🪂 Rocket Parachute Descent & Drift Calculator
🪂 Rocket Parachute Descent & Drift Calculator
Size a recovery parachute for a target descent rate, then see the impact speed, landing energy and wind drift from deployment altitude — with spill holes and clusters included.
Side view: altitude above the landing point against downwind drift. The line is the modelled descent from deployment to touchdown.
Sizing for a target rate
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| Target rate | Diameter per chute | Status |
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How the descent and drift are calculated, and what a parachute rate does not tell you
Drag equilibrium. A parachute reaches terminal velocity when drag equals weight: ½ρv²CdA = mg, so v = √(2mg/(ρCdA)). The effective area here is the sum of the canopies, with the area of any spill hole removed. Air density comes from a standard-atmosphere approximation adjusted for site elevation and temperature. Because density rises as the rocket descends, the model integrates the descent in small altitude steps and uses the local terminal velocity at each step; that is why the impact speed is a little higher than the rate at deployment.
Cd is the whole system. The drag coefficient of a canopy depends on its shape, porosity, suspension-line geometry, how it inflates and what it is carrying. A flat sheet is often quoted around 0.75–1.0, a hemispherical canopy around 1.2–1.5, and a toroidal or annular chute can be higher. A cluster does not simply add area: canopies interfere, and a realistic cluster may deliver less than the sum of its parts. The tool assumes no interference, so treat a cluster result as an optimistic upper bound on drag.
Spill holes. A spill hole reduces projected area but also reduces oscillation and opening shock. The area removed is πd²/4, which is why a 20 cm hole in a 60 cm canopy removes about 11% of its area — not 33%. A small hole often improves descent stability at a modest cost in rate.
Drift and landing energy. Drift is wind speed multiplied by descent time. Wind aloft is usually stronger than at the surface, so the true drift can be substantially larger; this model uses one constant wind. Landing energy is ½mv², and the equivalent drop height v²/2g is often the more intuitive number. There is no single safe impact speed: airframe mass, ground, recovery layout and what is inside the rocket all matter. Model-rocketry practice commonly aims for roughly 3.5–5.5 m/s, with high-power airframes often a little faster, but a heavy rocket at 5 m/s is not the same event as a 300 g rocket at 5 m/s.
Not modelled. Opening shock and inflation distance, reefing, canopy oscillation, wind shear, thermal updraughts, landing on a hard surface, or the rocket swinging under the chute. A deployment rate calculation is not a shock-load calculation.
Last reviewed: 2026-10-19. A recovery planning aid, not a substitute for a drop test and range safety assessment.