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Calcimator

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.

Inputs

Results

Exit velocity (m/s)

6.33

Exit velocity (ft/s)20.78
Safe minimum exit (m/s)15
Thrust-to-weight ratio3.06
Net acceleration (G)2.05
Time on guide (s)0.32
Min rod length for safe exit (m)5.61
Recommended Guide3/16" launch rod
How to Use This Calculator
  1. Enter Rocket mass (kg), Average thrust (N), and Rod/rail length (m).
  2. Set Wind speed (m/s) and Launch angle from vertical (°).
  3. Review the Exit velocity (m/s) result.
  4. Use Exit velocity (ft/s) and Safe minimum exit (m/s) to inform your decision.

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

  1. Identify Input Parameters
    4 parameters
    Rocket mass (kg) = 0.5, Average thrust (N) = 15, Rod/rail length (m) = 1, Wind speed (m/s) = 5 = 5 input(s) provided
  2. Calculate Exit velocity
    6.33 = 6.33
  3. Calculate Exit velocity
    Exit velocity = exitVelocityMs * 3.28084
    20.78 = 20.78
  4. Calculate Safe minimum exit
    Safe minimum exit = max(15, windSpeedMs * 3)
    15 = 15

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