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This question is inspired by two, linked questions.

Skyrockets (of the pyrotechnic/firework kind) have guide sticks, which are (according to internet sources) to enhance stability: "The guide stick. A typical bottle rocket, with no fins or other stabilizers, relies entirely on its stick for stability in flight. The stick's length and weight are chosen to achieve this."

Once upon a time, when I built these things, it was explained to me that the purpose of the stick was to to ensure that the center of gravity was behind the point generating the thrust. But this seems now like the pendulum rocket fallacy. Clearly however, skyrockets do have sticks, and they presumably therefore serve a useful purpose. How do they promote stability, and could a similar construction be used with larger (space) rockets in order to promote stability? (without going as far as the Goddard rocket)

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    $\begingroup$ Its mainly because a longer rocket is more stable than a pancake. And CoM higher = better because you have a longer lever, thats true even for "real" rockets. $\endgroup$ – Polygnome Mar 28 at 10:06
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    $\begingroup$ The stick will help stabilize by producing drag behind the center of mass. $\endgroup$ – GdD Mar 28 at 11:32
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    $\begingroup$ @Polygnome that article implies that stability results from the CP (Center of Pressure) being below the CG (Center of Gravity). Does this imply the addition of the stick lowers the CP by more than it lowers the CG? $\endgroup$ – abligh Mar 28 at 12:19
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    $\begingroup$ @abligh Yes. the stick is fairly light, thus doesn't lower the CoM all that much (the heavy end is at the top), but adds drag below the CoM. $\endgroup$ – Polygnome Mar 28 at 12:53
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    $\begingroup$ In particular, the stick doesn't actively stabilize (return the angled flight to vertical) as was expected in the pendulum rocket fallacy - it just reduces the rate at which the trajectory changes. It will keep a rocket flying straight up to go straight up just as efficiently as it will help one launched horizontally stay horizontal. $\endgroup$ – SF. Mar 29 at 0:46
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...ensure the center of gravity is behind the point generating thrust

Should read center of drag is behind the center of gravity relative to the direction of flight.

In the air, rockets follow the same directional stability rules as their aircraft cousins, and indeed, arrows.

Putting a crude stick on a rocket will increase its drag but enhance its directional stability. It also makes a handy holder while you're lighting it.

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  • $\begingroup$ Just in case anyone was wondering how to fly a rocket "sideways", well, you need vertical lift, which can come from a wing, body, and/or vertical component of thrust. Directional stability requirements are the same. $\endgroup$ – Robert DiGiovanni Mar 29 at 15:58

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