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When Falcon 9 was on its way to ascend, flames appeared around the bottom of the booster. What are they?

It seems to begin around about 20:36 in the video, T+ 00:41 3.6 km altitude and then appear at more and more locations

video frame grab cropped

video frame grab

video cued at T+ 00:35:

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I think that is exhaust gas recirculation. Exhaust products gather in the low-pressure area around the base of the rocket.

Diagram showing low-pressure area at base of rocket

The gas generator exhausts are grouped in a ring around the center engine. I've marked 8 exhausts (photo source), nr 9 is not visible here:

enter image description here

Their flames are long and narrow, and not visible when all engines are running. In a video of a landing stage, you can see the gas generator exhaust flame.

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That is the exhaust of the gas generator. This is basically an engine that burns the same fuel/oxidizer to turn the turbo pump that makes sure enough fuel/oxidizer gets fed to the engine.

Smoke Exhaust HansK pointing out gas generator

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  • $\begingroup$ Thank you! I had no idea that there is a second engine down there. $\endgroup$
    – Alexus
    Commented Dec 5, 2018 at 0:58
  • $\begingroup$ See space.stackexchange.com/q/22305/12102 as well. btw you may want to link to your answer here: space.stackexchange.com/a/31130/12102 as well. $\endgroup$
    – uhoh
    Commented Dec 5, 2018 at 2:28
  • $\begingroup$ In other words, instead of answering by simply linking to your answer, perhaps in this case closing is the correct option. The idea is to point to a good answer rather than re-answer. $\endgroup$
    – uhoh
    Commented Dec 5, 2018 at 2:55
  • $\begingroup$ @Alexus There are 9 engines down there. But it is not so much an 'engine' as it is a generator, for the power needed to turn the turbopumps $\endgroup$
    – geoffc
    Commented Dec 5, 2018 at 14:08

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