Rocket Static Stability Calculator
Enter the center of gravity, center of pressure and body diameter to find the static stability margin in calibers. Aim for 1 to 2 calibers, with the CP behind the CG, for a stable flight.
How to check your rocket's stability
- Measure the center of gravity from the nose tip, with the motor loaded.
- Find the center of pressure from the nose tip, then enter the body diameter.
- Read the static margin. Aim for 1 to 2 calibers with the CP behind the CG.
Examples
Well-behaved sport rocket
CG = 300 mm, CP = 340 mm, diameter = 25 mm
margin = 1.6 calibers, stable
Marginal rocket needing nose weight
CG = 300 mm, CP = 305 mm, diameter = 25 mm
margin = 0.2 calibers, understable
Frequently asked questions
What is a good static margin for a model rocket?
The classic rule of thumb is 1 to 2 calibers, where one caliber equals the body tube diameter. The center of pressure should sit behind the center of gravity.
How is the static margin calculated?
Subtract the CG distance from the nose from the CP distance from the nose, then divide by the body diameter. Both points are measured from the nose tip in the same units.
What does a negative margin mean?
It means the center of pressure is ahead of the center of gravity, so the rocket is unstable and will tumble. Add nose weight to move the CG forward and fix it.
Can a rocket be too stable?
Yes. Above about 2 calibers a rocket is overstable and tends to weathercock, turning sharply into the wind. Moving the CG back a little reduces the margin.
How do I find the center of gravity?
Balance the fully loaded rocket on an edge or hang it from a string. The point where it balances is the CG. Measure its distance from the nose tip.
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