Stability Calculator (Rocket)

Static stability is what makes a rocket weathercock back into straight flight after a gust rather than tumble, and it comes down to one thing: the center of pressure (CP) must sit behind the center of gravity (CG). This calculator applies the Barrowman equations, the standard method for locating CP on a simple fin-and-nose-cone airframe, originally derived by NASA/Catholic University aerodynamicist James S. Barrowman in his 1966-67 thesis "The Practical Calculation of the Aerodynamic Characteristics of Slender Finned Vehicles" and still the basis for the CP calculations in essentially every rocketry simulation tool used today. The nose cone's contribution is modeled with a fixed normal-force coefficient of 2.0 acting at 0.466 of its length back from the tip — the classic ogive-nose approximation — while the fins' contribution uses the full Barrowman normal-force formula, driven by the semi-span-to-diameter ratio and the mid-chord sweep relative to root and tip chord, located using the standard fin-CP formula that accounts for sweep and taper.

The two contributions are combined as a weighted average (by their respective normal-force coefficients) to get a single overall CP location, and the body tube itself is assumed to contribute negligible normal force, per the standard Barrowman simplifying assumption. Stability margin is then just the CP-to-CG distance divided by body diameter, expressed in calibers — the industry convention, since 1 caliber of margin means the same thing on a 29mm minimum-diameter rocket as it does on a 4-inch airframe. The generally accepted safe range is 1.0 to 2.5 calibers; much less risks instability, much more can overcorrect and cause excessive weathercocking into the wind. Because this uses the simplified single-fin-set formula, it assumes a standard tail-fin (not canard or ring-fin) layout, and CG must be measured on the fully loaded rocket — motor installed — not the empty airframe.