Predicting rocket drift distance during recovery

Drift is the horizontal distance your rocket travels during descent, carried by wind from apogee to landing. For a launch field, drift defines the recovery radius: land too far from the pad and you lose the rocket, cross the field boundary and you may lose launch privileges. This calculator takes apogee altitude, descent rate and wind speed and reports total drift distance plus total descent time, for both single-parachute and dual-deployment configurations.

The drift equation

Drift is straightforward: horizontal distance = descent time × wind speed. Descent time depends on altitude divided by descent rate. For single deployment: drift = (apogee / descent rate) × wind speed. For dual deployment the formula splits: drogue phase from apogee down to main deploy altitude, then main phase from main deploy altitude to ground. The main parachute drifts slower per metre of altitude because its descent rate is slower, but covers much less vertical distance, so total drift is dominated by the drogue phase, which is exactly the point of dual deployment.

Why dual deployment matters for high altitude flights

Consider a flight to 3000m apogee in a steady 5 m/s wind. With a single parachute descending at 5 m/s, total descent is 600 seconds and drift is 3 kilometres, almost certainly off any practical launch field. With dual deployment (drogue at 20 m/s, main at 300m AGL at 5 m/s), drogue phase is (3000-300)/20 = 135 seconds and main phase is 300/5 = 60 seconds, total 195 seconds of descent and only 975m of drift. The same flight recovered from the same field. Dual deployment becomes essentially mandatory above 1000m apogee on windy days or smaller fields.

Choosing the main deploy altitude

Lower main deploy altitude means less drift but also less time for the main parachute to fully inflate before landing. If the main fails to deploy or tangles, the rocket hits the ground at drogue speed, survivable with a hardy airframe, catastrophic with a delicate one. 300m (about 1000ft) is a sensible default. Use 400 to 500m for heavy rockets, complex deployment systems (piston, dual-bag) or if the main parachute is large and needs more time to inflate. Go below 250m only on very calm days, on small fields, and only with well-tested deployment hardware.

Wind speed: what value to use

Surface wind is rarely the full story. Wind speed increases with altitude (wind gradient), so the wind your rocket experiences at 1000m is often noticeably stronger than the surface reading. For drift planning, use the forecast wind at a height of several hundred metres above the launch site if available (aviation weather is a good source), or take the surface reading and add 30 to 50%. Include a safety margin of at least 50% on predicted drift when checking against field size, gusts, wind direction shifts and unexpected weather all push drift up.

Matching the flight to the field

Most rocketry clubs have minimum field size guidelines per motor impulse class. Check your launch site's rules. As a first approximation, field radius should exceed predicted drift by a comfortable safety margin, 50% extra is a reasonable rule. If drift exceeds the available field, reduce the flight: smaller motor, dual deployment, higher descent rate, or a less windy day. Launching into wind that makes drift exceed field size is how rockets end up on roads, in trees, on houses and in neighbouring farmland.

Frequently asked questions

How do I predict drift distance? Descent time × wind speed. The calculator handles both single and dual-deployment scenarios.

How much does dual deployment reduce drift? Often by 70-80% on high altitude flights. Essential above 1000m on windy days.

What altitude should I deploy the main? 200-500m AGL, with 300m a common default. Higher for heavy rockets or complex deployment.

What wind speed should I plan for? Forecast wind at flight altitude, or surface wind plus 30-50%. Add safety margin.

How big a field do I need? Predicted drift plus 50% safety margin. Check your rocketry body's guidelines.