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On nearly every SpaceX webcast since 2020 they explain that the Falcon 9 booster has soot on it because, during the entry burn, it flies through its own exhaust plume very quickly depositing the carbon based fuel on the booster. Recently, on this webcast at 7:25 i noticed that the fairing also has soot. Why is this? They don't perform on burn while flying through an exhaust plume. So, what deposits the soot there?

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SpaceX has released video of the fairings being released and falling through the second-stage plume (this link goes to a Scott Manley video that contains the footage; I'm not quickly finding the original video, so maybe it was on SpaceX's Twitter). Falcon 9's upper stage is also kerolox (see the sidebar at bottom), so it's a credible source of soot.

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  • $\begingroup$ Why so much though (its around as much as the booster) @Erin Anne $\endgroup$ Mar 2, 2023 at 0:03
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    $\begingroup$ @Starshipisgoforlaunch in your video in the question, note that one fairing is white, and the other has discolouration on it - so one is being reused and the other is probably new (Im going to guess theres someone out there that tracks these things). What this indicates is.... just from looking at the video, we have no idea how many times the fairing has been reused vs the booster, so we dont know that they are comparable in terms of sootiness. We may also be seeing scorching on the fairing, given it separates at 8500KM/h and is slowed by the atmosphere alone during its initial descent. $\endgroup$
    – Moo
    Mar 2, 2023 at 23:08
  • $\begingroup$ Okay. Thank you. @Moo $\endgroup$ Mar 2, 2023 at 23:17
  • $\begingroup$ @Moo interesting point on scorching. I think it's been asked before why SpaceX doesn't wash all the soot off the rockets, but actual scorching seems like it'd be damage to the fairing's coating and much less acceptable. hmm. $\endgroup$
    – Erin Anne
    Mar 2, 2023 at 23:42
  • $\begingroup$ @ErinAnne depends on the level of scorching as to whether its an issue or not - if its surface level only, with minimal effect on structural integrity or aerodynamics, then theres no reason to do anything about it. $\endgroup$
    – Moo
    Mar 3, 2023 at 0:59

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