During the Falcon 9 second stage firing of the Merlin 1D Vac engine, the video shows an exhaust plume ejecting from near the top of the nozzle bell, and canted outward (say 10 degrees) from the bell. I was thinking that was the Turbo Exhaust, but based on this article, I see that that is not the case. What in the world is that exhaust plume?

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    $\begingroup$ Welcome aboard! Can you edit your post to tell us which article, which video, and at what time in the video? $\endgroup$ Commented Oct 25, 2020 at 21:39
  • $\begingroup$ See the latest video from yestday's SpaceX Falcon Launch at: youtube.com/watch?v=UM8CDDAmp98 $\endgroup$ Commented Oct 25, 2020 at 21:45
  • $\begingroup$ Such move the video bar to the right till you see the Merlin 1D Vac engine firing. There are two camera positions viewing the Nozzle. One of the positions clearly shows a large plume of what appears to be Turbo Pump exhaust (I thought) but now I don't know. Just curious what it is. $\endgroup$ Commented Oct 25, 2020 at 21:47
  • $\begingroup$ I figured it out. The plume in question, starts after the last Merlin 1D "2" second insertion burn. After that, the second stage was used to put a spin on the satellite delivery section such that the satellites would separate properly. That plume, is most likely Gas (nitrogen jet) or even O2 being exhausted to initiate the spin. You can see frozen gas accumulation at the tiny nozzle where the gas is being ejected. The YouTube I posted, is actually the 18 Oct 2020 Starlink 14 launch, not yesterdays launch. $\endgroup$ Commented Oct 25, 2020 at 22:32

1 Answer 1


The Merlin 1D VACUUM uses its turbopump exhaust as engine nozzle film coolant, it it recirculated. Discussion of Merlin 1D Vac here

If you are referring to the gaseous exhaust from the small pipe at the top of the bell/below the engine. Seen here at mission time 43:54 , that is just venting the excess pressure in the oxidizer, following engine cooldown for planned firing and continuing after the last burn. You can see it active both before and after the final burn. The spin-up for deployment occurs only seconds before deployment, and is done using N2 thrusters.


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