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

  • 4
    $\begingroup$ Because it's excited $\endgroup$ – Richard Apr 5 '17 at 22:06

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.

  • $\begingroup$ Do you mean like a uniformly loaded beam flexing (e.g. 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? $\endgroup$ – uhoh Apr 5 '17 at 14:44
  • $\begingroup$ 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! $\endgroup$ – uhoh Apr 5 '17 at 14:55
  • 1
    $\begingroup$ I've asked the follow-up qestion Just how much can tall skinny rockets bend? (roughly, safely). $\endgroup$ – uhoh Apr 5 '17 at 15:33
  • $\begingroup$ @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. $\endgroup$ – Ilmari Karonen Apr 5 '17 at 16:11
  • $\begingroup$ @IlmariKaronen which is why I was careful to only say "appear", "appears" and "appear". $\endgroup$ – uhoh Apr 5 '17 at 16:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.