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I just saw the video from the YouTube channel SpaceX Edits titled SpaceX B1021: The First Rocket to Fly Twice (a supercut of highlights mentioned in the Pod Bay, and noticed the Falcon 9 for the CRS-8 launch appears to sway back and forth.

What is happening? Why does the rocket appear to move?

enter image description here

geoffc
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uhoh
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1 Answers1

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Because it is.

It is a very tall structure, first stage alone is 140 feet, plus second stage, plus fairing. It is considered at the limit of how tall and thin they can make it. Wikipedia says the total height is 230 ft (70m). That is very tall, and 12 feet wide is quite thin.

The only connection holding it down, is via the Octoweb clamped to the launch mount at the bottom.

It is windy, as you can see from the LOX venting and how it is blowing away. The clamp on the upper stage has retracted in the snippet above, so this is normal swaying of a long structure. Just like buildings also sway in the wind, considering they are only attached at the ground level.

Lightness Races in Orbit
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geoffc
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  • Do you mean like a uniformly loaded beam flexing (e.g. https://i.stack.imgur.com/XfvA4.gif) or are certain flexible sections doing most of the deflection? It's not rocking off the pad or anything like that, right? – uhoh Apr 05 '17 at 14:44
  • This answer discusses the F9 flying at 1000 m/s (at 0.06 bar) with an angle of attack between 4 and 5 degrees. At that point it must be bending a whole lot more than what we're seeing here! – uhoh Apr 05 '17 at 14:55
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    I've asked the follow-up qestion Just how much can tall skinny rockets bend? (roughly, safely). – uhoh Apr 05 '17 at 15:33
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    @uhoh: I wouldn't be so sure about the rocket bending in flight, or at least not in the same way. To get this kind of "swaying in the wind", you need two things: a restoring force to permit oscillation, and vortex shedding to couple to the oscillation and excite it. On the launchpad, the former is provided by the clamps still connecting the base of the rocket to the ground, while the latter is provided by the wind (blowing at about 90° to the axis of the rocket). In flight, there's no restoring force, and the wind angle is very different. – Ilmari Karonen Apr 05 '17 at 16:11
  • @IlmariKaronen which is why I was careful to only say "appear", "appears" and "appear". – uhoh Apr 05 '17 at 16:18
  • @IlmariKaronen did you mean to leave your comment on the follow-up question rather than here? I don't talk about "bending" here, only apparent swaying. Bending is in the other question, where I did not use the word "sway". – uhoh Apr 05 '17 at 16:21
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    @uhoh: The main reason why I posted my comment on your follow-up question was to determine whether my comment above might be (part of) an appropriate answer there. Based on your response, it seems that it probably wouldn't be, so I just left it as a comment here. If you feel it would make a decent answer, I can always turn it into one. But I have no idea what the design limits of the rocket body might be, which is what you seemed to be mostly asking about. (And you did use the word "bending" in your first comments right here.) – Ilmari Karonen Apr 05 '17 at 16:28
  • @IlmariKaronen if you have a value for a deflection in centimeters, or degrees, and can back it up with a link to or citation of a reliable source, then it will be a good answer, even if there is a gorilla in it. If it is just a few paragraphs about how one might go about deriving or estimating an answer in degrees or centimeters, then it may still get up voted, including by me. – uhoh Apr 05 '17 at 16:34