๐Ÿš€ 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 โ€” then calculate the nose ballast needed to reach a stable margin.

Fin set โ€” trapezoidal, root trailing edge flush with the body end

Mass, balance and target

Barrowman is a subsonic, slender-body method. Always swing-test and follow your club or range safety code.
Centre of pressureโ€”โ€”
Centre of gravityโ€”โ€”
Static marginโ€”โ€”
Verdictโ€”โ€”

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 โ€” 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

CNฮฑ(fin) = 4ยทNยท(s/d)ยฒ / (1 + โˆš(1 + (2ยทl/(a+b))ยฒ))

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+bโˆ’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 โˆ’ CG)/d, expressed in calibers โ€” body diameters โ€” 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.