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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
  • $\begingroup$ Many jet engines have centrifugal high pressure compressors, including modern designs that are currently in production. $\endgroup$ – J Walters Sep 23 at 1:51
  • $\begingroup$ @JWalters Can you name a few? Even if they are small business jet engines, I'd still like to look at them. $\endgroup$ – DrZ214 Sep 23 at 12:01
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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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The NK-12 is a single shaft turboprop with a counter rotating gearbox which drives the co-axial propeller. However, there are a few real counter rotating jet engines. For example the P&W F119PW100 from F-22 Raptor, the F135 from the F-35 family, the Yak-141's Soyuz R-79V-300, the MiG 1.44's AL-41F and the Harrier's Pegasus engine as well. These are twin spool engines. There are two main big benefits: - Higher compression ratio by the higher relative airflow speed at the meeting point of the two spools; - Almost zero precessive force due to the balanced torque of each rotors. This one especially good for the VTOL aircrafts because at zero speed there is no rotating momentum.

The three spool RB199 in the Tornado jets has also counter rotating shafts by the way.

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Torque reduction DOES NOT MATTER and is NOT a factor either way. What counter rotation does for a turbofan (in the present non-geared designs) is

  1. Increase the efficiency of the compressor by switching the 'swirl direction' between the fan & the first compressor stage. This has the effect of making the compressor close to unstallable, which means you can run WAY higher compression ratios, thus increasing efficiency.
  2. Between the high pressure & low pressure exhaust stages, counter rotating blades extract more energy from the exhaust gases, thus delivering more torque to the bypass fan. For example the GE genx.
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The real benefits of the counter rotating spool jet engines are the following:

  • Increase the efficiency by the increased compressing ration of the compressor, due to the counter rotation at intersection between the low and the high pressure stages. And just there. Between the last stage ofthe low pressure spool and the first stage of the high one, no need a stator, because of the opponent orinetation of the counter rotating blades. The counter rotatating blades, according to what is rotating in the front of it (but to the opponent direction) acting like a stator. How to build you up the pressure step on a compressor stage? You have a rotor which speeds up the airflow, and a stator, which slows it down, but meanwhile increases the pressure. So if you have a rotor with for example 10000 RPM, the stator "RPM" is zero, comapre to it. Now, if you have a counter rotating shaft, its rotor with another 10000 RPM means that the relative rotating speed going to be 20000 RPM. So the step up will be much higher, the pressure ratio at there is increased. But only at there. At the other stages of the LP and the HP compressors has to contain a stator. This is the same rule of the turbine as well, with the same benefit.
  • The second benefit is not the Torque-reduction. This is not make sense at the jet engine world, only in the propeller engine world (piston engine or turboprop). However, the real thing is the precession reduction, which is essential for the high AoA maneuvering. The rotating mass has its own precession, which tries to turning around the shaft of the rotating mass. This is obvious, if you have a low speed run at the runway and you hit the throttle on a propeller plane. The propeller torque is just one thing, because it would like to turning your plane around the longitudinal axis (but the gears holding it). But the precession of it would like to ruddering out you from the runway. This departure attitude is depending on the rotating mass and the RPM changes. The only thing which could stabilize it is the airflow around the plane, when your speed is building up. But inside a fighter jet, (F-22 or the F-35), the rotating mass of the shafts would like to break the plane out during a extremly low speed, high angle of attack maneuver, because of the rotating axis and the moving vector is not pointed in the same direction. It is critical situation. If you check the russian Su-30MKI...SM or the Su-35S, or Su-57 videos, you will clearly see, how the huge rudders and the TVC nozzles try to save the pilot's day, during a post stall maneuvering, driven by the very sophisticated Fly By Wire system. And very often, you can see a lot of unwanted slides, especially at the top of these low speed maneuvers, because of the AL-31FP or AL-41F1S engines similar direction spools. More on this, on those aircrafts, the engines are quite far away from the longitudinal axis, (unlike the F-22), so have a quite large rotating momentum (torque) as well.

And of course, due to the issue of the precession, all of the VTOL aircrafts has counter rotating spools, because it could preventing that the aircraft starts to rotating slowly around the vertical axis. And no, this is not the same fenomena as the helicopters have, by the main rotor torque. It is not from the torque. It is the precession.

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  • $\begingroup$ Because of the difficult flow calculation and relatively small numbers of really new engines. By other words, everyone just use their already-on-stock design to reduce the costs. $\endgroup$ – Mihály Laczkó Apr 9 at 5:39
  • $\begingroup$ I answered to your question. $\endgroup$ – Mihály Laczkó Apr 10 at 18:52

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