feat: hobby/model-rocket simulator with multi-body support, design validator, and CSV/JSON export

[rramboer]

Summary

Repositions the package from a heavy-lift launch-vehicle simulator (Saturn V, Falcon 9, etc.) to a single-stage Estes/Quest-class hobby-rocket simulator with multi-body launch support, then adds a pre-launch design validator, CSV/JSON time-series export, and the missing 1/2A6-2 motor preset. Two commits, one PR.

The previous framing was pedagogically misleading: in a 1-D vertical, constant-mass, drag-free model, real heavy-lift vehicles can never reach orbit. The 1-D model is physically correct for hobby model rockets, so that is the appropriate scope. Multi-body support is preserved across Earth, Moon, Mars, Venus, and Titan — the same physics is meaningful in any atmospheric and gravitational environment.

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Commit 1 — feat: pivot to hobby/model-rocket simulator with multi-body support

Substantive physics changes

• Real, sampled thrust curves with linear-time interpolation. Built-in motor 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.
• Mass loss during burn via a constant-Isp flow model (dm/dt = thrust(t) / (Isp · g₀)). Propellant is 30–40 % of total mass for hobby rockets, so constant-mass produces visibly wrong apogees.
• Atmospheric drag in BOOST and COAST (using the airframe's Cd · A) and in DESCENT (using the recovery device's Cd · A). Each body has its own exponential atmosphere; Moon is vacuum.
• Semi-implicit drag treatment in the integrator, so the simulator stays stable when a parachute deploys at high speed.
• Recovery deployment timing based on the motor's delay-grain (burn_time + delay_seconds, the realistic default), with an opt-in deploy_mode="apogee" for design-exploration runs.

New public API

Type	Purpose
Motor	Sampled thrust curve, propellant mass, total mass, delay grain.
Atmosphere	Exponential-density model with .earth(), .mars(), .venus(), .titan() factories.
Parachute(diameter, Cd) / Streamer(length, width, Cd)	Recovery devices. None = ballistic.
Rocket(name, dry_mass_kg, motor, diameter_m, drag_coefficient, recovery, body)	Composition of airframe + motor + recovery + launch body.
FlightPhase enum	Phase tracking on SimulationState.
SimulationResult	Reports apogee, burnout altitude/velocity/time, max acceleration, recovery deployment time, landing velocity, and a deployed_below_ground flag.

get_kit(name) and get_motor(name) return mutation-isolated copies. parse_eng_file(text) and load_motor_file(path) accept the standard .eng motor-file format used by Thrustcurve.org and OpenRocket.

Built-in presets

• 7 motor presets covering the popular Estes range (A through F).
• 4 rocket-kit presets (Estes Alpha III, Big Bertha, Mosquito, V-2).
• 5 celestial-body presets: EARTH, MOON, MARS, VENUS, TITAN.

CLI

Rewritten flag set: --kit, --motor, --motor-file, --dry-mass, --diameter, --cd, --recovery, --body, --launch-altitude, --deploy-mode. --list-motors/--list-kits replace the old --list-presets.

Removals

• The old Engine, RocketConfig, IntegrationMethod types
• The 13 heavy-lift rocket presets
• Physics.drag_force / Physics.atmospheric_density standalone helpers (drag and density are wired into the integrator and the Atmosphere class)
• The dead PlotConfig dataclass
• SimulationConfig.log_level

Physics.escape_velocity and Physics.orbital_velocity are kept as standalone utilities for educational comparisons.

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Commit 2 — feat: design validator, CSV/JSON export, 1/2A6-2 motor preset

Pre-launch design validator

validate_design(rocket) -> list[DesignWarning] runs heuristic checks on a configured Rocket and returns advisories. The CLI runs it automatically (skip with --no-validate) and exits with code 2 if any error-level warning fires.

Code	Severity	Trigger
motor_too_big	error	Motor diameter exceeds airframe diameter
will_not_lift	error	Peak thrust-to-weight ratio < 1
lawn_dart	error	Ground impact before recovery deploys
low_twr	warning	Peak TWR between 1 and 5
low_apogee	warning	Predicted apogee < 30 m
transonic	warning	Predicted max velocity > 343 m/s
ballistic_descent	warning	No recovery system configured
delay_too_short	info	Ejection fires more than 2 s before apogee
delay_too_long	info	Ejection fires more than 2 s after apogee

CSV / JSON export

SimulationResult gains .to_csv(path), .to_dict(), .to_json(path). CLI flags --csv FILE and --json FILE. Output formats:

• CSV: one row per simulation step, columns time_s, altitude_m, velocity_ms, acceleration_ms2, mass_kg, thrust_n, drag_n, phase.
• JSON: { "rocket_name", "summary": {...}, "states": [...] }.

1/2A6-2 motor preset

The Mosquito kit was previously documented as using 1/2A6-2, but the motor was missing from MOTORS so the kit silently fell back to A8-3 via a _KIT_MOTOR_FALLBACKS table. The preset is now present and the fallback plumbing is gone.

Bonus fix

SimulationResult.deployed_below_ground previously only fired when the ejection charge tried to deploy below ground level. It now also fires when the rocket lands before recovery ever deploys — which is what the new lawn_dart validator check needs.

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Modelling caveats (documented in README)

• Motion is 1-D vertical only — no wind, no horizontal velocity, no weathercocking
• Single stage only
• Built-in motor curves are simplified ~5–7-sample approximations; expect apogee predictions accurate to ~30 % vs. a properly-instrumented real flight
• No CG/CP stability analysis

The README points users to OpenRocket for higher-fidelity simulation.

Test plan

• pytest — 114 tests pass
• ruff check src tests — clean
• ruff format --check src tests — clean
• mypy src — no issues
• python -m build — produces rocket_sim-0.1.0.tar.gz and the wheel
• twine check dist/* — both pass
• CLI smoke: rocket-sim --kit alpha-iii --motor c6-5 --no-plot --csv f.csv --json f.json runs validation, prints summary, writes both files
• CLI smoke: motor-too-big-for-airframe triggers motor_too_big error and exits non-zero
• Multi-body sanity: same kit on Moon (vacuum, low gravity) reaches ~12 km; on Mars (thin atmosphere, low gravity) reaches ~1.3 km
• Lawn-dart sanity: a small rocket with a 30-second delay grain on a 1-second motor lands before recovery deploys
• CI green on this PR