Launch Lug Sizing Calculator
Calculate rod/rail exit velocity and minimum guide length for a safe launch. Accounts for thrust, gravity, friction, and wind speed.
About this calculator
This calculator works out how fast your rocket is actually moving when it clears the launch rod or rail, then checks whether that's fast enough to fly straight. It converts thrust, weight, and a lug-friction term into a net acceleration along the rod, tilted by your launch angle (gravity resists more as the rod leans back toward vertical, while a small sliding-friction force is modeled as scaling with the sine of that tilt). From that acceleration it applies the constant-acceleration kinematic relationship v = sqrt(2·a·L) to get exit velocity at the top of the rod, then compares it against a safety floor: the greater of 15 m/s or three times the current wind speed. Below that threshold, a crosswind can weathercock the rocket off its intended heading before the fins have enough airflow to correct it, risking an unstable or wandering flight path.
The calculator also backs out the minimum rod length that would hit that same safe-exit speed given your rocket's actual thrust-to-weight, and it maps your liftoff mass onto a standard rod/rail size chart (from 1/8" rod up through 2020 rail) as a rule-of-thumb starting point. Treat the thrust input as the motor's average thrust, not peak — using peak thrust will overstate exit velocity, especially for motors with a strong initial spike. Friction and rod tilt are simplified estimates, not a substitute for standing the rocket up and confirming it moves cleanly along the full length of guide before committing to a longer rod.
Inputs
Results
Exit velocity (m/s)
6.33
How to Use This Calculator
- Enter Rocket mass (kg), Average thrust (N), and Rod/rail length (m).
- Set Wind speed (m/s) and Launch angle from vertical (°).
- Review the Exit velocity (m/s) result.
- Use Exit velocity (ft/s) and Safe minimum exit (m/s) to inform your decision.
How the result changes with Rocket mass (kg)
| Rocket mass (kg) | Exit velocity (m/s) |
|---|---|
| 0.25 | 10.01 |
| 0.38 | 7.75 |
| 0.75 | 4.49 |
| 1.25 | 2.03 |
What each input means
- Rocket mass (kg)
- Total liftoff mass including motor.
- Average thrust (N)
- Average motor thrust from the data sheet.
- Rod/rail length (m)
- Length of the launch rod or rail. Common: 1m (3ft), 1.8m (6ft), 2.4m (8ft).
- Wind speed (m/s)
- Surface wind speed. Exit velocity should be at least 3x wind speed for stability.
- Launch angle from vertical (°)
- Tilt angle of the launch rod from vertical. 0-10° typical.
What each result means
- Exit velocity (m/s)
- Rocket speed when leaving the launch guide.
- Exit velocity (ft/s)
- Exit velocity in feet per second.
- Safe minimum exit (m/s)
- Minimum safe exit velocity based on wind and 15 m/s floor.
- Thrust-to-weight ratio
- Motor thrust divided by rocket weight. Minimum 5:1 recommended.
- Net acceleration (G)
- Net acceleration on the guide in multiples of gravity.
- Time on guide (s)
- How long the rocket rides the guide before release.
- Min rod length for safe exit (m)
- Minimum guide length needed to reach safe exit velocity.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRocket mass (kg) = 0.5, Average thrust (N) = 15, Rod/rail length (m) = 1, Wind speed (m/s) = 5 = 5 input(s) provided
- Calculate Exit velocity6.33 = 6.33
- Calculate Exit velocityExit velocity = exitVelocityMs * 3.2808420.78 = 20.78
- Calculate Safe minimum exitSafe minimum exit = max(15, windSpeedMs * 3)15 = 15
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator use three times the wind speed as a safety floor instead of a fixed number?
A fixed 15 m/s minimum is fine in calm air, but a stronger crosswind pushes the rocket sideways off the rod faster than the fins can build up corrective airflow at low speed. Scaling the safe-exit threshold to three times the current wind speed keeps the margin proportional to the actual disturbance, so the calculator takes whichever number is larger — the 15 m/s floor or 3x wind — as your target exit velocity.
Why does increasing the launch angle from vertical reduce my exit velocity?
Tilting the rod changes how gravity and friction load the rocket as it slides. The gravity-opposing component scales with the cosine of the tilt angle (so it barely drops at small angles), while the friction force is modeled as scaling with the sine of that same angle, adding a small additional drag term that grows as the rod leans over. Both effects work against net acceleration, so a more tilted rod produces a slightly lower exit velocity for the same rod length.
My thrust-to-weight ratio looks fine — why does the calculator still recommend a longer rod?
Thrust-to-weight tells you how hard the rocket accelerates, but exit velocity also depends on how much distance it has to build up speed. A high-thrust, lightweight rocket on a short rod can still leave the guide below the safe-exit threshold if there isn't enough rod length for v = sqrt(2·a·L) to reach that speed. The minimum rod length output solves for exactly the length needed to hit your safe-exit velocity given your actual net acceleration.
Should I enter the motor's peak thrust or its average thrust?
Enter average thrust, which is what the calculator's kinematics assume. Most motors have a thrust curve with an initial spike well above their average, and feeding in peak thrust will overstate net acceleration and exit velocity, potentially leading you to pick a shorter rod or lighter guide than the rocket actually needs during the slower parts of the burn.
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