🛫 Rocket Launch Safety Check — TWR, Rail Speed & Wind
🛫 Rocket Launch Safety Check — TWR, Rail Speed & Wind
Check the three numbers that decide whether a rocket leaves the rail safely: liftoff thrust-to-weight, rail-exit speed and the wind limit — with drag included and a pass/fail checklist.
Pre-flight checklist
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Why 5:1, why four times the wind, and what this model leaves out
Thrust-to-weight. The widely used club rule is an average thrust of at least five times the all-up weight at liftoff. Below that, a rocket spends a long time on the rail, is more sensitive to tip-off and wind, and may not reach a stable speed before it leaves the launcher. The ratio uses the motor's average thrust; a motor whose thrust falls away late in the burn can still be marginal even when the average looks healthy. Some codes and some high-power classes use different minimums — the 5:1 figure is a starting screen, not a universal law.
Rail-exit speed. A rocket needs enough airspeed when it clears the rail for its fins to take control. The common model-rocketry guideline is at least four times the surface wind speed; high-power practice often adds a floor around 30 mph (13.4 m/s). This calculator integrates the actual drag force along the rail, so a short rail, a wide body or a high Cd all show up as a lower exit speed. It uses the average thrust and holds Cd constant — real thrust and drag both vary during the half-second that matters.
Wind. Many safety codes cap launches at about 20 mph (9 m/s) at the pad, and wind aloft can be stronger than the surface reading. The limit shown here is the smaller of 20 mph and one quarter of your calculated exit speed. It is a screening number, not a clearance: wind direction, gusts, recovery area, fire risk and visibility all matter.
What is not modelled. Thrust curve shape, motor ignition transients, rail whip and tip-off, launch angle, base drag, fin flutter, air density changes over the rail, and the certification or waiver under which you are flying. Use the manufacturer's thrust data and your national organisation's safety code. In the UK, model and high-power rocketry is governed through the CAA exemption and UKRA/NAR/TRA arrangements; in the US, NAR/TRA safety codes and FAA waivers apply.
Last reviewed: 2026-10-19. A pre-flight screening aid, not a substitute for range safety officers, waivers or your organisation's safety code.