Calculator inputs
Enter rocket body, mass, and fin planform data. The calculator estimates stability and suggests a revised fin semi-span for the target margin.
Estimate stable fin sizing for hobby rockets. Review span, sweep, area, and margin values with practical outputs for safer early design decisions.
Enter rocket body, mass, and fin planform data. The calculator estimates stability and suggests a revised fin semi-span for the target margin.
Use these sample values to test the page or compare common hobby configurations.
| Rocket | Body Ø (mm) | Nose (mm) | Body (mm) | Mass (g) | Fins | Root (mm) | Tip (mm) | Span (mm) | Sweep (mm) | CG (mm) | Target Margin |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Falcon Hobby Mk-I | 54 | 160 | 700 | 780 | 3 | 150 | 70 | 85 | 45 | 430 | 1.5 |
| Range Scout 38 | 38 | 120 | 520 | 410 | 3 | 110 | 48 | 62 | 30 | 310 | 1.6 |
| Lift Test 75 | 75 | 220 | 980 | 1650 | 4 | 190 | 95 | 105 | 65 | 610 | 1.8 |
This calculator uses a first-order Barrowman-style stability approach for slender rockets with trapezoidal fins. It is useful for early sizing and comparison, not final flight certification.
Afin = 0.5 × (Cr + Ct) × S
Here, Cr is root chord, Ct is tip chord, and S is semi-span.
MAC = (2/3) × [(Cr + Ct) − (CrCt / (Cr + Ct))]
CNfin = [4N(S/D)²] / [1 + √(1 + (2S/(Cr + Ct))²)]
This estimates how strongly the fins shift the overall center of pressure rearward.
XCP = (CNnoseXnose + CNfinXfin) / (CNnose + CNfin)
Static Margin = (XCP − XCG) / D
The result is expressed in calibers, meaning body diameters.
q = 0.5ρV²
Fd = q × Aref × Cd × Safety Factor
This load is used to create a quick fin-loading estimate.
Important: Always validate with a trusted simulator, ground checks, and local safety rules before any real launch.
Static margin is the distance between center of pressure and center of gravity, divided by body diameter. Higher values usually improve restoring stability, but too much margin may cause weathercocking into the wind.
For many hobby rockets, 1.5 to 2.5 calibers is a practical starting range. Lightweight or very fast designs may need more detailed review in simulation software before flight.
Semi-span strongly affects fin area and the fin normal-force contribution. Increasing span usually moves the center of pressure rearward and raises static margin faster than small chord changes.
You can use it for early sizing, but not as the only design method. High-power rockets require stronger structural checks, flutter review, motor effects, and trusted flight simulation.
This page models a simple trapezoidal fin set. Keeping tip chord at or below root chord avoids inverted shapes that would need different geometry handling and extra checks.
No. It estimates static stability and a basic aerodynamic load. Launch guidance, wind shear, thrust curve, and rail exit speed must be checked separately before building.
It is a first-order estimate using design velocity, reference area, drag coefficient, and a safety factor. It helps compare designs, but it is not a substitute for CFD or flight testing.
Exports help document design changes, compare iterations, and share early sizing results with teammates, clubs, or clients without copying each metric manually.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.