Motor Retention Calculator

A rocket motor tries to leave its mount twice during flight, and this calculator computes both forces so you can pick hardware rated to stop the worse of the two. The first is the ejection charge itself: the code assumes a typical black-powder ejection pressure of about 100 psi (689 kPa) and multiplies it by the motor casing's cross-sectional area to get the force pushing the motor forward when the recovery charge fires. The second is inertial: at burnout the rocket is still decelerating hard from aerodynamic drag, computed from the standard drag equation (0.5 × air density × velocity² × drag coefficient × an approximated body reference area) using your estimated max velocity, and that deceleration — expressed in G's — is applied to the motor's own mass to get the force trying to fling it forward inside the airframe.

The calculator takes the larger of these two forces as the worst case, doubles it for a safety margin, and checks that figure against rough force ratings for common retention methods: friction fit (good to roughly 10 N), tape wrap (~40 N), and a clip or spring retainer (~120 N) — above that, the code assumes you need a threaded retainer, which is treated as sufficient in essentially all cases. These force ratings are rough rules of thumb from the rocketry community, not lab-measured specs for any particular hardware, so treat a "friction fit OK" result as a starting point to double-check against your specific motor mount and always err toward a more robust retention method when your worst-case force sits near a threshold. Peak thrust and Ejection delay are collected but do not currently factor into any of the calculated forces -- worst-case retention force is driven by ejection charge pressure and burnout-deceleration drag alone, not by the motor's thrust curve or the timing of the ejection charge.