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This question already has an answer here:

I see something interesting the webcast of the NROL-76 launch. At around T+00:00:55 it looks like flames from the exhaust start jumping around the bases of the ring of eight nozles in a circular sweep. Until this point the exhaust is only below the nozzles, after this point it seems everywhere. The process takes place between 4 and 6 km of altitude, at a speed 250 to 300 m/sec.

The video is keyed to start ten seconds earlier, where it looks to me like the exhaust on the left side of the image starts getting "jittery" before it begins to jump around the bases of the nozzles where they connect to the combustion chambers.

The image samples are for convenience only. If you can, view the video itself. If you need more screen shots, explain what you need and I"ll try to add it for you. Also, remember that YouTube allows you to play at half and quarter speed, and in HD.

What is happening here? What causes this change in the shape and location of the engine plume?

In the GIF the images are spaced by about 1 second, so it is approximately normal speed (though under-sampled)

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marked as duplicate by uhoh, Mark Omo, Nathan Tuggy, DrSheldon, Jan Doggen Dec 6 '18 at 8:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I've voted to close as duplicate; I think answers at the duplicate nicely answer my question as well. As far as I know there's nothing wrong with duping to a newer question. $\endgroup$ – uhoh Dec 5 '18 at 23:33
  • $\begingroup$ There's also another answer that addresses this here. $\endgroup$ – uhoh Dec 5 '18 at 23:36
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So, rocket nozzle geometry is designed differently for different ranges of atmospheric pressure because as pressure decreases, the direction of rocket gasses escaping moves from downward vertical toward more and more horizontal. By the time rockets designed to operate in the atmosphere reach the vacuum of space, the exhaust is exiting pretty much perpendicular to the engine bell. The jittery-ness you see, is atmospheric drag on the nose cone slowing the overall craft down a little more than the lighter upward moving exhaust gasses. The gasses are actually passing the rocket nozzle in the upward direction. This phenomenon is really visible watching Saturn V launches. Views from ground camera's really capture how far up the first stage the engine plume is travelling while the overall rocket is experiencing atmospheric drag. In the movie Apollo 13, Tom Hanks say's, "Get ready for a little jolt boys." When the first stage shuts down. Pressure/drag on the nose cone RAPIDLY slows the craft and from the outside you can see the engine plume appear to completely pass the vehicle. Pretty cool stuff.

On Apollo(s) 15, 16 and 17 the geometry of the engine bell on the Lunar Module was widened to accommodate for plume direction in a vacuum. (Also to minimize engine bell blow back inches from the Lunar ground, which can rupture the engine.) (Hmm, Apollo 15 performing new engine shape flight testing on the first operational all up science mission? Pretty gutsy move.)

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  • $\begingroup$ Thanks for the nice answer! For the following: "The jittery-ness you see, is atmospheric drag on the nose cone slowing the overall craft down a little more than the lighter upward moving exhaust gasses. The gasses are actually passing the rocket nozzle in the upward direction." are you thinking that this is buoyancy, or just low pressure, or flow separation? I'm wondering why it happens suddenly in one place, then over a few seconds moves around the base of the rocket. $\endgroup$ – uhoh Jul 16 '17 at 15:34
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    $\begingroup$ "Are you thinking this is buoyancy, or just low pressure, or flow separation?" I don't want to answer in the affirmative if I don't know the exact reason. I would guess some or all of the above. I would also add possible delayed ignition of combustion bubbles. That's what all the sparks you see near the Space Shuttle engine bells are doing right before lift off. Burning off escaping combustion bubbles. It is interesting to note, you said you see this activity primarily in the 250 to 300 m/s range as this is the trans-sonic range. There could be what in aerodynamics is called critical mach turb $\endgroup$ – W. Peek Jul 16 '17 at 16:01
  • $\begingroup$ I've removed the accept because this and currently this answer offer a more likely source. Do you see a way to better distinguish between the two explanations for the case shown in this question? Thanks! $\endgroup$ – uhoh Dec 5 '18 at 4:44

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