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Can an S-duct fit a turbofan exhaust instead of the intake? The idea here is to merge 2 exhausts into one at the exact center of the tail. This would be for twin-engine aircraft that have both engines at the tail, like the Boeing 717.

There are of course S-ducts for intakes, like on the Tu-154. I have never seen them for exhausts, however, probably because it's much harder due to the heat.

I know it is engineeringly possible somehow, because I've seen vectored thrust devices with variable geometry, like the iris types and swivel types. For a large commercial jet, it would be easier because the geometry would be static--no moving parts.

But I want to know if it is economically feasible. To take the heat, I think it just needs some super-pure graphite, which does not sound so expensive to me. But I am not an expert.

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Yes they can, and it has been done in the Garrett AiResearch ATF3. Although the engine was not hugely successful, it is still being used in aircraft today.

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From this answer:

The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon.

You can see several S-ducts leading the hot exhaust gases internally into the little staircase-like contraptions behind the fan.

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  • $\begingroup$ Why is a low IR signature valuable in a business jet? $\endgroup$ – Adam Jun 23 at 3:58
  • $\begingroup$ @Adam for those flying their business jets to the fun parts of the world. $\endgroup$ – AEhere Jun 23 at 12:05
  • $\begingroup$ @Adam the engine was developed for a UAV $\endgroup$ – Koyovis Jun 23 at 19:07
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Not really. Any type of curve in an exhaust would raise pressure in the duct. Remember a jet engine extracts power from the exhaust stream to turn the fan and compressor as well. Any additional exhaust "plumbing" would be an unnecessary engineering nightmare in addition to adding weight.

But the British do have a liking for stacking jet engines vertically as seen in the English Electric Lightning to eliminate yawing if one failed. This design also significantly reduces drag as the engines are housed in the fuselage instead of in exterior nacelles.

S ducts are ok for air intake provided adequate air can be drawn to feed the engine. They also may have the interesting property of reducing a 250 knot 15 pound goose into a loose mass of flesh BEFORE it hits the compressor intake by bouncing it off the duct wall, which can absorb the considerable kinetic energy of its impact.

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  • $\begingroup$ Raise or reduce pressure? $\endgroup$ – Carlo Felicione Jun 22 at 1:56
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    $\begingroup$ On the exhaust side the "S" would raise pressure in the exhaust duct, there for less pressure drop for exhaust turbines to extract at the engine outlet. $\endgroup$ – Robert DiGiovanni Jun 22 at 2:04
  • $\begingroup$ Actually an S-Duct was clause a pressure loss in the jet pipe, reducing the thrust the diffuser can produce. $\endgroup$ – Carlo Felicione Jun 22 at 3:17

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