Additionally, the first stage of the low-pressure turbine should show improved efficiency (like the second propeller of a contra-rotating propeller).

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  • $\begingroup$ Can you explain a little more why with little benefit in regular operation.? In particular, the engines of the Tu-95 are contra-rotating turboprops, and the turbines of turboprops are very similar to jets. Or maybe those engines did not have contra-rotating spools, but just contra-rotating propellers? $\endgroup$ – DrZ214 Sep 18 '16 at 8:42
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    $\begingroup$ @DrZ214: Yes, the NK-12 of the Tu-95 drives the propellers through a single shaft, and contra-rotation is achieved in the gearbox. Regarding little benefit: The second paragraph already discusses the lack of aerodynamic benefit, and the gyro effects are small compared to the damping and control forces available in regular flight. Only in a VTOL design would contra-rotation offer a tangible benefit. $\endgroup$ – Peter Kämpf Sep 18 '16 at 8:48
  • $\begingroup$ Do contra-rotating spools within jet turbines need stators though? To me it seems two blades sweeping right next to each other will produce enough compression or extract enough torque such that stators are unnecessary. $\endgroup$ – DrZ214 Sep 18 '16 at 8:55
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    $\begingroup$ @DrZ214, no, they are not rotating at the same RPMs. But when they are rotating in the same direction, the bearing between them only needs to handle the difference while when they are rotating in opposite direction, it needs to handle the sum. And that's the Peter's first argument. $\endgroup$ – Jan Hudec Sep 19 '16 at 8:17
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    $\begingroup$ @DrZ214 It's all here $\endgroup$ – Peter Kämpf Sep 19 '16 at 8:38
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Contra rotation in civil turbine engines was first introduced by Rolls Royce in the Trent 900, where its high pressure spool rotated in the opposite direction to the other spools.

http://www.prnewswire.co.uk/news-releases/rolls-royce-trent-900-engines-provide-power-for-first-a380-154499805.html

The GEnx engine family use contra rotating spools - the high pressure compressor is driven clockwise while the low pressure compressor is drive counterclockwise.

The Trent XWB engine family follow on from the Trent 900 and also use contra rotating spools - the high pressure compressor is driven clockwise, while the low pressure and intermediate pressure compressors are driven counterclockwise.

http://www.easa.europa.eu/system/files/dfu/EASA-TCDS-E.111_Rolls--Royce_plc_Trent_XWB_Series_engines-01-07022013.pdf

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  • $\begingroup$ Interesting. Is there any word on how much more efficient this is compared to a non-contra-rotating turbofan of the same thrust? $\endgroup$ – DrZ214 Sep 19 '16 at 9:02
  • $\begingroup$ Given that contra-rotation was only introduced very recently to the Trent line (with the 8115), what is the technical benefit of doing so? $\endgroup$ – Peter Kämpf Sep 21 '16 at 22:37
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It seems to me the contra-rotating parts would be more efficient in extracting power or compressing air.

With propeller, contra-rotating propellers are the only way to stop rotation of the slip-stream. However jet engines have static guide vanes that stop the rotation almost as efficiently with a fraction of the complexity of actual contra-rotation.

Another benefit would be reduced or eliminated gyroscopic forces hampering yaw or pitch torque

These are negligible compared to the aerodynamic forces in flight. The only aircraft where it matters is the Hawker Siddeley Harrier and that does, indeed, have engine with contra-rotating spools.

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If you're talking about making contra rotating spools in an axial flow jet engine, the major advantage there would be torque reduction. However, keep in mind that the torque applied to each spool may have a considerable disparity in in magnitude, effectively eliminating much of the benefit.

Making individual stages contra rotating would have a nightmarish complexity in terms of gearing and associated systems to drive this, in addition to a serious weight gain and reduced reliability. For those reasons such a design was never seriously considered.

Now contra rotating fans have been considered from time to time. Some examples which directly come to mind are the UDF aft unducted fan design by General Electric in the 1970s as an attempt to increase engine efficiency. And SENECMA is apparently working on a contra rotating turbofan fan for increased efficiency as well as reduced IR signatures for UAVs.

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  • $\begingroup$ the major advantage there would be torque reduction. What does "torque reduction" mean? If it's reducing the driving torque obtained from the turbine, that would reduce engine power and thus reduce efficiency. keep in mind that the torque applied to each spool may have a considerable disparity in in magnitude Why is that? If each turbine has the same number of blades, same shape, and right next to each other, how could there be considerable disparity just from rotating in opposite directions? (I may have to open a new question on how contra-rotating jets would work). $\endgroup$ – DrZ214 Sep 18 '16 at 0:48
  • $\begingroup$ BTW, thanks for pointing out axial flow jets. I totally forgot there were centrifugally-compressed jet engines which have all but disappeared. $\endgroup$ – DrZ214 Sep 18 '16 at 0:48

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