Simulate a model rocket's powered ascent and coast to predict apogee, time to apogee and the best motor delay — then see exactly where a chosen delay fires the ejection charge.
Altitude above the pad (m) against flight time (s). Green: powered ascent. Gold: coast and descent. Markers show apogee and your chosen ejection point.
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How the simulation works. A one-dimensional point mass is integrated from rest with a constant average thrust for the stated burn time, a linearly decreasing mass, and a drag force of ½ρv²CdA opposing motion. Air density and the speed of sound come from a standard-atmosphere approximation, adjusted for the launch-site elevation and the air temperature you enter. After burnout the motor case stays aboard; the model coasts under gravity and drag, finds the first moment vertical speed reaches zero, and then follows the ballistic descent until the rocket reaches the ground. There is no parachute in the trajectory, no wind, no staging, no rail tip-off and no thrust-curve shape.
Why the delay is measured from burnout. A motor designation such as C6-5 means a C-class motor, about 6 N average thrust, and a 5-second delay after propellant burnout before the ejection charge fires. The delay element is a slow-burning train inside the motor; it is not a timer you can set. If it fires before apogee, the parachute opens while the rocket is still climbing: the shock load can break the shock cord, zipper the body tube, or strip the chute. If it fires well after apogee, the rocket is already falling fast and may hit the ground before the chute is out.
What changes the answer. A heavier rocket, a lower average thrust, a larger diameter, a higher Cd or a warmer day all reduce apogee and shorten the coast. The best delay is therefore specific to the rocket, the motor and the day — not to the motor alone. The recommendation here is the nearest whole-second delay; motor delays are manufactured in discrete values and their actual burn time varies with age and storage. For dual-deployment or electronic ejection, use the predicted apogee time as a cross-check, not as the primary event.
Limits. Constant average thrust badly misrepresents a motor with a strong initial spike or a long tail. Drag coefficient varies with Mach; transonic rockets need a Mach-dependent Cd. Wind, launch angle and rail tip-off move the apogee and the landing point. Treat the output as a planning estimate and confirm with flight data.
Last reviewed: 2026-10-19. A planning aid; follow the motor manufacturer's delay recommendations and your range safety code.