# Session Summary - 2026-01-24

## Overview
Implemented Phases 1-3 of unified orbital elements configuration system.

## Changes Made

### Phase 1: Create Orbital Mechanics Module
- **Created** `src/orbital_mechanics.h` - Defines `OrbitalElements` struct with Keplerian elements
- **Created** `src/orbital_mechanics.cpp` - Implements `orbital_elements_to_cartesian()` conversion function
- Supports all orbit types: circular (e=0), elliptical (0<e<1), parabolic (e=1), hyperbolic (e>1)
- Planar orbits only (inclination=0), 3D orientation deferred to future work

### Phase 2: Update Data Structures
- **Modified** `CelestialBody` struct (simulation.h):
  - Added `OrbitalElements orbit` field
  - Renamed `position` → `global_position`
  - Renamed `velocity` → `global_velocity`
  - Reorganized field order for clarity

- **Modified** `Spacecraft` struct (spacecraft.h):
  - Added `OrbitalElements orbit` field
  - Renamed `position` → `global_position`
  - Renamed `velocity` → `global_velocity`

- **Renamed** `OrbitalElements` → `OrbitalMetrics` in simulation.h (output/analysis use case)
  - Updated all references in `simulation.cpp`, `config_loader.cpp`, `renderer.cpp`, `ui_renderer.cpp`

### Phase 3: Update Config Parser
- **Modified** `parse_toml_body()` (config_loader.cpp):
  - Replaced old `position` field requirement with `orbit` table parsing
  - Removed old `eccentricity` and `semi_major_axis` top-level fields
  - Parses all orbital element fields with defaults
  - Supports `altitude` convenience field for LEO/GEO orbits
  - Required: `semi_major_axis` or `altitude` in orbit table
  - Optional: `eccentricity` (default 0.0), `true_anomaly` (default 0.0), 3D orientation elements (default 0.0)
  - Position/velocity initialized to zero (calculated from orbital elements later)

- **Modified** `parse_toml_spacecraft()` (config_loader.cpp):
  - Replaced `position` + `velocity` fields with `orbit` table parsing
  - Uses same orbital element parsing logic as bodies
  - Position/velocity initialized to zero (calculated from orbital elements later)

- **Added** validation for orbital elements:
  - `semi_major_axis` must not be zero
  - `eccentricity` must be >= 0
  - For elliptical orbits (e < 1): `semi_major_axis > 0`
  - Validates both bodies and spacecraft

- **Modified** `load_spacecraft_from_toml()`:
  - Removed manual position/velocity calculation from parent
  - Now initializes both bodies and spacecraft in `initialize_orbital_objects()`

## Commits
1. **dc1f9f4** - Phase 1: Create orbital mechanics module
2. **121f1b9** - Phase 2: Update data structures with global_position/global_velocity
3. **ec1d115** - Phase 3: Update config parser to use orbital elements

## Net Line Count
- **+172 lines added** (orbital_mechanics module, updated structs, config parsing, validation)
- **-93 lines deleted** (removed manual velocity calculations, old config format support)

## Remaining Issues
1. **Phase 4 incomplete**: Need to update initialization to use `orbital_elements_to_cartesian()` function
   - Current state: `add_body_to_simulation()` calls old `calc_orbital_velocity()` (removed)
   - Old `initialize_bodies()` function still references removed function
   - Need to replace with `initialize_orbital_objects()` that calls `orbital_elements_to_cartesian()`

2. **Function name typo**: File has `orbital_mechanics.h` but code uses `calc_orbital_velocity` (should be `orbital` not `orbi`)

3. **Config format**: All existing test configs need to be updated to new `orbit` table format (13 config files)

4. **Renderer references**: Need to verify all `render_orbit()` calls are correct after struct changes

## Next Steps (Phase 4)
1. Remove old `calc_orbital_velocity()` function from simulation.cpp
2. Implement `initialize_orbital_objects()` using `orbital_elements_to_cartesian()`:
   - Bodies: Convert orbital elements to local position/velocity, compute global coordinates
   - Spacecraft: Handle altitude convenience, add parent radius, convert to local/global coordinates
   - Calculate SOI radii for bodies
3. Update `load_system_config()` to call `initialize_orbital_objects()` instead of `initialize_bodies()`
4. Update all 13 test configs to new format with `orbit` tables
5. Verify renderer displays orbits correctly
6. Run full test suite to ensure all changes work correctly

## Notes
- All orbital calculations use planar orbits (inclination=0), 3D orientation deferred
- `OrbitalElements` struct in separate module to enable clean separation of concerns
- Config parser now validates orbital elements at load time (better error messages)
- Altitude convenience simplifies LEO/GEO orbit specification (e.g., altitude=400km instead of semi_major_axis calculation)