Do jet engine exhausts usually glow orange?

Question

In these two lovely images:

^{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?

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. — Ron Beyer, May 12 '16 at 15:43
Closely related: How are temperature differences handled in a jet engine?. — Jan Hudec, May 12 '16 at 19:19
@RonBeyer - not disagreeing or arguing your points at all. My question of "is that normal for flight ops" still stands... — FreeMan, May 12 '16 at 19:39
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. — T.J.L., May 12 '16 at 19:53
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. — Peter Kämpf, May 12 '16 at 20:06
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. — OSUZorba, May 13 '16 at 03:34
Also engine cooling design as significantly improved in the decades since the J58 was developed and massive importance has been placed on reducing IR signature. — OSUZorba, May 13 '16 at 03:37
@PeterKämpf, I suppose that would be specific to model engines though since full-scale engines would be primarily cooled by the bypass air and that shouldn't differ significantly between static and flight run or even be a bit colder in static run. — Jan Hudec, May 13 '16 at 06:55
@JanHudec See my second comment, cooling has improved a lot and IR signature reduction is much more important today than when the J58 was developed. But both pictures show the outer nozzle casing, not a combustor liner. Modern nozzles use bypass air to reduce IR signature and case temperatures, but the J58 used compressor bleed air that was already hot (especially in flight). Besides the temperature difference there is a lot more by-pass air the compressor bleed air. — OSUZorba, May 15 '16 at 04:27
I used to lie under the departures and watch aircraft take off.. on most airliners you wouldn't see anything but when a fully laden 747 took off you would see the inner part of the exhaust dimly glowing. One that sticks in my mind was the CI 747-200F.. — Anilv, Feb 12 '20 at 01:11

score 11 · Accepted Answer · edited Apr 13 '17 at 12:59

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.

score 0 · Answer 2 · answered Feb 11 '20 at 23:25

To supplement Jan Hudec's answer, you are also looking at the engine casing itself. When the engine is installed in the aircraft there is additional nacelle material surrounding it so that even at full power the metal glowing will not be seen looking at it from the side. (although in afterburner you will see the flame)

But the answer is yes, a typical jet engine will heat up enough to glow during normal flight operations. However, is it much much dimmer than your photos, really just a dull orange. And you usually only notice it if you are looking almost directly up the tailpipe at the turbine section.

score 0 · Answer 3 · answered Feb 12 '20 at 04:01

Q1: Yes the afterburner pipe and nozzle interiors can hit upwards of 1800-2000° F during full throttle operations.

Q2: Most engines have ceramic linings in the afterburner can and nozzle petals to protect the metallic parts from the high heat. In his book Sled Driver, Maj Brian Shul remarks about witnessing J58 engine test runs post maintenance out at Beale AFB, where the engines were brought up the maximum power for several minutes and then shut down. He stated the engine jet pipes would glow orange hot during operations (and took the photos to prove it!), yet within a minute or so of shutdown the ceramic linings were cool enough to touch!

Do jet engine exhausts usually glow orange?

3 Answers3