What is the Model Rocket Flight Calculator?
This calculator estimates how high a model rocket will fly (its apogee, or peak altitude), how fast it is going at motor burnout, its top speed in flight, and how long it takes to reach the peak — all with air drag included. It puts the three forces acting on the rocket (gravity, thrust and drag) into the equations of motion and integrates them numerically over small time steps, producing a realistic flight curve rather than a single simplistic formula. A separate descent mode computes the parachute's terminal (descent) speed and tells you whether it lands at a safe rate.
How to use
1. In Flight mode, enter the airframe mass (g), body diameter (mm) and drag coefficient (Cd).
2. Enter the motor's average thrust (N) and burn time (s), or tap a motor preset (A8, B6, C6, etc.) to fill them in.
3. See the peak altitude, burnout velocity, maximum velocity and time to apogee, plus an altitude-versus-time profile.
4. Switch to Descent mode and enter mass, parachute diameter (mm) and parachute Cd to get the descent rate and a safe-zone verdict.
How it is calculated
The rocket feels an upward thrust T, a downward weight m·g, and a drag force D opposing its motion. Drag is D = ½·ρ·Cd·A·v², where ρ is air density, Cd the drag coefficient, A the rocket's frontal area and v its speed. During the burn (0 to burn time) thrust acts; after burnout thrust is zero and the rocket coasts upward under gravity and drag alone. The net acceleration a = (T − m·g − D)/m is integrated over tiny time steps to update velocity and altitude; the point where velocity reaches zero is the apogee (peak altitude). For parachute descent the terminal velocity v = √(2·m·g / (ρ·Cd·A)) gives the steady descent speed where weight and drag balance.
How to read the results
Peak altitude (apogee) helps you judge safety margins and the recovery area. Burnout velocity is how fast the rocket is moving as it switches from powered flight to coasting, indicating whether it left the launch rod fast enough for stable flight. The altitude profile is colour-split into the powered phase (solid) and the coast phase (dashed) so you can see the climb to apogee. In descent mode, a descent rate inside the 3–6 m/s safe zone is a good balance: not so slow that it drifts away in the wind, not so fast that landing damages the rocket.
Motor data, the altitude model, and how this compares with OpenRocket
Everything below describes exactly what this calculator does: the motor numbers it starts from, the equations it integrates, and the points where its answer will part company with a full simulator. Model rocketry is one of the few hobbies whose reference data is publicly certified and free to read, so there is no reason to guess.
Reading a motor code: A8-3
A hobby motor code has three parts. The letter is the total impulse class, the total push of the motor measured in newton-seconds. The first number is the nominal average thrust in newtons. The number after the dash is the ejection delay in seconds, the pause between burnout and the charge that pushes the parachute out. So an A8-3 is an A-class motor with a nominal 8 N average thrust and a 3-second delay. Each letter doubles the upper limit of the class, so a C motor carries up to twice the impulse of a B and four times that of an A.
Total impulse classes (NFPA 1125 / NAR)
| Class | Total impulse | Typical size and use |
| 1/4A | 0.313 - 0.625 N·s | 13 mm micro motors |
| 1/2A | 0.626 - 1.25 N·s | 13 mm |
| A | 1.26 - 2.50 N·s | 18 mm, first flights and small fields |
| B | 2.51 - 5.00 N·s | 18 mm |
| C | 5.01 - 10.0 N·s | 18 mm, the workhorse of the hobby |
| D | 10.01 - 20.0 N·s | 24 mm |
| E | 20.01 - 40.0 N·s | 24 or 29 mm |
| F | 40.01 - 80.0 N·s | 29 mm |
| G | 80.01 - 160 N·s | The top of model rocketry |
| H and above | 160.01 N·s and up | High power, certification required |
What the presets contain, and what the certified data says
The preset buttons above fill in a round average thrust and a round burn time whose product is the total impulse. Certification testing measures the real thing, and the two do not always agree: the number in a motor code is a nominal figure, not a measured average. The C6 is the clearest case. The code says 6 N, but the certification data gives 4.7 N averaged over a 1.9 s burn. Feeding the higher thrust and the shorter burn into the equations produces a faster, steeper boost and a higher apogee than the motor actually delivers.
| Motor | Class | Preset avg. thrust | Preset burn | Preset impulse | Certified impulse | Certified avg. thrust |
| A8 | A | 8.0 N | 0.30 s | 2.4 N·s | 1.93 N·s | — |
| B6 | B | 5.0 N | 0.80 s | 4.0 N·s | 4.23 N·s | — |
| C6 | C | 6.0 N | 1.60 s | 9.6 N·s | 8.8 N·s | 4.7 N |
| D12 | D | 12.0 N | 1.65 s | 19.8 N·s | 16.84 N·s | 10.2 N |
| E9 | E | 9.0 N | 3.00 s | 27.0 N·s | 25.08 N·s | — |
| F15 | F | 15.0 N | 3.50 s | 52.5 N·s | — | — |
Certified figures are National Association of Rocketry certification and static-test data as published through ThrustCurve.org. A dash means we have not verified that figure and would rather leave it blank than print a guess. All six presets are Estes black-powder motors. An AeroTech composite motor of the same letter spreads the same total impulse over a longer, flatter burn, so it lifts a heavy airframe higher than a black-powder motor of the same class. For a specific motor, look up its certified thrust curve and type its measured average thrust and burn time in by hand.
The altitude model, written out
The rocket is treated as a point mass flying straight up. Three forces act on it: thrust T from the motor, weight m·g downward, and aerodynamic drag D = ½·ρ·Cd·A·v² opposing the motion, where A = π·(d/2)² is the frontal area. Writing k = ½·ρ·Cd·A, the equation of motion is m·dv/dt = T − m·g − k·v² while the motor burns, and m·dv/dt = −m·g − k·v² after burnout.
The classic closed-form solution to that pair, the one behind the Estes technical report TR-10 Altitude Prediction Charts, gives the burnout velocity vb = √((T − m·g)/k) · tanh( (tb/m) · √(k·(T − m·g)) ) and the extra height gained while coasting, hc = (m/2k) · ln( (m·g + k·vb²) / (m·g) ). This calculator does not evaluate those formulas. It integrates the equations of motion directly in 0.01 s steps, which agrees with them for a constant thrust and additionally produces the altitude-against-time curve drawn above.
Assumptions, and therefore the limits
- •Thrust is held constant at the average value for the whole burn. A real black-powder motor opens with a sharp spike several times its average, which gets the rocket off the pad faster than a flat average does.
- •Mass is held constant. A real motor loses its propellant as it burns, so the rocket is lighter, and accelerating harder, at burnout than this model assumes.
- •Air density is fixed at ρ = 1.225 kg/m³, the ISA sea-level value. Higher up, and at high-altitude launch sites, the air is thinner and drag is lower.
- •Gravity is fixed at g = 9.80665 m/s².
- •Flight is purely vertical: no wind, no weathercocking, no launch-rod or rail effects, no angle of attack.
- •Cd is the single number you type in. In reality it changes with speed and with the finish of the airframe; 0.75 is the usual starting estimate for a simple model.
- •Stability is not modelled at all. This tool will compute an apogee just as happily for a rocket that would be unstable and unsafe in flight.
How this compares with OpenRocket
OpenRocket is the free, open-source design and simulation package that most of the hobby uses. It is a different class of tool, and the differences are worth knowing before you trust either number.
| This calculator | OpenRocket |
| Motor thrust | One average value | The full measured thrust curve (.eng / .rse) |
| Mass | Constant | Falls as the propellant burns |
| Air density | Fixed sea-level value | Varies with altitude |
| Drag | One Cd you type in | Built up per component (Barrowman), varies with speed |
| Motion | Vertical, one dimension | Six degrees of freedom, with wind and launch rail |
| Stability | Not modelled | Centre of pressure, centre of gravity, static margin |
| Recovery | Steady descent rate | Full descent with wind drift |
| Setup | Seconds | Build the rocket component by component |
For a simple single-stage low-power rocket in still air, expect the two to land within roughly 10 to 20 percent of each other, with this calculator usually reading high because of the flat thrust curve and the constant mass. The gap widens with peaky black-powder thrust curves, with wind, and with staging or clusters, none of which appear here. Use this page to sanity-check a design in seconds, or to teach the physics with every number visible; use OpenRocket when you are committing a real rocket and a real motor to the pad.