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This is a followup to a question about why the Falcon 9 second stage flame was invisible when out of the atmosphere. See: Why is the flame of the Falcon 9's 2nd stage invisible?.

That was well answered, relative to the fireball one sees at lift-off. However, it raised the secondary question of why the extremely hot gas and/or plasma of large rocket engines is not brightly glowing in the visible spectrum, whether in a vacuum or not?

Specifically, the temperatures of large space-capable rocket engines are said to be up to ~3500K. Why is it not glowing brightly with yellow to warm-white coloring just below the nozzle here?

Space Shuttle Main Engine (RS-25) Test, 1981, NASA

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    $\begingroup$ Note that’s a hydrogen-oxygen engine (SSME/RS25) which has a much fainter visible plume than hydrocarbon engines do. I know we have several QAs about exhaust plume appearance on the site; have you searched those? $\endgroup$ – Russell Borogove May 31 at 21:55
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    $\begingroup$ The visible part is the first shock diamond, caused by a standing wave in an overexpanded supersonic exhaust. If you scroll down in this question, uhoh shared some beautiful pictures showing a larger view. space.stackexchange.com/questions/16926/… $\endgroup$ – Greg May 31 at 23:34
  • $\begingroup$ I think this will be answered already in answers to one of these $\endgroup$ – uhoh Jun 1 at 4:16
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    $\begingroup$ @RussellBorogove, I have searched quite a bit. I see a variety of answers discussing the different chemical properties and how different fuels burn with different visible results. But I have not found one that discusses why a 2000K - 3500K black box (i.e. any 2000K - 3500K exhaust) would not glow due to heat even if not combusting. $\endgroup$ – HumanJHawkins Jun 1 at 16:22
  • $\begingroup$ A good related question would be "what are the visible elements of the flame?". For me, gases should be transparent and thus the flame always invisible. Also note that your question does not apply to SRB. $\endgroup$ – Manu H Jun 1 at 17:31
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I have not found [an answer on the site] that discusses why a 2000K - 3500K black box (i.e. any 2000K - 3500K exhaust) would not glow due to heat even if not combusting.

According to this paper, gases don't behave like ideal blackbodies.

When heat is injected into a gas, the energy is primarily redistributed into translational degrees of freedom and is not used to drive emission ... Once reflection and translation are properly considered, it is simple to understand why gases can never emit as blackbodies.

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If you look closely enough , all 'flames' are transparent at the point of oxidation. It is only slightly later in time, and thus a bit away in a spatial dimension, that the heated products of oxidation shed photons as they cool, resulting in what we view as 'flame.' You can see this on a gas stove, or a strongly burning hunk of wood.

When it comes to current rocket engines, I do not know just how far the exhaust particles travel before cooling enough to generate visible photons. As some of the comments point out, some of the fuels don't generate visible flame -- same as with certain race car fuels.

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