Motor Ejection Delay Calculator
Work out the ideal ejection delay so the recovery charge fires right at apogee, when the rocket is barely moving. Enter the burnout velocity directly, or let the tool derive it from motor thrust, burn time and mass.
How to choose your ejection delay
- Enter the burnout velocity in m/s if you know it, or switch to the motor inputs.
- For the motor inputs, enter average thrust, burn time and the loaded mass.
- Read the coast time to apogee, then pick the motor delay closest to it.
Examples
From a known burnout speed
burnout velocity = 50 m/s
coast to apogee = 5.10 s, pick a 5 second delay
Derived from the motor
thrust = 10 N, burn = 1.5 s, mass = 0.1 kg
burnout = 135.3 m/s, coast to apogee = 13.80 s
Frequently asked questions
What ejection delay should I pick?
Pick the motor delay closest to the coast time to apogee. Firing a little early is safer than late, since a late charge fires while the rocket is heading back down fast.
How is the coast time calculated?
After burnout, gravity alone removes the upward velocity in a no drag model, so the time to apogee is burnout velocity divided by g, where g is 9.80665 m/s^2.
What if I do not know the burnout velocity?
Switch to the motor inputs. The tool finds burnout velocity from a constant net force boost: acceleration is (thrust minus weight) over mass, times the burn time.
Does drag change the ideal delay?
Yes. This is a no drag estimate. Real drag slows the rocket faster during coast, so the true time to apogee is shorter and a slightly shorter delay is better.
Why is ejecting at apogee best?
At apogee the rocket is moving slowest, so the parachute opens under the least stress. Deploying while still fast can shred the chute or rip out the recovery gear.
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