STY clarify FlightBody proxy assumptions and notebook realism notes

Agent-Logs-Url: https://github.com/aZira371/RocketPy/sessions/dd7a22c8-7e00-4af1-8f3f-0320d902b368

Co-authored-by: aZira371 <99824864+aZira371@users.noreply.github.com>

Author: Copilot

docs/multistage_example.ipynb

@@ -428,6 +428,7 @@
    },
    "outputs": [],
    "source": [
+    "# Keep stage-1 short so stage-2 starts near burnout/separation, not at apogee.\n",
     "mission.set_flight_inputs(\n",
     "    \"first_stage\",\n",
     "    inclination=85,\n",
@@ -498,7 +499,8 @@
    "source": [
     "## 8) Execute the full mission (including payload)\n",
     "\n",
-    "The same mission object configured above is executed directly by `MissionExecutor`. This run includes both powered stages and the deployable payload `FlightBody`, preserving mission continuity.\n"
+    "The same mission object configured above is executed directly by `MissionExecutor`.\n",
+    "This run includes both powered stages and the deployable payload `FlightBody`, preserving mission continuity.\n"
    ]
   },
   {

rocketpy/mission/mission_executor.py

@@ -102,20 +102,34 @@ def _extract_rocket(body: Any):
 
     @staticmethod
     def _build_point_mass_from_flight_body(body: FlightBody) -> PointMassRocket:
-        """Build a PointMassRocket proxy from a FlightBody mission item."""
+        """Build a PointMassRocket proxy from a FlightBody mission item.
+
+        Notes
+        -----
+        This proxy uses the `FlightBody` snapshot at ``t=0`` for dry mass and
+        center of mass. The goal is to provide an immediate Flight-compatible
+        representation for mission execution of deployables, especially passive
+        payloads and recovery-only bodies.
+        """
         radius = MissionExecutor._extract_reference_radius(body.geometry)
+        # Use t=0 as a stable initialization snapshot for point-mass proxy setup.
         mass = body.mass(0.0)
         center_of_mass = body.center_of_mass(0.0)
+        # Conservative default Cd for a generic bluff-body payload proxy.
+        # These can be superseded by parachute/recovery deployment dynamics.
+        default_drag_coefficient = 0.75
         rocket = PointMassRocket(
             radius=radius,
             mass=mass,
             center_of_mass_without_motor=center_of_mass,
-            power_off_drag=0.75,
-            power_on_drag=0.75,
+            power_off_drag=default_drag_coefficient,
+            power_on_drag=default_drag_coefficient,
         )
         rocket.name = body.name
 
         orientation = body.coordinate_system_orientation()
+        # PointMassRocket currently has no public API to re-evaluate `_csys`
+        # after initialization, so we synchronize both orientation fields here.
         rocket.coordinate_system_orientation = orientation
         rocket._csys = 1 if orientation == "tail_to_nose" else -1
 

tests/unit/mission/test_mission.py

@@ -103,6 +103,10 @@ def center_of_mass(self, t):
         return 0.0
 
 
+class UnsupportedBody:
+    """Intentional unsupported mission body type for executor error-path tests."""
+
+
 def _make_rocket_adapter(name="rocket_body"):
     """Create a RocketAdapter with a minimal fake rocket object."""
     return RocketAdapter(FakeRocket(name=name))
@@ -556,7 +560,7 @@ def test_execute_runs_stage_and_deployable(self):
     def test_execute_raises_for_non_rocket_body(self):
         """execute raises TypeError when body is not RocketAdapter-backed."""
         mission = Mission()
-        mission.add_stage(_make_stage(name="stage_1", body=object()))
+        mission.add_stage(_make_stage(name="stage_1", body=UnsupportedBody()))
 
         executor = MissionExecutor(
             mission=mission,