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Recalling what little physics I know, this could be explained by two factors:

  • The blue color is given by the characteristic EM radiation of C-H bond breaking, C- and H- radicals forming and recombining with O- radicals to form CO2 and H2O. This is independent of temperature and is the reason why a new gas oven's flames are blue.

  • The orange color is definitely the characteristic blackbody radiation wavelength of about 1500K, ~1200 degree Celsius. Now the problem is what the source of this blackbody radiation is. The way I see it it's possibly from three sources:

    1. The hot afterburner inner wall,

    2. The soot produced by partial combustion, or

    3. The incandescent exhaust itself.

If the above recap is correct, this means either

  • the Western afterburners either run a lot dirtier than Eastern afterburners, which is unlikely because F35 and F22 have orange afterburner flames and they must be running squeaky clean to not have their afterburner inner walls coated with soot (amorphous carbon) thus having a huge IR signature because of the high emissivity of carbon, and we do not see black smoke coming from any jet since F4,

or

  • Western afterburners are a lot hotter than Eastern afterburners, to the point that the blackbody radiation color, which is dependent on temperature, dominates the radical color, which is independent of temperature. This seems to be the logical conjecture, given that the Russians have always been pretty poor on metallurgy.

Does anyone have authoritative knowledge on this subject? This rumor seems to be circulating on the internet forever.

F-35 on afterburner

F-22 on afterburner

Su-27 on afterburner

Updates

  • OK, the exhaust may seem a little purplish, that is caused by the emissive characteristic of H2O.

  • Or, maybe the Su-27 and the MiG-29 just had a cold start and the inner walls of their afterburners are still cold?

  • On closer inspection, the orange color seems to be solely coming from the glowing afterburner's inner wall, as opposed to the exhaust, although the exhaust is glowing also. This seems to confirm my guess that Western afterburners run hotter.

  • OK, I've spoken to some guys from one of the ex-Soviet design bureaus. One of them said the blue color is the result of cold afterburner and Sukhoi afterburner design, the fuel being dumped at acute angles into the center of the cylinder using a few big injectors.

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    $\begingroup$ I'm guessing it has something to do with the fuel. $\endgroup$
    – John K
    Jan 19, 2019 at 19:52
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    $\begingroup$ Three photos are, unfortunately, far from representative. Light conditions can make rather big difference to representation of colors, as can exposition (on night photos, the flames usually look white simply because they are partially overexposed). It would be best to compare photos from consecutive take-offs on the same air-show on the same day (that will also minimise difference in fuel as everybody will fuel from the same tank—the fuel standards are not exactly the same, but similar enough that nobody bothers with special fuel). $\endgroup$
    – Jan Hudec
    Jan 19, 2019 at 22:36
  • $\begingroup$ I've seen jets take off from aircraft carriers in full burner with both orange and blue visual signature. likewise fully loaded F-16's and F-15's, day and night. (the most pronounced visual impact is an night, of course). This question does not fit real life. I think it's place on SE, if it has any place at all, is in physics se. $\endgroup$
    – KorvinStarmast
    Jan 20, 2019 at 3:07
  • $\begingroup$ While not an important point, the F-14 was pretty smoky, and the B-52 still is (though not afterburning) $\endgroup$
    – MD88Fan
    May 14, 2021 at 22:10
  • $\begingroup$ I also wonder why some of the Chinese fighters are blue while others are orange, even among the ones that uses supposedly the same engine. $\endgroup$
    – user3528438
    Sep 1, 2022 at 2:31

2 Answers 2

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It all depends. I don’t think there’s a distinctive color between the two. I would submit that the photos of the Flanker may be in a early stage of the afterburners lighting versus the photos of the Raptor or the F-35, which appear to be in full afterburner. The color of the exhaust gas is a function of the stoichiometric mixture of fuel and air ratio. It full burner the mixture is quite rich, resulting in a orange colored flame. Lower stages of afterburner are probably much closer to peak stoichiometric mixture. Incidentally these blue colored flames are indicative of a much hotter exhaust gas temperature than an orange colored exhaust. I’m not sure of the reason why one is selected over the other. I can submit that there are plenty of photos of Russian aircraft in afterburner with orange colored exhaust and other pics of western military airplane with blue colored exhaust as seen below.

enter image description here enter image description here

enter image description here

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    $\begingroup$ The blue colour is not indicative of hotter flame—no flame is hot enough to reach that colour of blackbody radiation. It is explained in the question and that explanation is somewhat clearly correct. $\endgroup$
    – Jan Hudec
    Jan 19, 2019 at 22:08
  • $\begingroup$ The middle photo is probably blue because of this effect. Unfortunately that reasoning does not seem to apply to the third image in the question. $\endgroup$
    – Jan Hudec
    Jan 19, 2019 at 22:39
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    $\begingroup$ no the blue color comes from characteristic emission of CH+O2 reaction, not blackbody radiation $\endgroup$
    – Meatball Princess
    Jan 20, 2019 at 5:59
  • $\begingroup$ If the color were solely due to lighting, one should be able to find photos of planes with both colors, right? $\endgroup$
    – StephenS
    Feb 25, 2022 at 14:02
  • $\begingroup$ It’s unusual but there are a few shots of Fat Amy with a blue AB flame. i.pinimg.com/originals/9e/9b/20/… $\endgroup$
    – Romeo_4808N
    Feb 25, 2022 at 14:50
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The light problem is quite easy, during the day we don't see much of the blue colour as there are other sources of light brighter. At night as seen it shows as a source of light. Then the question arises, why can't we see the Balck body radiation, simple because the AB exhaust is brighter.

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