Rocket Motor Selection Calculator

This calculator estimates the minimum motor impulse needed to reach a target altitude using an energy-based approach: it first finds the delta-V for a frictionless ballistic coast to apogee via deltaV = sqrt(2 * g * altitude), the same physics as tossing a ball straight up and asking how fast it needs to leave your hand. That delta-V is then converted to a required total impulse (rocket mass times delta-V), inflated by a drag-loss factor (your estimated percentage of impulse lost to air resistance, typically 20-40% for model rockets) and a safety margin for wind, temperature, and motor-to-motor manufacturing variance. The resulting impulse in Newton-seconds is mapped onto the real National Association of Rocketry (NAR) letter classification used industry-wide on every commercial motor label, where each letter roughly doubles the impulse range of the one before it (A ≈ 1.26-2.5 N-s, B ≈ 2.5-5, C ≈ 5-10, and so on up through O) — the code finds this by taking log base 2 of your impulse over the bottom of the A range.

It also reports a minimum safe thrust (5x the rocket's weight, the widely-used rule of thumb for stable, safe liftoff off the pad) and an estimated average thrust assuming a typical burn time for that motor class. Because this is an energy-based estimate rather than a full flight simulation, it ignores wind, launch-rod dynamics, and thrust curve shape — it's meant to get you into the right motor class ballpark, not to replace a simulator like OpenRocket for a final flight-readiness check, especially near your rocket's altitude or motor-mount limits.