rocket-sim

A small, scriptable hobby/model-rocket flight simulator. Predict the apogee, peak velocity, peak acceleration, and recovery behaviour of a single-stage Estes/Quest-class rocket — and, just for fun, see what the same rocket would do on the Moon, Mars, Venus, or Titan.

Features

• Real (interpolated) thrust curves — solid motors aren't constant thrust, and rocket-sim doesn't pretend they are. Built-in presets cover A8-3, B6-4, C6-3, C6-5, D12-5, E9-6, F15-6, with manufacturer-consistent total impulse, burn time, and peak thrust.
• Drag-aware trajectory — exponential atmosphere with body-specific surface density and scale height; drag opposes motion throughout the flight.
• Mass loss during burn — propellant is a meaningful fraction of hobby-rocket mass, so the simulator integrates a constant-Isp mass flow rather than treating mass as constant.
• Multi-body launches — Earth, Moon, Mars, Venus, Titan ship built-in. Custom CelestialBody instances are supported.
• Recovery modelling — Parachute, Streamer, or ballistic. Realistic motor-delay-grain timing by default; --deploy-mode apogee for the idealised case.
• Kit presets — Estes Alpha III, Big Bertha, Mosquito, V-2, each configured with its recommended motor and recovery.
• .eng motor file loader — drop in any motor from Thrustcurve.org.
• Pre-launch design validator flags marginal thrust-to-weight, underpowered configurations, transonic flight, motors that don't fit the body tube, and "lawn dart" timing.
• CSV / JSON export for the trajectory time-series and summary stats — pipe straight into a Jupyter notebook or spreadsheet.
• CLI and library — use rocket-sim as a command-line tool or import rocket_sim from your own scripts.

Modelling caveats

This is a deliberately simplified simulator:

• Motion is 1-D vertical only — no pitch-over, no horizontal velocity, no weathercocking, no wind.
• Single stage only — no boost-dart or multi-stage configurations.
• Built-in motor thrust curves are simplified ~5-7-sample approximations of public manufacturer data; expect apogee predictions accurate to ~30 % relative to a properly-instrumented real flight.
• No center-of-gravity / center-of-pressure stability analysis.
• Physics.escape_velocity and Physics.orbital_velocity are exposed as standalone utilities; they are not used in the trajectory loop (hobby rockets do not approach orbital speeds).

For higher-fidelity simulation (6-DOF, wind, multi-stage, optimisation, GUI), see OpenRocket — the gold standard in hobby-rocket simulation.

Quick start

Install

pip install rocket-sim

Command line

# Launch an Estes Alpha III with a C6-5 motor on Earth
rocket-sim --kit alpha-iii --motor c6-5

# Same rocket, on the Moon
rocket-sim --kit alpha-iii --motor c6-5 --body moon

# Multi-panel dashboard, save as PNG
rocket-sim --kit big-bertha --dashboard -o flight.png

# Use a downloaded thrustcurve.org motor file
rocket-sim --kit alpha-iii --motor-file ./Estes_C6.eng

# Save the trajectory time-series for analysis
rocket-sim --kit alpha-iii --csv flight.csv --json flight.json --no-plot

# List available motors and kits
rocket-sim --list-motors
rocket-sim --list-kits

Python library

from rocket_sim import get_kit, simulate_rocket, Plotter

rocket = get_kit("alpha-iii")
result = simulate_rocket(rocket)

print(f"Apogee:      {result.apogee_m:.1f} m at t = {result.apogee_time_s:.2f} s")
print(f"Max velocity:{result.max_velocity_ms:.1f} m/s")
print(f"Max accel:   {result.max_acceleration_ms2 / 9.80665:.2f} g")

Plotter().plot_trajectory(result, filename="alpha-iii.png")

Multi-body comparison

from dataclasses import replace
from rocket_sim import Physics, get_kit, simulate_multiple, Plotter

rocket = get_kit("alpha-iii")
worlds = [Physics.EARTH, Physics.MOON, Physics.MARS]
rockets = [replace(rocket, body=b, name=f"Alpha III on {b.name}") for b in worlds]
results = simulate_multiple(rockets)
Plotter().plot_multiple_trajectories(results, title="Same rocket, three worlds")

Custom rocket

from rocket_sim import Rocket, Parachute, get_motor, simulate_rocket

rocket = Rocket(
    name="My Rocket",
    dry_mass_kg=0.060,           # 60 g airframe
    motor=get_motor("D12-5"),
    diameter_m=0.029,            # 29 mm body tube
    drag_coefficient=0.7,
    recovery=Parachute(diameter_m=0.45),
)

result = simulate_rocket(rocket)
print(result.summary())

Validate a design

from rocket_sim import get_kit, validate_design, format_warnings

rocket = get_kit("alpha-iii")
warnings = validate_design(rocket)
print(format_warnings(warnings))
# → "No design warnings."

Export the time-series

from rocket_sim import get_kit, simulate_rocket

result = simulate_rocket(get_kit("alpha-iii"))
result.to_csv("flight.csv")
result.to_json("flight.json")

Built-in motor presets

Designation	Total impulse	Burn time	Peak thrust	Delay grain
1/2A6-2	~1.25 N·s	0.32 s	~7 N	2 s
A8-3	~2.5 N·s	0.5 s	~13 N	3 s
B6-4	~5.0 N·s	0.86 s	~12 N	4 s
C6-3	~10.0 N·s	1.85 s	~14 N	3 s
C6-5	~10.0 N·s	1.85 s	~14 N	5 s
D12-5	~16.8 N·s	1.6 s	~30 N	5 s
E9-6	~28.5 N·s	2.83 s	~24 N	6 s
F15-6	~50 N·s	3.45 s	~30 N	6 s

Curves are simplified educational approximations of public manufacturer data. For exact certified-motor data, download .eng files from Thrustcurve.org and load them with load_motor_file(path) or --motor-file path.eng.

Built-in kit presets

Kit	Dry mass	Diameter	Recommended motor	Recovery
Estes Alpha III	34 g	24.7 mm (BT-50)	C6-5	12-inch parachute
Estes Big Bertha	77 g	41.3 mm (BT-60)	C6-5	18-inch parachute
Estes Mosquito	4.5 g	13.2 mm (BT-5)	1/2A6-2	30 × 2.5 cm streamer
Estes V-2	64 g	41.3 mm (BT-60)	C6-3	12-inch parachute

Built-in celestial bodies

Body	Surface gravity	Atmosphere	Notes
Earth	9.82 m/s²	1.225 kg/m³ surface, 8.5 km scale height	Default
Moon	1.62 m/s²	none (vacuum)	Lots of altitude, no chute drag
Mars	3.72 m/s²	0.020 kg/m³ surface, 11.1 km scale height	Hits hard at landing
Venus	8.87 m/s²	65 kg/m³ surface, 15.9 km scale height	Soft launch, thermal hellscape
Titan	1.35 m/s²	5.4 kg/m³ surface, 21 km scale height	Thick cold atmosphere

Physics model

The integrator advances the state vector (altitude, velocity, mass) through three flight phases (BOOST / COAST / DESCENT):

F_thrust(t) - drag(v, ρ(h), Cd, A) - m(t) · g(h)

• Gravity uses the inverse-square law: g(h) = G·M / (R + h)².
• Drag is ½·ρ(h)·v²·Cd·A, opposing the velocity vector. During BOOST and COAST, Cd·A comes from the airframe; during DESCENT, it comes from the deployed recovery device.
• Atmospheric density follows an exponential model: ρ(h) = ρ₀ · exp(-h / H).
• Mass-flow is computed by assuming constant Isp: dm/dt = thrust(t) / (Isp · g₀), equivalent to "propellant burns proportionally to delivered impulse."
• The integrator is symplectic Euler with semi-implicit drag, which keeps it stable when a parachute deploys at high speed.

Project structure

rocket-sim/
├── src/rocket_sim/
│   ├── __init__.py        # Public exports
│   ├── physics.py         # Atmosphere, CelestialBody, Physics
│   ├── motors.py          # Motor type, .eng loader, motor presets
│   ├── models.py          # Rocket, Parachute, Streamer
│   ├── presets.py         # Kit presets
│   ├── simulation.py      # 3-phase trajectory integrator
│   ├── config.py          # SimulationConfig
│   ├── validation.py      # Pre-launch design validator
│   ├── visualization.py   # Plotter
│   └── cli.py             # rocket-sim CLI
├── tests/
└── examples/

Development

git clone https://github.com/rramboer/rocket-sim.git
cd rocket-sim
pip install -e ".[dev]"
pre-commit install

# Test, lint, type-check
pytest
ruff check src tests
ruff format src tests
mypy src

License

MIT — see LICENSE.

Acknowledgements

• Motor thrust-curve data is informed by the public dataset at Thrustcurve.org.
• Kit specifications are drawn from public Estes catalogs and product pages.
• For a full-featured open-source aerospace simulator, see OpenRocket.