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I've just watched Scott Manley's video SpaceX's Water Landing Reveals Rocket "Secrets" (or, What We Learned from CRS-16) (again) and he mentions the second stage "pusher" in the center of the view (screen shot below).

I've added a small GIF made from the video, of the clip which shows the stages separating.

In the black-and-white clip, I can see the white bit of the pusher extending. I am assuming that the end of the pusher already extends all the way through the long vacuum nozzle and expansion chamber and in contact with the back of the combustion chamber even before the stages start to separate, and that this is just more of it being exposed to sunlight.

Question: Roughly how long is the pusher in totality? Once separation begins, how far does it extend to accelerate the separation, just a few centimeters for a hard push, or does it continue to extend for tends of cm or even a meter to gently accelerate the separation?

Scott Manley F9 CRS-16 2nd stage pusher

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    $\begingroup$ I'm pretty sure it presses into the throat of the nozzle, not the back of the combustion chamber (wouldn't want to shove on the injector heads, probably). $\endgroup$ – Saiboogu Jan 21 '19 at 13:53
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    $\begingroup$ Much as I'm a Manley fan, he's not 'in the know' on SpaceX matters. Neither am I of course, but I'm comfortable with my assumption based on the shape of the pusher head - also visible in your video around the same time. Seems the mating surface is around the round rim of the pusher. Check the pinned comment from Scott on the same video, he confirms the general impression. $\endgroup$ – Saiboogu Jan 21 '19 at 18:46
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    $\begingroup$ @Saiboogu Yes indeed, there it is: Some important updates on things which I probably got wrong:... Most people believe the pusher mates with the narrow throat of the engine rather than the top of the combustion chamber. $\endgroup$ – uhoh Jan 21 '19 at 22:13
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    $\begingroup$ Here's a nice picture of the business end of the pusher. It looks to me like the throat theory is correct. teslarati.com/… $\endgroup$ – Organic Marble Jan 22 '19 at 0:17
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    $\begingroup$ Very much interested in an answer to this question. Even a rough figure would be nice. And what kind of pusher is this? Because in Organic Marble's picture, there seems to be a telescopic cylinder with multiple sections, similar to those that extend the legs. $\endgroup$ – user36480 Jun 11 '20 at 7:20
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Here's a still from a video with a better view of the cylinder as it extends.

enter image description here

You can clearly see the wide rim at the tip of the center pusher extending. That white rim is not due to changes in shading as in other footage.

If you look back at Organic Marble's picture, you'll see that the tip of the pusher is roughly a foot off the top of the quadrapod support when retracted.

enter image description here

Judging from the video still, the tip of the pusher must move about a couple of feet from its retracted position. Rough number, but still better than any offered so far, and still useful since it rules out a few cm and a few m as extension ranges.

It seems fair to say a meter wouldn't be too far off.

Plus! You can clearly see what the center pusher must press against in the second stage engine. This has been source of debate, so it's nice to see it so clearly here:

enter image description here

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