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:


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.


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

  • $\begingroup$ Thank you! I had no idea that there is a second engine down there. $\endgroup$
    – Alexus
    Dec 5 '18 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
    Dec 5 '18 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
    Dec 5 '18 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
    Dec 5 '18 at 14:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.