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Why does a rocket spin during launching?

https://www.youtube.com/watch?v=BuPY5l45UC8

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The thrust from the rocket is not quite "straight down", but at an angle - and slightly offset from the center. If the (solid) fuel burns ever-so-slighly unevenly, the surface that the burnt fuel pushes against will be at a slight angle; and since the rocket has relatively small angular momentum about the long axis, it will readily spin.

Note - once the fuel is almost spent, the rotation stops (aided by the fins which try to keep the rocket straight).

If you are trying to aim a rocket, imparting a bit of rotation might be a good idea. Just as the rifling of a gun barrel cause the bullet to spin about its axis, this has two effects: first, if the shape of the rocket/bullet is slightly asymmetrical, the rotation will cause a "self-correction" of the deviation this incurs. You will go up-right-down-left and end up pointing in (roughly) the right direction. Second, the rotation will stabilize the bullet against the torque caused by the drag when the bullet is not pointing straight in the direction of travel. Instead of "tipping on its side" (think about how a playing card falls when you drop it "straight"), the rotation will keep the bullet pointing in the direction of travel.

In summary - it's accidental in this case; but when it is done deliberately, it's a good thing.

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  • $\begingroup$ Doesn't the jet engine inside liquid fuel rockets provide some of the angular momentum? I am not sure if it reaches max rpm prior to liftoff but if it did not, it could cause the rocket body to turn in response to the torque created by the accelerating engine shaft. $\endgroup$ – honeste_vivere Feb 8 '17 at 0:26
  • $\begingroup$ @honeste_vivere - did you watch the video. This is a solid fuel model rocket we're talking about. $\endgroup$ – Floris Feb 8 '17 at 0:32
  • $\begingroup$ Ah, no I did not glance at it. So there is no liquid fuel parts? Okay, then my comment is clearly moot. Thanks... $\endgroup$ – honeste_vivere Feb 8 '17 at 0:33
  • $\begingroup$ Plus, there are no "jet engines" in liquid fuel rockets, either. A pair of turbopumps per nozzle, yes, but that's a different story altogether. $\endgroup$ – Pirx Feb 8 '17 at 0:55
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In a nutshell, the rotation slows the rocket's reaction to disturbances which might throw it off course.

This is similar to how a spinning gyroscope can remain upright and seems to resist forces which would seem to topple it.

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  • $\begingroup$ Did you watch the video before writing this? $\endgroup$ – Floris Feb 8 '17 at 0:07
  • $\begingroup$ Any rocket will rotate about its CG, which due to axisymmetric rocket design is along the roll axis. If a slight perturbation starts the rocket spinning, it will tent to continue to do so due to this design. If the rocket is intentionally sent spinning, it gains the benefit I mentioned. I couldn't get the video to load, so I went with a general answer. If you look at the question, it isn't specific to the linked video. $\endgroup$ – Jake Watrous Feb 8 '17 at 0:16

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