Is there an optimal position for the CP?

There is ample discussion about the requirement for CG to be ahead of CP and what is sufficient static margin. However, I have not been able to learn if there is an optimal location for the CP, which indirectly locates the CG. Consider these two example, a Darkstar and Zodiac:


The Darkstar's CP is at the tip of its fins, while the Zodiac's CP is a bit more than a caliper forward of the fins. Both are set up with 2 cal stability for this comparison.

Is there any benefit to a more-forward or more-rearward CP regarding overall stability, weathercocking, or other flight characteristics? I ask because the CG "follows" the CP for stability reasons and the rocket rotates around its CG when perturbed.

Or is the rocket's behavior determined primarily by the static margin regardless of CP location?

Technically the further aft you are with cp the better in terms of stability. The issue becomes one where you can get into the realm of the so called over stable rockets.

The Cp is going to be a function of many design parameters. This is one of many variables to consider in your designs- everything is a trade off.

Also remember that the faster you go the Cp tends to move forward. So, if designing rockets for higher speed it is best not to design them with a static ratio of less then say 1.5. Between the Cp change the the changing Cg as the propellant burns one needs to be cognizant if there relationship at all points/velocities.


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Thanks, Mark.

markkoelsch said:

Technically the further aft you are with cp the better in terms of stability. The issue becomes one where you can get into the realm of the so called over stable rockets.

The Cp is going to be a function of many design parameters. This is one of many variables to consider in your designs- everything is a trade off.

Also remember that the faster you go the Cp tends to move forward. So, if designing rockets for higher speed it is best not to design them with a static ratio of less then say 1.5. Between the Cp change the the changing Cg as the propellant burns one needs to be cognizant if there relationship at all points/velocities.


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+1, CP is also one of the harder points to move unlike the CG (add or remove nose weight), as there are so many design parameters affecting it and it won't move very far either (which is why the cardboard cutout method for determining CP works as well as it does).

I would think that having the center of pressure located such that the center of gravity location results in the lowest angular moment of inertia would be best.


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I would think that having the center of pressure located such that the center of gravity location results in the lowest angular moment of inertia would be best.

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That's what seems correct to me: reducing the combined moment arm of torque... or said another way, finding the balance point where the moment arm is equal on both sides of the CG. Then the rocket will react to angle-of-attack changes most quickly.

It appears that the absolute location of CP is not as relevant as the position of CG and the static margin.