Dual Deploy Calculator

Dual deploy splits recovery into two stages — a small drogue chute that opens at apogee to keep the rocket descending fast and stable (typically 15-25 m/s) so it doesn't drift for miles, followed by a larger main chute at a lower altitude that slows things to a gentle 3-5 m/s for touchdown. This calculator sizes both canopies from the same drag-equation approach: D = sqrt(8mg / (π × ρ × Cd × v²)), solving for the diameter needed so aerodynamic drag exactly balances the rocket's weight at your target descent rate, using a hemispherical chute's drag coefficient of 1.5 and sea-level air density. It also works out total descent time (drogue phase plus main phase, each altitude range divided by its respective descent rate), the opening shock the airframe experiences when the main chute snaps open while still falling at drogue speed (a physics estimate scaled by a 1.5x opening-shock factor), and landing kinetic energy so you can check it against the roughly 75 J threshold that tends to keep components from cracking on impact.

For the ejection charges that physically separate the airframe and deploy the chutes, it uses a simplified rule of thumb — about 1 gram of black powder per 20 cubic inches of sealed compartment volume for 15 psi — and sizes the main charge 1.5x larger than the drogue charge for a more forceful, reliable separation on the more heavily-packed main bay. Because these are simplified sizing rules rather than a full charge-pressure simulation, always ground-test your actual ejection charges before flight — compartment volume, wadding, and chute-packing all shift the real pressure needed.