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# Session Summary: Mission Planning - LEO Spacecraft and Impulse Burn |
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**Date:** January 18, 2026 |
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**Session Length:** ~2 hours |
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**Branch:** mission-planning |
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**Goals:** |
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1. Replace dynamic spacecraft spawning with config-based LEO spacecraft |
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2. Implement patched conics impulse burn for Hohmann transfer |
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3. Add comprehensive test verification |
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--- |
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## Work Completed |
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### Phase 0: Git Workflow ✅ |
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- Stashed debug changes on main branch |
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- Switched to mission-planning branch |
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- Applied debug printf statements to mission-planning branch |
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- All debug output from spacecraft parent investigation preserved |
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### Phase 1: Configuration File ✅ |
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**File:** `tests/configs/earth_mars_simple.toml` |
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**Changes:** |
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- Added Spacecraft body to config |
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- Configured with placeholder position/velocity (set at runtime) |
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- Parent set to Earth (index 1) |
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- Initial semi-major axis placeholder: 6.571e6 m (Earth radius + 200km) |
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### Phase 2: Mission Planning Module ✅ |
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**Function Declarations Added:** |
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```cpp |
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void initialize_spacecraft_leo(CelestialBody* spacecraft, CelestialBody* parent, |
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double altitude_m); |
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void apply_transfer_burn(SimulationState* sim, int spacecraft_idx, |
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int departure_idx, TransferParameters* params); |
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double calculate_phase_angle(SimulationState* sim, int departure_idx, int arrival_idx); |
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``` |
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**Function Implementations:** |
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1. **`initialize_spacecraft_leo()`** - Sets circular LEO orbit at specified altitude |
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- Calculates orbital radius = Earth radius + altitude |
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- Positions spacecraft radially outward from Sun |
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- Calculates LEO velocity: v = sqrt(G * M_earth / r) |
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- Sets prograde orientation (tangential to Earth-Sun line) |
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- Verified to produce correct LEO velocity (~7788 m/s at 200km altitude) |
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2. **`calculate_phase_angle()`** - Computes phase angle between two bodies |
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- Calculates angular positions relative to Sun |
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- Returns phase difference normalized to [0°, 360°) |
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- Used for launch window verification |
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3. **`apply_transfer_burn()`** - Applies impulse burn for Hohmann transfer |
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- Calculates required heliocentric velocity magnitude from transfer parameters |
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- Calculates prograde direction (tangential to Earth-Sun line) |
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- Computes delta-v vector: Δv = v_target - v_current |
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- Applies impulse to spacecraft velocity |
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- Updates local velocity relative to departure body |
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### Phase 3: Comprehensive Test Case ✅ |
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**Test Structure:** |
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``` |
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1. Load config with 4 bodies (Sun, Earth, Mars, Spacecraft) |
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2. Initialize spacecraft in 200km LEO around Earth |
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3. Verify LEO orbit stability (parent, position, velocity, energy) |
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4. Calculate Hohmann transfer parameters |
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5. Wait for Earth-Mars launch window (within 1°) |
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6. Verify phase angle accuracy |
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7. Apply impulse burn for transfer |
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8. Verify post-burn energy >= 0 (escape trajectory) |
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9. Simulate transfer for 110% of expected duration |
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10. Track SOI transitions (Earth→Sun→Mars) |
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11. Verify final parent and energy conservation |
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12. If Mars SOI entry, verify distance |
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``` |
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**Test Results (Current Status):** |
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✅ PASSED (8 assertions): |
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- Config loading (4 bodies loaded) |
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- Spacecraft loaded correctly |
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- Spacecraft parent = Earth (index 1) |
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- LEO position within expected radius (<1km error) |
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- LEO velocity matches expected (<10 m/s error) |
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- LEO total energy negative (bound to Earth) |
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- Launch window opened after ~94 days |
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- Phase angle error < 1° |
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❌ FAILED (1 assertion): |
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- Post-burn heliocentric energy >= 0.0 (expected) |
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- Actual: -3.5e8 J (negative, still bound) |
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- Expected: ≥ 0 J (positive, escape trajectory) |
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### Phase 4: Build System ✅ |
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- Makefile already configured for mission_planning.o |
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- Test executable builds successfully |
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- All warnings noted (unused variables, harmless) |
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### Phase 5: Cleanup ⏸️ |
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- Not yet started (waiting on test fix) |
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--- |
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## Test Status |
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### Before Session |
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- Previous mission planning implementation with spacecraft spawning |
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### After Session |
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- Test framework complete with LEO spacecraft initialization |
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- 8/9 assertions passing in Hohmann transfer test |
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- 1 assertion failing: post-burn energy validation |
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--- |
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## Code Statistics |
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### Files Modified (3) |
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- `tests/configs/earth_mars_simple.toml` - Added spacecraft body (+13 lines) |
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- `src/mission_planning.h` - Added function declarations (+3 lines) |
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- `src/mission_planning.cpp` - Implemented new functions (~100 lines) |
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- `tests/test_hohmann_transfer.cpp` - Added comprehensive test (~175 lines) |
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### Net Changes |
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- **~+291 lines** (estimated, not yet committed) |
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--- |
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## Documentation |
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### Updated |
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- `docs/mission_planning.md` - Implementation plan for LEO spacecraft approach |
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- Added test configuration reference |
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- Added future work section |
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- Added notes and references |
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--- |
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## Current Issue |
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### Problem: Incorrect Delta-V Direction After Multi-Day Wait |
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**Symptom:** |
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- Spacecraft enters LEO orbit correctly with negative energy (bound to Earth) |
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- Waits 94 days for Earth-Mars launch window |
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- During wait period, spacecraft completes ~6.3 LEO orbits |
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- LEO orbit phase changes significantly over 94 days |
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- After wait, `apply_transfer_burn()` applies delta-v assuming spacecraft is at Earth's current orbital phase |
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- Result: Delta-v applied in wrong direction, resulting in retrograde burn |
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- Post-burn energy remains negative (spacecraft still bound to Earth) |
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**Root Cause Analysis:** |
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The `apply_transfer_burn()` function calculates: |
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1. Required heliocentric transfer velocity magnitude: `v_transfer = 32,697 m/s` |
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2. Prograde direction based on Earth's current position: `transfer_dir = prograde(t_current)` |
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3. Target velocity: `v_target = v_transfer * transfer_dir` |
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However, after 94 days: |
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- Earth has moved to different orbital phase |
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- Spacecraft in LEO is still orbiting Earth |
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- Spacecraft's current heliocentric velocity includes Earth's motion + LEO motion |
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- The calculated transfer direction is based on Earth's instantaneous position, not spacecraft's actual heliocentric velocity vector |
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- This results in delta-v that doesn't account for spacecraft's phase in LEO |
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**What Should Happen:** |
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1. Calculate spacecraft's current heliocentric velocity vector: `v_current` |
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2. Calculate required heliocentric velocity for transfer orbit: `v_transfer` |
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3. Apply delta-v: `Δv = v_transfer - v_current` (vector subtraction, not magnitude-based) |
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**What Currently Happens:** |
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1. Assumes spacecraft starts at Earth's orbital position (ignores LEO phase) |
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2. Calculates transfer direction based on Earth's current prograde vector |
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3. Applies magnitude-based delta-v without considering spacecraft's actual velocity direction |
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4. Results in incorrect burn direction |
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### Solution Required |
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Modify `apply_transfer_burn()` to: |
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1. **Calculate spacecraft's actual heliocentric velocity:** |
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```cpp |
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Vec3 v_current_helio = spacecraft->velocity; // Already in global frame |
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``` |
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2. **Calculate required heliocentric transfer velocity:** |
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```cpp |
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double v_transfer_mag = params->departure_velocity; // ~32,697 m/s |
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// Direction: prograde to Sun (same as Earth's orbital direction) |
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Vec3 sun_to_earth = vec3_sub(departure->position, sun->position); |
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Vec3 sun_to_earth_norm = vec3_normalize(sun_to_earth); |
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Vec3 transfer_dir = (Vec3){-sun_to_earth_norm.y, sun_to_earth_norm.x, 0.0}; |
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Vec3 v_transfer_helio = vec3_scale(transfer_dir, v_transfer_mag); |
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``` |
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3. **Calculate delta-v as vector difference:** |
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```cpp |
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Vec3 delta_v = vec3_sub(v_transfer_helio, v_current_helio); |
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``` |
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4. **Apply impulse:** |
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```cpp |
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spacecraft->velocity = vec3_add(spacecraft->velocity, delta_v); |
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spacecraft->local_velocity = vec3_sub(spacecraft->velocity, departure->velocity); |
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``` |
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**This approach:** |
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- Accounts for spacecraft's actual heliocentric velocity (includes LEO phase) |
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- Uses vector subtraction instead of magnitude-based calculation |
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- Produces correct delta-v direction regardless of LEO phase |
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- Should result in positive post-burn energy (escape trajectory) |
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--- |
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## Next Steps |
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### High Priority |
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1. **Fix Delta-V Direction Bug** |
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- Modify `apply_transfer_burn()` to use vector subtraction |
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- Test with actual spacecraft heliocentric velocity |
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- Verify post-burn energy becomes positive |
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- Estimated time: 30 minutes |
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2. **Complete Full Transfer Test** |
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- Verify Mars SOI entry after fix |
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- Validate energy conservation during transfer |
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- Confirm SOI transitions (Earth→Sun→Mars) |
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- Estimated time: 1 hour |
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### Medium Priority |
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3. **Phase 5: Cleanup** |
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- Remove `spawn_spacecraft_on_transfer()` function |
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- Update documentation |
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- Estimated time: 20 minutes |
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--- |
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## Technical Decisions |
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### 1. Config-Based vs Dynamic Spawning |
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- **Decision:** Use config-based LEO spacecraft initialization |
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- **Rationale:** Cleaner approach, spacecraft in config from start |
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- **Result:** Spacecraft loaded with placeholder position/velocity, set at runtime |
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### 2. Single-Impulse vs Patched Conics |
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- **Decision:** Single impulse as initial approximation |
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- **Rationale:** Simpler to implement, adequate for initial testing |
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- **Future:** Can refine to two-impulse burn for higher accuracy |
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### 3. LEO Orbit Phase |
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- **Issue:** Spacecraft LEO phase changes during multi-day wait |
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- **Impact:** Delta-v must account for spacecraft's actual heliocentric velocity |
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- **Fix:** Use vector subtraction with spacecraft's actual velocity |
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--- |
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## Achievements |
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1. ✅ Config-based LEO spacecraft initialization |
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2. ✅ Three new functions in mission planning module |
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3. ✅ Comprehensive test framework with SOI transition tracking |
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4. ✅ LEO orbit validated (position, velocity, energy) |
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5. ✅ Launch window detection working |
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6. ✅ Root cause identified for delta-v direction bug |
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7. ✅ Clear solution path defined |
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--- |
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## Risks and Blockers |
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### Current Blockers |
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1. **Delta-V Direction Bug** - Blocks completion of transfer test |
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- Impact: Can't validate full Hohmann transfer |
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- Severity: Low - solution is clear and simple |
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- Mitigation: Fix identified, implementation straightforward |
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--- |
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## Conclusion |
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Session focused on implementing config-based LEO spacecraft initialization and impulse burn for Hohmann transfer. Core implementation complete, test framework working. One bug identified (delta-v direction) with clear solution path. |
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**Session Outcome:** Productive with clear fix required for final test validation. |
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**Estimated Time to Complete:** 1.5-2 hours (fix bug, complete test, cleanup) |
