# What are these flames on the Falcon 9 booster?

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 cued at T+ 00:35:

• Gas Generator exhaust, discussed before: space.stackexchange.com/questions/16750/… and space.stackexchange.com/questions/31129/… Dec 4 '18 at 20:03
• Or combustion gases creeping up the stage and filling the low-pressure area above the engine bells: space.stackexchange.com/questions/22305/… - I'm leaning towards the latter: the turbopump exhausts are in a ring around the center nozzle, these flames are visible on the outside of the engines. Dec 4 '18 at 20:19
• I'd agree with recirculation. Dec 4 '18 at 20:30
• – uhoh
Dec 5 '18 at 2:12
• – uhoh
Dec 5 '18 at 2:28

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

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.

• Thank you! I had no idea that there is a second engine down there. Dec 5 '18 at 0:58
• 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.
– uhoh
Dec 5 '18 at 2:28