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Did I see the F9 2nd stage nozzle vibrate at about 2Hz at SECO-1? It looked like the bottom of the bell vibrated in an elliptical shape. Did I imagine that? Has this happened before?

$\hskip 3.4 cm$It's just after T+ 09:17, playing at half speed:

$\hskip 3.5 cm$enter image description here

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    $\begingroup$ Watching the video the vibration happens right at SECO and it looks like it is a transient as the engine shuts down. $\endgroup$ – geoffc Aug 24 '17 at 19:21
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This video from the Iridium-2 launch definitely shows a squash-and-stretch oscillation at SECO-1, after 23:45 into the video.

I see a less distinct version of the same oscillation in CRS-11 after 29:00.

The Merlin vacuum engine nozzle is a quite thin shell (getting down to 1/64" thickness according to one member of this SE) and subject to substantial forces from the exhaust; it would be surprising if it didn't vibrate and flutter a bit depending on the throttle state.

Merlin Vac nozzle extension

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