BUG: correct the nozzle gyration tensor parallel axis term

rocketpy/rocket/rocket.py

@@ -1004,16 +1004,16 @@ def evaluate_nozzle_to_cdm(self):
 
     def evaluate_nozzle_gyration_tensor(self):
         """Calculates and returns the nozzle gyration tensor relative to the
-        rocket's center of dry mass. The gyration tensor is saved and returned
-        in units of kg*m².
+        rocket's center of dry mass. The gyration tensor is a second moment of
+        area per unit area, so it is saved and returned in units of m².
 
         Returns
         -------
         self.nozzle_gyration_tensor : Matrix
             Matrix containing the nozzle gyration tensor.
         """
         S_noz_33 = 0.5 * self.motor.nozzle_radius**2
-        S_noz_11 = S_noz_22 = 0.5 * S_noz_33 + 0.25 * self.nozzle_to_cdm**2
+        S_noz_11 = S_noz_22 = 0.5 * S_noz_33 + self.nozzle_to_cdm**2
         S_noz_12, S_noz_13, S_noz_23 = 0, 0, 0  # Due to axis symmetry
         self.nozzle_gyration_tensor = Matrix(
             [

tests/acceptance/test_defiance_rocket.py

@@ -11,8 +11,8 @@
 MAX_RELATIVE_APOGEE_ERROR = 0.01
 REFERENCE_MAX_SPEED = 444.24
 REFERENCE_MAX_ACCELERATION = 10400.76
-REFERENCE_IMPACT_X = 1625.55
-REFERENCE_IMPACT_Y = 81.78
+REFERENCE_IMPACT_X = 1609.40
+REFERENCE_IMPACT_Y = 87.03
 REFERENCE_METRIC_RELATIVE_TOLERANCE = 0.01
 REFERENCE_IMPACT_ABSOLUTE_TOLERANCE = 3.0
 

tests/unit/rocket/test_rocket.py

@@ -515,9 +515,9 @@ def test_evaluate_nozzle_to_cdm(calisto):
 
 def test_evaluate_nozzle_gyration_tensor(calisto):
     expected_gyration_tensor = np.array(
-        [[0.3940207, 0, 0], [0, 0.3940207, 0], [0, 0, 0.0005445]]
+        [[1.5752660, 0, 0], [0, 1.5752660, 0], [0, 0, 0.0005445]]
     )
-    atol = 1e-3 * 1e-2 * 1e-2  # Equivalent to 1g * 1cm^2
+    atol = 1e-7  # equivalent to 0.1 mm^2
     assert np.allclose(
         expected_gyration_tensor, np.array(calisto.nozzle_gyration_tensor), atol=atol
     )
@@ -526,6 +526,34 @@ def test_evaluate_nozzle_gyration_tensor(calisto):
     assert np.allclose(expected_gyration_tensor, np.array(res), atol=atol)
 
 
+@pytest.mark.parametrize(
+    "rocket_fixture",
+    ["calisto", "calisto_liquid_modded", "calisto_hybrid_modded"],
+)
+def test_evaluate_nozzle_gyration_tensor_matches_the_exit_disk(rocket_fixture, request):
+    """The tensor is the exit disk second moment per unit area, about the CDM."""
+    rocket = request.getfixturevalue(rocket_fixture)
+    radius = rocket.motor.nozzle_radius
+    offset = rocket.nozzle_to_cdm
+
+    tensor = np.array(rocket.evaluate_nozzle_gyration_tensor())
+
+    lateral = radius**2 / 4 + offset**2
+    assert tensor[0, 0] == pytest.approx(lateral, rel=1e-12)
+    assert tensor[1, 1] == pytest.approx(lateral, rel=1e-12)
+    assert tensor[2, 2] == pytest.approx(radius**2 / 2, rel=1e-12)
+    assert (tensor[0, 1], tensor[0, 2], tensor[1, 2]) == (0, 0, 0)
+
+
+def test_evaluate_nozzle_gyration_tensor_parallel_axis_term(calisto):
+    """Taking the disk term out leaves the whole squared offset, not a fraction."""
+    tensor = np.array(calisto.evaluate_nozzle_gyration_tensor())
+
+    parallel_axis = tensor[0, 0] - tensor[2, 2] / 2
+
+    assert parallel_axis == pytest.approx(calisto.nozzle_to_cdm**2, rel=1e-12)
+
+
 def test_evaluate_com_to_cdm_function(calisto):
     atol = 1e-3  # Equivalent to 1mm
     assert np.allclose(

tests/unit/simulation/test_flight.py

@@ -316,7 +316,7 @@ def test_export_sensor_data(flight_calisto_with_sensors):
     [
         ("t_initial", (0.25886, -0.649623, 0)),
         ("out_of_rail_time", (0.792028, -1.987634, 0)),
-        ("apogee_time", (-0.509420, -0.732933, -2.089120e-14)),
+        ("apogee_time", (-0.519917, -0.734918, -1.005368e-18)),
         ("t_final", (0, 0, 0)),
     ],
 )
@@ -355,7 +355,7 @@ def test_aerodynamic_moments(flight_calisto_custom_wind, flight_time, expected_v
     [
         ("t_initial", (1.654150, 0.659142, -0.067103)),
         ("out_of_rail_time", (5.052628, 2.013361, -1.75370)),
-        ("apogee_time", (2.321838, -1.613641, -0.962108)),
+        ("apogee_time", (2.322999, -1.643037, -0.950316)),
         ("t_final", (-0.019802, 0.012030, 159.051604)),
     ],
 )
@@ -396,7 +396,7 @@ def test_aerodynamic_forces(flight_calisto_custom_wind, flight_time, expected_va
         ("out_of_rail_time", (0, 2.248540, 25.700928)),
         (
             "apogee_time",
-            (-14.826350, 15.670022, -0.000264),
+            (-14.593411, 15.743567, -0.000409),
         ),
         ("t_final", (5, 2, -5.660155)),
     ],