BUG: sample StochasticFlight inputs once per simulation (#1090)

rocketpy/simulation/monte_carlo.py

@@ -461,12 +461,15 @@ def __run_single_simulation(self):
         Flight
             The flight object of the simulation.
         """
+        rocket = self.rocket.create_object()
+        environment = self.environment.create_object()
+        flight_inputs = self.flight._sample_flight_inputs()
         return Flight(
-            rocket=self.rocket.create_object(),
-            environment=self.environment.create_object(),
-            rail_length=self.flight._randomize_rail_length(),
-            inclination=self.flight._randomize_inclination(),
-            heading=self.flight._randomize_heading(),
+            rocket=rocket,
+            environment=environment,
+            rail_length=flight_inputs["rail_length"],
+            inclination=flight_inputs["inclination"],
+            heading=flight_inputs["heading"],
             initial_solution=self.flight.initial_solution,
             terminate_on_apogee=self.flight.terminate_on_apogee,
             time_overshoot=self.flight.time_overshoot,
@@ -1377,7 +1380,7 @@ def export_ellipses_to_kml(  # pylint: disable=too-many-statements
             except KeyError as e:
                 raise KeyError("No impact data found. Skipping impact ellipses.") from e
 
-        (apogee_ellipses, impact_ellipses) = generate_monte_carlo_ellipses(
+        apogee_ellipses, impact_ellipses = generate_monte_carlo_ellipses(
             impact_x,
             impact_y,
             apogee_x,

rocketpy/stochastic/stochastic_flight.py

@@ -122,21 +122,28 @@ def _validate_initial_solution(self, initial_solution):
             else:
                 raise TypeError("`initial_solution` must be a tuple of numbers")
 
-    # TODO: these methods call dict_generator a lot of times unnecessarily
+    def _sample_flight_inputs(self):
+        """Sample rail_length, inclination, and heading in a single draw.
+
+        Returns
+        -------
+        dict
+            Mapping with keys ``rail_length``, ``inclination``, and ``heading``.
+            Also updates ``last_rnd_dict``.
+        """
+        return next(self.dict_generator())
+
     def _randomize_rail_length(self):
         """Randomizes the rail length of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["rail_length"]
+        return self._sample_flight_inputs()["rail_length"]
 
     def _randomize_inclination(self):
         """Randomizes the inclination of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["inclination"]
+        return self._sample_flight_inputs()["inclination"]
 
     def _randomize_heading(self):
         """Randomizes the heading of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["heading"]
+        return self._sample_flight_inputs()["heading"]
 
     def create_object(self):
         """Creates and returns a Flight object from the randomly generated input
@@ -147,12 +154,11 @@ def create_object(self):
         flight : Flight
             Flight object with the randomly generated input arguments.
         """
-        generated_dict = next(self.dict_generator())
-        # TODO: maybe we should use generated_dict["rail_length"] instead
+        generated_dict = self._sample_flight_inputs()
         return Flight(
             rocket=self.obj.rocket,
             environment=self.obj.env,
-            rail_length=self._randomize_rail_length(),
+            rail_length=generated_dict["rail_length"],
             inclination=generated_dict["inclination"],
             heading=generated_dict["heading"],
             initial_solution=self.initial_solution,

tests/unit/stochastic/test_stochastic_flight.py

@@ -45,3 +45,39 @@ def test_stochastic_flight_optional_attributes(flight_calisto_robust):
     assert obj.terminate_on_apogee is True
     assert obj.time_overshoot is True
     assert obj.max_time == 987.6
+
+
+def test_create_object_matches_last_rnd_dict(flight_calisto_robust):
+    """Regression for #1090: create_object must use one dict_generator draw.
+
+    Spreads are set on all three flight inputs so a second draw would diverge
+    from ``last_rnd_dict``.
+    """
+    stochastic_flight = StochasticFlight(
+        flight=flight_calisto_robust,
+        rail_length=(5.2, 0.5),
+        inclination=(84.7, 1),
+        heading=(53, 2),
+    )
+    stochastic_flight._set_stochastic(4242)
+
+    flight = stochastic_flight.create_object()
+    sampled = stochastic_flight.last_rnd_dict
+
+    assert flight.rail_length == sampled["rail_length"]
+    assert flight.inclination == sampled["inclination"]
+    assert flight.heading == sampled["heading"]
+
+
+def test_monte_carlo_single_simulation_matches_flight_last_rnd_dict(
+    monte_carlo_calisto,
+):
+    """Regression for #1090: MonteCarlo must fly the same sample it logs."""
+    monte_carlo_calisto.flight._set_stochastic(4242)
+
+    flight = monte_carlo_calisto._MonteCarlo__run_single_simulation()
+    sampled = monte_carlo_calisto.flight.last_rnd_dict
+
+    assert flight.rail_length == sampled["rail_length"]
+    assert flight.inclination == sampled["inclination"]
+    assert flight.heading == sampled["heading"]