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Calcimator

Motor Retention Calculator

Calculate ejection and deceleration forces on your rocket motor to choose the right retention method -- friction fit, tape, clip, or threaded retainer.

Inputs

Results

Worst-case retention force (N)

455.1

Ejection charge force (N)455.1
Deceleration inertial force (N)1.2
Peak deceleration (G)2
Required retention (N, 2x safety)910.2
Friction Fit OkNo
Tape OkNo
Clip OkNo
How to Use This Calculator
  1. Enter Motor diameter (mm), Motor mass (g), and Peak thrust (N).
  2. Set Estimated max velocity (m/s), Rocket total mass (kg), and Ejection delay (s).
  3. Review the Worst-case retention force (N) result.
  4. Use Ejection charge force (N) and Deceleration inertial force (N) to inform your decision.

How the result changes with Motor diameter (mm)

Motor diameter (mm)Worst-case retention force (N)
1165.5
36701.3
652,286.3
904,383.2

What each input means

Motor diameter (mm)
Motor casing outer diameter. Common: 18mm, 24mm, 29mm, 38mm, 54mm, 75mm.
Motor mass (g)
Total motor mass including propellant and casing.
Peak thrust (N)
Maximum thrust from the motor data sheet.
Estimated max velocity (m/s)
Peak speed at motor burnout. Used to estimate deceleration forces.
Rocket total mass (kg)
Total liftoff mass of the rocket.
Ejection delay (s)
Delay between burnout and ejection charge firing.

What each result means

Worst-case retention force (N)
Maximum force trying to push the motor out (ejection or deceleration, whichever is greater).
Ejection charge force (N)
Force from ejection gas pressure on the motor casing.
Deceleration inertial force (N)
Inertial force on the motor during peak aerodynamic deceleration.
Peak deceleration (G)
Estimated deceleration in multiples of gravity at burnout.
Required retention (N, 2x safety)
Minimum retention force with 2x safety factor.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Motor diameter (mm) = 29, Motor mass (g) = 60, Peak thrust (N) = 30, Estimated max velocity (m/s) = 100 = 6 input(s) provided
  2. Calculate Worst-case retention force
    Worst-case retention force = max(ejectionForceN, inertialForceN)
    455.1 = 455.1
  3. Calculate Ejection charge force
    Ejection charge force = ejectionPressurePa * motorAreaM2
    455.1 = 455.1
  4. Calculate Deceleration inertial force
    Deceleration inertial force = motorMassKg * decelerationG * g
    1.2 = 1.2

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