Sources and references
Where the numbers in Rocket Builder come from: the published methods and data it uses, and the choices we made ourselves.
Items marked Our choice are engineering assumptions or rules of thumb picked for this tool, not taken from a published standard. All results are estimates for planning; check a design in OpenRocket and follow the safety code when you fly.
Stability
- Center of pressure (CP): the Barrowman equations
- The nose cone and fins are summed with the Barrowman method; the body tube adds nothing at small angles. J. Barrowman and J. Barrowman, “The Theoretical Prediction of the Center of Pressure”, NAR R&D report, NARAM-8, 1966. Background: NASA Glenn, Rocket Center of Pressure.
- Center of gravity (CG)
- From each printed part's own volume and centroid (from the 3D model), plus the loaded motor and an allowance for the parachute and shock cord. Measured from the nose tip, as OpenRocket reports it. NASA Glenn, Rocket Center of Gravity.
- Stability margin in calibers; 1 caliber minimum, 1–2 the sweet spot, over 3 over-stable Our choice
- Margin = (CP − CG) ÷ body diameter. A minimum of 1 caliber is the common hobby rule of thumb; the zones on the score meter are our guidance. The check uses the heaviest motor that fits the bay (the least stable case) unless you pick one. Why CG must be ahead of CP: NASA Glenn, Rocket Stability.
- Fins that grow until the rocket is stable Our choice
- Standard fins are scaled (span and sweep) up to 2.5× until the margin reaches about 1.15 calibers with the heaviest motor.
Motors
- Motor data
- Total impulse, average and peak thrust, burn time, propellant and loaded mass, and delays for Estes motors of each size.
ThrustCurve.org motor database; motors by Estes Industries. Stored in
geometry/rocketgen/data/web_motors.csv. - Thrust curve shape Our choice
- A quick rise to the peak thrust in the first 5% of the burn, a fall to a steady level by 25%, then steady to burnout, with the steady level set so the area equals the total impulse (flat thrust when no peak is listed).
Flight
- Altitude: a one-dimensional flight simulation
- Thrust, weight (falling as propellant burns) and air drag, stepped every 2 ms until the rocket stops climbing. Drag: NASA Glenn, The Drag Equation. Air density 1.225 kg/m³ (standard sea level): NASA Glenn, Earth Atmosphere Model.
- Drag coefficient 0.65 Our choice
- A typical value for a small model rocket; you can change it in Advanced mode. OpenRocket works it out from the shape if you want a closer number.
- Safety checks: 5× thrust-to-weight, 15 m/s off the launch rod, 30 m minimum altitude Our choice
- Common hobby rules of thumb, set on the cautious side: enough push for a straight climb, fast enough off the rod for the fins to steer, and high enough for the parachute to open.
- Ejection delay
- The motor's listed delay closest to the simulated coast time (burnout to the top), so the parachute comes out near the highest point.
Parachute
- Descent speed
- Steady descent where drag on the canopy equals the rocket's weight (the drag equation solved for speed). NASA Glenn, The Drag Equation.
- Flat chute drag coefficient 0.75, a 4.5 m/s (about 15 ft/s) landing, standard 12–36 inch sizes Our choice
- The smallest standard size that lands the rocket (with the empty motor casing) at 4.5 m/s or slower. A streamer is suggested under 40 g.
Launch field and wind
- Minimum field size by motor class; no launches in winds over 20 mph
- 1/4A–1/2A: 50 ft (15 m) · A: 100 ft (30 m) · B: 200 ft (60 m) · C: 400 ft (120 m) · D: 500 ft (150 m) · E–F: 1,000 ft (300 m), on the field's shortest side. National Association of Rocketry, Model Rocket Safety Code, Launch Site Dimensions table.
- Parachute drift and the recommended field Our choice
- The report assumes a 10 mph breeze and drift = height ÷ descent speed × wind speed; the field is the larger of the safety-code size and that drift.
Launch simulator
- Two-dimensional flight in a steady wind Our choice
- The same thrust, mass and drag as the altitude simulation, in two dimensions with the wind along the ground. Off the rod the rocket points into its own airflow (a stable rocket turns into the wind, called weathercocking); the parachute opens over half a second at burnout + delay and drifts with the wind. The field covers the landing spot with 25% extra for gusts, and is never smaller than the safety-code size.
- Unsafe designs: wobble and tumble Our choice
- So the simulator can show why a design isn't safe, a rocket under 1 caliber of stability gets a simple pitch model instead of pointing straight into its airflow: the fins' restoring push grows with the margin and the square of the airspeed, with light damping, and a small nudge as it leaves the rod. Under 1 cal it wobbles off course; under 0 cal (push point ahead of the balance point) it tumbles, and side-on to the air its drag is many times higher, so it barely climbs. The numbers are tuned to look right, not measured: treat these flights as examples of what goes wrong, not predictions. A safe landing is taken as under 10 m/s.
Parts and mass
- Filament densities: PLA 1.24, PETG 1.27, ASA 1.07 g/cm³ Our choice
- Typical values from filament makers' data sheets. Masses assume solid parts, so real prints with less infill weigh a little less.
- Recovery allowance Our choice
- Parachute or streamer, shroud lines and shock cord: 0.3 g per mm of body diameter (about 6 g at 18 mm).
- Rocket geometry
- Body sizes, motor mounts, couplers and fin shapes come from OS_Rocket_Builder, our Onshape FeatureScript rocket generator that Rocket Builder was ported from.
Check your design
- OpenRocket
- Free, open-source rocket simulator; Rocket Builder reports CG and CP the same way so you can compare. openrocket.info · documentation.