# Propagation Call Chain Analysis

## Session Date
2026-04-20

## Objective
Audit all call sites of `propagate_orbital_elements()` for spacecraft and trace the `update_simulation()` call chain through `execute_pending_maneuvers()` and `update_spacecraft_physics()` to identify inefficiencies and confusing branching.

## Call Chain: `update_simulation()`

```
update_simulation()
│
├── reset_spacecraft_tracking()
│
├── update_bodies_physics()
│   └── for each body:
│       ├── find_dominant_body()
│       ├── orbital_elements_to_cartesian() → expected_vel
│       ├── velocity drift check (every frame)
│       ├── cartesian_to_orbital_elements() (if drift > 1e-6)
│       └── propagate_orbital_elements()
│
├── compute_global_coordinates()
│
├── execute_pending_maneuvers()
│   └── for each unexecuted maneuver:
│       ├── check_maneuver_trigger()
│       │   ├── TRIGGER_TIME: sim->time >= trigger_value
│       │   └── TRIGGER_TRUE_ANOMALY:
│       │       ├── propagate_orbital_elements() ← per-frame probe
│       │       └── Kepler equation solving
│       └── if triggered:
│           ├── propagate_orbital_elements(dt_to_burn)
│           ├── execute_maneuver()
│           │   ├── apply_impulsive_burn()
│           │   └── cartesian_to_orbital_elements()
│           └── propagate_orbital_elements(remaining_dt)
│
├── update_spacecraft_physics()
│   └── for each craft NOT handled:
│       ├── orbital_elements_to_cartesian() → expected_vel
│       ├── velocity drift check
│       ├── cartesian_to_orbital_elements() (if drift)
│       └── propagate_orbital_elements()
│
├── compute_spacecraft_globals()
└── sim->time += sim->dt
```

## Issues Found

### Issue 1: Redundant Propagation for True-Anomaly Triggers

**Location:** `src/maneuver.cpp`, line 147 in `check_maneuver_trigger()`

For every frame that a true-anomaly maneuver is pending, `propagate_orbital_elements()` is called as a "look-ahead" probe to determine if the target angle is approaching.

```cpp
// maneuver.cpp:147
OrbitalElements future_elements = propagate_orbital_elements(craft->orbit, sim->dt, parent->mass);
```

**Impact:** For a Hohmann transfer with a ~244,000s wait time and DT=10s, this results in ~24,400 redundant propagations — each one solving Kepler's equation — before the maneuver even fires. The spacecraft's orbit state is not modified, but the computational cost is paid every single frame.

### Issue 2: Mixed Concerns in `execute_pending_maneuvers()`

**Location:** `src/simulation.cpp`, `execute_pending_maneuvers()`

The function performs two distinct responsibilities:
1. **Checking** trigger conditions (calls `check_maneuver_trigger()`)
2. **Executing** the burn (propagation → burn → propagation)

```cpp
void execute_pending_maneuvers(SimulationState* sim) {
    for (...) {
        if (check_maneuver_trigger(maneuver, craft, sim)) {  // CHECKING
            // ... propagation, burn, more propagation         // EXECUTING
        }
    }
}
```

The trigger-checking logic (especially the true-anomaly branch with its Kepler equation solving) is interleaved with the execution logic. This makes it difficult to reason about what happens in each phase and complicates testing.

### Issue 3: Ambiguous `scheduled_dt` Semantics

The `scheduled_dt` field in `Maneuver` has different meanings depending on trigger type:

| Trigger Type | `scheduled_dt` meaning | Set by |
|-------------|----------------------|--------|
| `TRIGGER_TIME` | Always 0.0 (never set) | Never |
| `TRIGGER_TRUE_ANOMALY` | Seconds until exact burn position | `check_maneuver_trigger()` |

In `execute_pending_maneuvers()`:

```cpp
double dt_to_burn = maneuver->scheduled_dt;
if (dt_to_burn > 0.0) {
    craft->orbit = propagate_orbital_elements(craft->orbit, dt_to_burn, ...);
}
// ... burn ...
double remaining_dt = sim->dt - dt_to_burn;
craft->orbit = propagate_orbital_elements(craft->orbit, remaining_dt, ...);
```

For `TRIGGER_TIME`, `scheduled_dt == 0` is a coincidence — the field is never set. The code works, but the reason is opaque. The branching (`if dt_to_burn > 0`) exists only for true-anomaly triggers, but a reader cannot tell this from the code alone.

### Issue 4: Time-Triggered Burns Propagate from Wrong State

For time triggers, the sequence is:

```
Frame N:  sim->time = 310.0  (trigger_value = 305.0)
          check: 310 >= 305 → true
          dt_to_burn = 0
          remaining_dt = 10
          propagate craft by 10s starting from sim->time=310
```

The craft's orbit state is at `sim->time=310`, not at the trigger time `305`. The burn fires 5s late and the post-burn propagation starts from the wrong orbital position. This is the root cause of the burn timing quantization problem documented in `docs/planning/hohmann-rendezvous-quantization-fix.md`.

### Issue 5: Hardcoded Array Size

**Location:** `src/simulation.cpp`, static declaration

```cpp
static bool spacecraft_handled_this_frame[256];
```

A global array with a hardcoded size of 256. If more than 256 spacecraft are added, the tracking silently truncates. It should be part of `SimulationState` or dynamically sized. There is already a FIXME comment on this line.

### Issue 6: Duplicated Propagation Logic

The "normal" propagation path in `update_spacecraft_physics()` and the maneuver path in `execute_pending_maneuvers()` are essentially duplicates:

**Normal path** (`update_spacecraft_physics`):
```cpp
propagate_orbital_elements(craft->orbit, sim->dt, ...);
orbital_elements_to_cartesian(...);
```

**Maneuver path** (`execute_pending_maneuvers`, when `dt_to_burn == 0`):
```cpp
propagate_orbital_elements(craft->orbit, 0, ...);     // no-op
orbital_elements_to_cartesian(...);
execute_maneuver(...);
propagate_orbital_elements(craft->orbit, sim->dt, ...);
orbital_elements_to_cartesian(...);
```

When no sub-step is needed, the maneuver path is: propagate → convert → burn → propagate → convert. The propagation and coordinate conversion logic is duplicated across both code paths.

## Call Sites Summary

| Location | Function | Context | dt value |
|----------|----------|---------|----------|
| `simulation.cpp:287` | `update_bodies_physics()` | Normal body propagation | `sim->dt` |
| `simulation.cpp:315` | `update_spacecraft_physics()` | Normal craft propagation | `sim->dt` |
| `simulation.cpp:343` | `execute_pending_maneuvers()` | Pre-burn sub-step propagation | `dt_to_burn` (0 to `sim->dt`) |
| `simulation.cpp:350` | `execute_pending_maneuvers()` | Post-burn remaining propagation | `sim->dt - dt_to_burn` |
| `maneuver.cpp:147` | `check_maneuver_trigger()` | True-anomaly look-ahead probe | `sim->dt` |

Total: 5 call sites, 3 distinct contexts (normal propagation, sub-step execution, probe).