🚀 Model Rocket Stability, CP & Ballast
🚀 Model Rocket Stability, CP & Ballast
Find the centre of pressure with the Barrowman method, compare it with your centre of gravity and read the static margin in calibers — then calculate the nose ballast needed to reach a stable margin.
Side view, nose to the left. CG and CP are drawn at their calculated stations.
Ballast to reach the target margin
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How Barrowman finds the centre of pressure — and where this model stops
The method. Barrowman's 1966 method treats the rocket as a slender body at a small angle of attack and adds the normal-force contributions of the nose and each fin set, ignoring body lift and interference. The nose is taken as a pure slender body with a normal-force slope of 2 per radian. Each trapezoidal fin set contributes
CNα(fin) = 4·N·(s/d)² / (1 + √(1 + (2·l/(a+b))²))
where N is the number of fins, s the semi-span from the body surface, d the body diameter, a and b the root and tip chords, and l the length of the fin mid-chord line. The fin centre of pressure is measured from the fin root leading edge as
x(fin) = m(a+2b)/(3(a+b)) + (a+b−ab/(a+b))/6
with m the leading-edge sweep. The total CP is the normal-force-weighted average of the nose and fin CPs. The static margin is (CP − CG)/d, expressed in calibers — body diameters — because the same margin in millimeters means something different on a 25 mm and a 100 mm rocket.
What it does not know. Barrowman's equations are subsonic and assume attached, slender flow. They do not model transonic CP shift, base drag, body lift, fin-body interference, vortex lift at high angle of attack, or the change in CP as the motor burns. A rocket at 1.5 calibers on the bench can still be marginal at Mach 0.9. Add a margin for the heaviest motor you intend to fly, check the CG with that motor installed, and confirm with a swing test or a six-degree-of-freedom simulator before flight.
Ballast. The calculator treats ballast as a point mass at the station you enter and solves the two-point CG equation exactly. Epoxy and washers in the nose are not a point mass; place the real ballast as far forward as practical and re-measure the CG afterwards. A heavier rocket also needs more thrust: check thrust-to-weight separately.
Sources: J. S. Barrowman, Theoretical and Practical Aerodynamics of Rockets (1966); NAR and UKRA model-rocketry safety guidance. Last reviewed: 2026-10-19. A design aid, not a substitute for a swing test, range safety check or flight simulation.