The Wikipedia article on the Pratt & Whitney F119 that powers the F-22 says the following:

  • The high pressure and low pressure turbines are single stage and counter-rotating, thus shortening the engine and saving weight.
  • Turbine: Counter-rotating 1-stage high-pressure, 1-stage low-pressure turbines
  • Type: Twin-spool, axial flow augmented turbofan

We also find:

Thrust augmentation is using additional fuel in a turbofan's cold bypass air only, instead of the combined cold and hot gas flows as in a conventional afterburning engine.— Afterburner

  1. What's the benefit of lighting the bypass air compared to relighting the exhaust? Wouldn't the cooling benefits of the bypass air be lost?
  2. Does that mean that the F-22's "supercruising" could be "augmented"?
  3. How does by counter-rotating the two-spools, the engine becomes shorter and lighter?
  4. How are the turbines counter-rotated?

While the answers are probably buried deep in the Integrated High Performance Turbine Engine Technology program, hopefully some educated guesses can entertain my curiosity.

My only guess for 4. is the use of aggressively curved and converging stators, which would otherwise stall a normal none counter-rotating LP turbine. Or perhaps the lack of a stator in the turbine section, since it's a single LP stage, which would also answer 3.

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    $\begingroup$ Much better! Thank you for helping keeping the quality of this site as high as it is. Will delete earlier comment to clean up. $\endgroup$
    – DeltaLima
    Commented Nov 28, 2016 at 16:56
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    $\begingroup$ heating cold air is thermodynamically more efficient than heating hot air. Similar to why flying in colder air increases efficiency. $\endgroup$ Commented Nov 28, 2016 at 21:09

1 Answer 1

  1. The afterburner is aft of the gas core of the engine in the jet pipe and bypass air from the fan would have already been mixed with the exhaust gases from the core prior to passing through the afterburner flame holders.

  2. Supercruise is by definition supersonic cruise flight without the use of afterburning, so augmentation would not be in use here

  3. Counter rotation of the spools reduces net torque from the spools upon the structure of the engine requiring less metal and therefore lightens it. The length of the engine is reduced by making use of a single stage, high efficiency turbine rotor per spool as opposed to multiple turbine stages per spool, which would make the hot section of the engine longer.

  • $\begingroup$ Agreed, the length of the engine is irrelevant to the counter-rotating spools. The number of stages in the compressor, turbine and the efficiency of the combustion chamber ultimately determine the length of the engine. $\endgroup$
    – LuftBier
    Commented Nov 28, 2016 at 12:55
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    $\begingroup$ On a low bypass afterburning turbofan engine, the fan air is mixed with the exhaust after the final turbine stage prior to entering the afterburner. There is no separation of the air streams at that point. Also augmentation is afterburning; I don't know who wrote that Wikipedia article but they are wrong about that - it does happen from time to time. $\endgroup$ Commented Nov 28, 2016 at 15:05
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    $\begingroup$ You're talking about the PCB systems on experimental RR Pegasus engines - that's a little bit of a different animal than the reheat system on a low bypass turbofans used for fighter aircraft. The F-119 does not utilize PCB capabilities in its design. $\endgroup$ Commented Nov 28, 2016 at 18:28

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