What it does
This simulator flies a model rocket straight up and tells you how high it goes. At every millisecond it works out the forces on the rocket:
- thrust from the motor’s thrust curve (straight lines between the points you give);
- drag = ½ ρ v² Cd A, with the air density ρ falling with height as in the standard atmosphere;
- weight, with the mass falling as propellant burns, in step with the impulse delivered.
It steps the motion forward with the fourth-order Runge-Kutta method until after apogee and the ejection charge. The rocket waits on the pad until the thrust exceeds its weight.
You get the apogee and when it happens, the top speed and Mach number, peak acceleration, the speed off the launch rod, the burnout height and speed, the best ejection delay and what happens with the delay you chose, with plots and a CSV of the whole flight.
How to use it
- Type the rocket’s lift-off mass including the motor, the propellant mass (from the motor’s data sheet), the body diameter and a drag coefficient.
- Paste the motor’s thrust curve as time and thrust pairs, or open a CSV or text file. The numbers in a RASP .eng file paste as they are. Or choose a made-up shape with a total impulse and burn time to explore.
- Give the ejection delay printed on the motor, the launch rod length, the site elevation and the temperature.
Worked example
The made-up example: a 120 g rocket, 25 mm across, Cd 0.75, on a made-up 8.3 N·s curve (a C-class impulse) with 10.8 g of propellant. The page shows apogee 163 m at 6.25 s. The rocket leaves a 1 m rod at 12.8 m/s (a little slow: the page suggests a longer rod), reaches a top speed of 51.8 m/s at burnout, 56.5 m up, then coasts for 4.5 s. So the best delay is 4.5 s; with a 4 s delay the ejection comes at 5.75 s, 0.5 s before apogee, at 162 m and 4.9 m/s, which is fine.
Tips
- Pick the delay nearest the best delay. Too short and the parachute opens while the rocket is still fast; too long and it opens on the way down, fast.
- Drag matters a lot for small, fast rockets. Halving Cd can add a third to the apogee.
- Hot or high sites have thinner air, so rockets go a little higher.
FAQ
Why do real flights go lower? Wind makes rockets turn into it (weathercocking), launch angles, a rough finish and motor-to-motor spread all cost height. Treat the result as an upper estimate.
Where do I get a thrust curve? From the motor maker’s data sheet or certification data. The example here is made up.
Is this a safety tool? It is for study and planning. Always follow your national rocketry safety code and the motor maker’s instructions.
Where the numbers come from
- Equation of motion of a rocket in vertical flight (thrust, quadratic drag, gravity, mass falling as propellant burns) integrated with the classical fourth-order Runge-Kutta method: standard rocketry and numerical methods texts (equations only).
- Air density from the 1976 US Standard Atmosphere (US government, public domain), shifted to the temperature you give.
- Motor impulse classes (A up to 2.5 N·s, each letter doubling): the common hobby-rocketry convention.
- Checked against an independent SciPy solve_ivp (adaptive Runge-Kutta, tight tolerances) calculation with its own atmosphere and impulse integration. The example rocket and thrust curve are made up and are not a real motor.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.