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I was recently browsing photos of early Apollo missions and noticed that the plume of the Apollo 4 during launch is significantly bigger than that of Apollo 8. Wondering why is that?

apollo 4 and 8 comparisson

Apollo 4 (top) vs. Apollo 8 (bottom) plumes.

I am not a hoaxer or something like that. I don't have even a slightest shadow of a doubt that the people from the planet Earth walked on the surface of the Moon 6 times. The evidence to support this fact is overwhelming. Just really curious why such a difference. Does it depend on payload mass or different engine modes or something like that?

Photograph sources: Apollo 4 and Apollo 8

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    $\begingroup$ Incidentally, while this site is distinctly unwelcoming to moon-landing deniers and flat-earthers, we're fairly tolerant of good faith "what explains this phenomenon?"-type questions. Welcome to Space! $\endgroup$
    – Russell Borogove
    Commented Feb 29, 2020 at 4:27
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    $\begingroup$ Vaguely related: space.stackexchange.com/q/20916/195 $\endgroup$
    – Russell Borogove
    Commented Feb 29, 2020 at 4:35
  • $\begingroup$ We should know and compare the heights of Apollo 4 and 8 when the images were taken. The center engine of the first stage of the Saturn V was switched off after some time to limit acceleration. We should know if 5 or 4 engines were running for both images. $\endgroup$
    – Uwe
    Commented Feb 29, 2020 at 11:53
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    $\begingroup$ from the plume expansion, Apollo V appears to be higher @uwe, looking closely, plume induced flow separation seems to have started, so it's probably going about Mach 4 $\endgroup$
    – user20636
    Commented Feb 29, 2020 at 13:24
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    $\begingroup$ The plume expansion (and flow separation) is much greater when the centre engine is shut down @Uwe (e.g., in this video at 2:36 you can hear "inboard out, flight [director]; copy inboard out, right on time huh; rog" $\endgroup$
    – user20636
    Commented Mar 1, 2020 at 10:59

2 Answers 2

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In addition to the foreshortening illustrated by Organic Marble, the appearance of rocket plumes changes significantly with altitude. At higher altitudes, the pressure of the surrounding air is lower, the plume expands and dissipates faster, and less free oxygen is available for combustion with the fuel rich exhaust (which is the source of the bright yellow plume in kerosene-oxygen engines like those of the Saturn V’s first stage). I’m guessing that the Apollo 8 picture is taken at higher altitude than the other.

The chemistry of the modern Falcon 9’s engines is similar, and continuous high-quality video of F9 flights is readily available; you can watch a flight from liftoff to staging and study how the plumes evolve.

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  • $\begingroup$ Incidentally, nowadays, one often sees complex plume displays (ICBM tests, especially when they start to manoeuver, FalconX, etc) which then appear on Instagrams with people yelling "What is that" "OMG" etc. I can't remember seeing these back when the Shuttle was still going, Is it just an effect of ubiquitous cameras or did something change in propulsion? $\endgroup$
    – David Tonhofer
    Commented Mar 1, 2020 at 9:54
  • $\begingroup$ Ubiquitous cameras and social media to distribute the pictures/videos on, mostly. The very tail end of the shuttle program could have had a little mobile camera exposure on Facebook and Twitter (and even in Instagram’s first year) but the culture of photo sharing really exploded after 2011. $\endgroup$
    – Russell Borogove
    Commented Mar 1, 2020 at 17:24
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@russell-borogove explains that the plume changes during flight. We don't know the precise height when the two photographs were taken (both are labelled "climbing after lift-off" in the NASA archive).

For a better comparison, I combined the two images and adjusted size and contrast to be roughly the same:

enter image description here

My impression: There is a slight difference in the plumes (about a factor of 2), but most of the visual difference comes from the very harsh contrast in the Apollo 4 picture. There even the "smoky parts" of the plume that don't glow are overexposed, while in the Apollo 8 image only the innermost part of the plume is overexposed. This makes the plume seem much smaller although it isn't.

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