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Calcimator

Stability Calculator (Rocket)

Calculate your rocket's static stability margin using the Barrowman equations. Determines CP and CG separation in calibers for safe flight.

Inputs

Results

Stability margin (calibers)

4.02

CP from nose (mm)396.7
CP - CG distance (mm)116.7
Fin CN-alpha22.32
CG position (% of length)58.3
CP position (% of length)82.6
Lm70
How to Use This Calculator
  1. Enter Body diameter (mm), Nose cone length (mm), and Body tube length (mm).
  2. Set CG from nose tip (mm), Number of fins, and Fin root chord (mm).
  3. Adjust Fin tip chord (mm), Fin semi-span (mm) as needed.
  4. Review the Stability margin (calibers) result.
  5. Use CP from nose (mm) and CP - CG distance (mm) to inform your decision.

How the result changes with Body tube length (mm)

Body tube length (mm)Stability margin (calibers)
3010.89
1,05124.63
1,95053.08
2,70076.81

What each input means

Body diameter (mm)
Outside diameter of the body tube. Common: BT-20=18mm, BT-50=24mm, BT-55=33mm, BT-60=41mm.
Nose cone length (mm)
Length of the nose cone from tip to shoulder.
Body tube length (mm)
Total length of the body tube (excluding nose cone).
CG from nose tip (mm)
Measured center of gravity from the nose tip. Find by balancing the loaded rocket on a ruler.
Number of fins
Number of fins. Most common: 3 or 4.
Fin root chord (mm)
Length of fin where it meets the body tube.
Fin tip chord (mm)
Length of fin at the outer tip. 0 for a triangular fin.
Fin semi-span (mm)
Distance from body tube to fin tip.
Fin sweep distance (mm)
How far aft the leading edge of the tip is from the leading edge of the root.

What each result means

Stability margin (calibers)
Distance between CG and CP in body diameters. Ideal: 1.0-2.5 calibers.
CP from nose (mm)
Calculated center of pressure location from nose tip.
CP - CG distance (mm)
Physical separation between CG and CP.
Fin CN-alpha
Normal force coefficient contribution from fins.
CG position (% of length)
CG location as percentage of total rocket length.
CP position (% of length)
CP location as percentage of total rocket length.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Body diameter (mm) = 29, Nose cone length (mm) = 80, Body tube length (mm) = 400, CG from nose tip (mm) = 280 = 9 input(s) provided
  2. Calculate Stability margin
    Stability margin = (cpTotal - cgFromNoseMm) / d
    4.02 = 4.02
  3. Calculate CP from nose
    CP from nose = (cnNose * cpNose + cnFins * xFin) / (cnNose + cnFins)
    396.7 = 396.7
  4. Calculate CP - CG distance
    CP - CG distance = round((cpTotal - cgFromNoseMm) * 10) / 10
    116.7 = 116.7

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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