In these two lovely images:

Source: Wikimedia Commons

Source: wvi.com

The exhaust end of the engines are glowing bright orange from being so hot. Do they get that hot during normal flight operations, or are they hot because the engines are statically mounted for testing and have (relatively) minimal air flow to cool it?

If it's normally this hot, what is the exhaust made of, and how is it insulated to keep it from melting the nacelle around it?

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    $\begingroup$ Those are afterburning engines that dump raw fuel into the exhaust stream to produce additional thrust. Specifically the engines you posted are ramjet engines used in the SR-71. The SR-71 engine was basically 2 engines in 1, one turbojet for lower speed and then at higher speeds a ramjet. What you have posted are images of the turbojet in afterburner, so yes it can get that hot. The SR-71 was made with titanium materials that have very high melting points. $\endgroup$ – Ron Beyer May 12 '16 at 15:43
  • $\begingroup$ Closely related: How are temperature differences handled in a jet engine?. $\endgroup$ – Jan Hudec May 12 '16 at 19:19
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    $\begingroup$ I think @RonBeyer is trying to say that you've picked two pictures of a very special engine for a very special aircraft. There is nothing "normal" about the SR-71. What is "normal" for the SR-71 is utterly absurd for any other aircraft... Take for example, the fact that it leaks fuel on the ground. If it didn't, when the material expanded due to the skin heat generated in flight, it'd pop like an overfilled balloon. $\endgroup$ – T.J.L. May 12 '16 at 19:53
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    $\begingroup$ I have personal experience only with pulso jets on model aircraft. Yes, they, too, would glow orange in static operation, but when airborne the outer flow would cool them sufficiently to become "dark" again. $\endgroup$ – Peter Kämpf May 12 '16 at 20:06
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    $\begingroup$ Here is an image of the F136 in a special test chamber that simulated flight conditions, including flight pressure and flow. As you can see the afterburner is not glowing orange. That being said, many engine internals, especially the combustor and HPT glow orange the entire time the engine is running. $\endgroup$ – OSUZorba May 13 '16 at 3:34

Both images you posted are of Pratt&Whitney J58 afterburning turbojet that was used on the Lockheed A-12 and Lockheed SR-71 aircraft. That engine ran particularly hot.

The engine was designed for operation at Mach 3.2. The efficiency of turbojet increases with speed, but only to a certain point and somewhere above Mach 2 it starts to decline. However efficiency of afterburner continues to increase, so this engine was designed to operate continuously with afterburner and to generate higher fraction of power in the afterburner than other engines.

The afterburner combustor liner was able to withstand temperature up to 1,760°C (3,800°F). The core of the flame was much hotter still, but a colder air was fed along the walls, like in all turbine engines, as explained here. The engine was also cooled by incoming fuel similarly to how rocket engines are.

Additionally, at the top speed, the compression heating as the air was slowed to subsonic speed in the inlet heated it to over 400°C (800°F) at the compressor face already. Since in the static test the air was not that hot, the engine might have actually ran a bit colder in the test than in normal operation.

This was however specific to this one engine. The compression heating is negligible at subsonic speeds and grows faster at higher Mach numbers. Most supersonic aircraft fly around Mach 2, where the effect is already noticeable, but not nearly as pronounced. For most engines, they run at roughly the same temperature in test as they do in normal operation. It just isn't as high as in this special case.


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