Model Rocket Motor Calculator

This calculator estimates the total impulse a model rocket motor needs to reach a target altitude, using a simplified energy-balance model: the motor has to supply enough kinetic energy to lift the rocket's weight to that height, plus an estimate of aerodynamic drag loss along the way. From the resulting Required Impulse in Newton-seconds, it maps the result onto the standard NAR/Estes motor-class lettering (Class 1 = A through Class 7 = G-and-up), where each class represents roughly double the total impulse of the one below it. Rocket Weight is the input this model responds to most — required impulse scales with both the potential energy (mass x gravity x height) and the kinetic-energy term, so a heavier rocket needs a disproportionately larger motor even at the same target altitude. Body Diameter and Drag Coefficient feed into the drag-loss term, and for typical mid-weight rockets near this calculator's default inputs that drag contribution is small enough relative to the potential-energy term to leave the recommended motor class unchanged.

It stops being negligible, though, for a light, wide-bodied rocket aimed at a high target altitude — there, drag loss can account for a meaningful share of total energy and can genuinely push the recommended motor class up a step, so don't assume Drag Coefficient is always a rounding error. Two things worth reading carefully: Estimated Altitude simply echoes the Target Altitude you entered back as the output — it is not an independent physics prediction of how high the suggested motor will actually carry the rocket, so use it as a restated target, not a verification. And this model omits engine mass loss during burn, wind, and stability (center-of-pressure/center-of-gravity) effects that real rocket flights depend on — always cross-check a real motor's published thrust curve and total impulse rating before flying, and follow NAR/Estes safety codes for the field you're flying at.