BUG: Wrong Phi Initialization For nose_to_tail Rockets

CHANGELOG.md

@@ -45,6 +45,7 @@ Attention: The newest changes should be on top -->
 
 - BUG: Fix the handling of reference pressure for older rpy files. [#808](https://github.com/RocketPy-Team/RocketPy/pull/808)
 - BUG: Non-overshootable simulations error on time parsing. [#807](https://github.com/RocketPy-Team/RocketPy/pull/807)
+- BUG: Wrong Phi Initialization For nose_to_tail Rockets [#809](https://github.com/RocketPy-Team/RocketPy/pull/809)
 
 ## v1.9.0 - 2025-03-24
 

rocketpy/simulation/flight.py

@@ -1156,7 +1156,9 @@ def __init_flight_state(self):
             try:
                 self.phi_init += (
                     self.rocket.rail_buttons[0].component.angular_position_rad
-                    * self.rocket._csys
+                    if self.rocket._csys == 1
+                    else 2 * np.pi
+                    - self.rocket.rail_buttons[0].component.angular_position_rad
                 )
             except IndexError:
                 pass
@@ -1277,7 +1279,7 @@ def __set_ode_solver(self, solver):
                     f"Invalid ``ode_solver`` input: {solver}. "
                     f"Available options are: {', '.join(ODE_SOLVER_MAP.keys())}"
                 ) from e
-            
+
         self.__is_lsoda = issubclass(self._solver, LSODA)
 
     @cached_property

tests/fixtures/flight/flight_fixtures.py

@@ -93,6 +93,20 @@ def flight_calisto_robust(calisto_robust, example_spaceport_env):
     )
 
 
+@pytest.fixture
+def flight_calisto_nose_to_tail_robust(
+    calisto_nose_to_tail_robust, example_spaceport_env
+):
+    return Flight(
+        environment=example_spaceport_env,
+        rocket=calisto_nose_to_tail_robust,
+        rail_length=5.2,
+        inclination=85,
+        heading=0,
+        terminate_on_apogee=False,
+    )
+
+
 @pytest.fixture
 def flight_calisto_robust_solid_eom(calisto_robust, example_spaceport_env):
     """Similar to flight_calisto_robust, but with the equations of motion set to

tests/fixtures/rockets/rocket_fixtures.py

@@ -184,6 +184,33 @@ def calisto_robust(
     return calisto
 
 
+@pytest.fixture
+def calisto_nose_to_tail_robust(
+    calisto_nose_to_tail,
+    calisto_nose_cone,
+    calisto_tail,
+    calisto_trapezoidal_fins,
+    calisto_main_chute,
+    calisto_drogue_chute,
+):
+    """Calisto with nose to tail coordinate system orientation. This is the same
+    as calisto_robust, but with the coordinate system orientation set to
+    "nose_to_tail"."""
+    csys = -1
+    # we follow this format: calisto.add_surfaces(surface, position)
+    calisto_nose_to_tail.add_surfaces(calisto_nose_cone, 1.160 * csys)
+    calisto_nose_to_tail.add_surfaces(calisto_tail, -1.313 * csys)
+    calisto_nose_to_tail.add_surfaces(calisto_trapezoidal_fins, -1.168 * csys)
+    calisto_nose_to_tail.set_rail_buttons(
+        upper_button_position=0.082 * csys,
+        lower_button_position=-0.618 * csys,
+        angular_position=360 - 0,
+    )
+    calisto_nose_to_tail.parachutes.append(calisto_main_chute)
+    calisto_nose_to_tail.parachutes.append(calisto_drogue_chute)
+    return calisto_nose_to_tail
+
+
 @pytest.fixture
 def calisto_air_brakes_clamp_on(calisto_robust, controller_function):
     """Create an object class of the Rocket class to be used in the tests. This

tests/integration/test_flight.py

@@ -521,3 +521,37 @@ def test_freestream_speed_at_apogee(example_plain_env, calisto):
         test_flight.free_stream_speed(test_flight.apogee_time), 0.0, atol=soft_atol
     )
     assert np.isclose(test_flight.apogee_freestream_speed, 0.0, atol=soft_atol)
+
+
+def test_rocket_csys_equivalence(
+    flight_calisto_robust, flight_calisto_nose_to_tail_robust
+):
+    """Test the equivalence of the rocket coordinate systems between two
+    different flight simulations.
+
+    Parameters
+    ----------
+    flight_calisto_robust : rocketpy.Flight
+        Flight object to be tested. See the conftest.py file for more info.
+    flight_calisto_nose_to_tail_robust : rocketpy.Flight
+        Flight object to be tested. See the conftest.py file for more info.
+    """
+    assert np.isclose(
+        flight_calisto_robust.apogee, flight_calisto_nose_to_tail_robust.apogee
+    )
+    assert np.isclose(
+        flight_calisto_robust.apogee_time,
+        flight_calisto_nose_to_tail_robust.apogee_time,
+    )
+    assert np.isclose(
+        flight_calisto_robust.x_impact,
+        flight_calisto_nose_to_tail_robust.x_impact,
+    )
+    assert np.isclose(
+        flight_calisto_robust.y_impact,
+        flight_calisto_nose_to_tail_robust.y_impact,
+    )
+    assert np.allclose(
+        flight_calisto_robust.initial_solution,
+        flight_calisto_nose_to_tail_robust.initial_solution,
+    )