ENH: Enable only radial burning

CHANGELOG.md

@@ -32,6 +32,8 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
+- ENH: Enable only radial burning [#815](https://github.com/RocketPy-Team/RocketPy/pull/815)
+
 ### Changed
 
 ### Fixed
@@ -73,6 +75,7 @@ Attention: The newest changes should be on top -->
 ## [v1.10.0] - 2025-05-16
 
 ### Added
+
 - ENH: Support for ND arithmetic in Function class. [#810] (https://github.com/RocketPy-Team/RocketPy/pull/810)
 - ENH: allow users to provide custom samplers [#803](https://github.com/RocketPy-Team/RocketPy/pull/803)
 - ENH: Implement Multivariate Rejection Sampling (MRS) [#738] (https://github.com/RocketPy-Team/RocketPy/pull/738)

rocketpy/motors/hybrid_motor.py

@@ -193,8 +193,12 @@ class HybridMotor(Motor):
     HybridMotor.reference_pressure : int, float
         Atmospheric pressure in Pa at which the thrust data was recorded.
         It will allow to obtain the net thrust in the Flight class.
+    SolidMotor.only_radial_burn : bool
+        If True, grain regression is restricted to radial burn only (inner radius growth).
+        Grain length remains constant throughout the burn. Default is False.
     """
 
+    # pylint: disable=too-many-arguments
     def __init__(  # pylint: disable=too-many-arguments
         self,
         thrust_source,
@@ -216,6 +220,7 @@ def __init__(  # pylint: disable=too-many-arguments
         interpolation_method="linear",
         coordinate_system_orientation="nozzle_to_combustion_chamber",
         reference_pressure=None,
+        only_radial_burn=True,
     ):
         """Initialize Motor class, process thrust curve and geometrical
         parameters and store results.
@@ -313,6 +318,11 @@ class Function. Thrust units are Newtons.
             "nozzle_to_combustion_chamber".
         reference_pressure : int, float, optional
             Atmospheric pressure in Pa at which the thrust data was recorded.
+        only_radial_burn : boolean, optional
+            If True, inhibits the grain from burning axially, only computing
+            radial burn. If False, allows the grain to also burn
+            axially. May be useful for axially inhibited grains or hybrid motors.
+            Default is False.
 
         Returns
         -------
@@ -364,6 +374,7 @@ class Function. Thrust units are Newtons.
             interpolation_method,
             coordinate_system_orientation,
             reference_pressure,
+            only_radial_burn,
         )
 
         self.positioned_tanks = self.liquid.positioned_tanks

rocketpy/motors/solid_motor.py

@@ -193,6 +193,9 @@ class SolidMotor(Motor):
     SolidMotor.reference_pressure : int, float
         Atmospheric pressure in Pa at which the thrust data was recorded.
         It will allow to obtain the net thrust in the Flight class.
+    SolidMotor.only_radial_burn : bool
+        If True, grain regression is restricted to radial burn only (inner radius growth).
+        Grain length remains constant throughout the burn. Default is False.
     """
 
     # pylint: disable=too-many-arguments
@@ -217,6 +220,7 @@ def __init__(
         interpolation_method="linear",
         coordinate_system_orientation="nozzle_to_combustion_chamber",
         reference_pressure=None,
+        only_radial_burn=False,
     ):
         """Initialize Motor class, process thrust curve and geometrical
         parameters and store results.
@@ -314,11 +318,19 @@ class Function. Thrust units are Newtons.
             "nozzle_to_combustion_chamber".
         reference_pressure : int, float, optional
             Atmospheric pressure in Pa at which the thrust data was recorded.
+        only_radial_burn : boolean, optional
+            If True, inhibits the grain from burning axially, only computing
+            radial burn. If False, allows the grain to also burn
+            axially. May be useful for axially inhibited grains or hybrid motors.
+            Default is False.
 
         Returns
         -------
         None
         """
+        # Store before calling super().__init__() since it calls evaluate_geometry()
+        self.only_radial_burn = only_radial_burn
+
         super().__init__(
             thrust_source=thrust_source,
             dry_inertia=dry_inertia,
@@ -500,17 +512,25 @@ def geometry_dot(t, y):
 
             # Compute state vector derivative
             grain_inner_radius, grain_height = y
-            burn_area = (
-                2
-                * np.pi
-                * (
-                    grain_outer_radius**2
-                    - grain_inner_radius**2
-                    + grain_inner_radius * grain_height
+            if self.only_radial_burn:
+                burn_area = 2 * np.pi * (grain_inner_radius * grain_height)
+
+                grain_inner_radius_derivative = -volume_diff / burn_area
+                grain_height_derivative = 0  # Set to zero to disable axial burning
+
+            else:
+                burn_area = (
+                    2
+                    * np.pi
+                    * (
+                        grain_outer_radius**2
+                        - grain_inner_radius**2
+                        + grain_inner_radius * grain_height
+                    )
                 )
-            )
-            grain_inner_radius_derivative = -volume_diff / burn_area
-            grain_height_derivative = -2 * grain_inner_radius_derivative
+
+                grain_inner_radius_derivative = -volume_diff / burn_area
+                grain_height_derivative = -2 * grain_inner_radius_derivative
 
             return [grain_inner_radius_derivative, grain_height_derivative]
 
@@ -521,32 +541,55 @@ def geometry_jacobian(t, y):
 
             # Compute jacobian
             grain_inner_radius, grain_height = y
-            factor = volume_diff / (
-                2
-                * np.pi
-                * (
-                    grain_outer_radius**2
-                    - grain_inner_radius**2
-                    + grain_inner_radius * grain_height
+            if self.only_radial_burn:
+                factor = volume_diff / (
+                    2 * np.pi * (grain_inner_radius * grain_height) ** 2
                 )
-                ** 2
-            )
-            inner_radius_derivative_wrt_inner_radius = factor * (
-                grain_height - 2 * grain_inner_radius
-            )
-            inner_radius_derivative_wrt_height = factor * grain_inner_radius
-            height_derivative_wrt_inner_radius = (
-                -2 * inner_radius_derivative_wrt_inner_radius
-            )
-            height_derivative_wrt_height = -2 * inner_radius_derivative_wrt_height
 
-            return [
-                [
-                    inner_radius_derivative_wrt_inner_radius,
-                    inner_radius_derivative_wrt_height,
-                ],
-                [height_derivative_wrt_inner_radius, height_derivative_wrt_height],
-            ]
+                inner_radius_derivative_wrt_inner_radius = factor * (
+                    grain_height - 2 * grain_inner_radius
+                )
+                inner_radius_derivative_wrt_height = 0
+                height_derivative_wrt_inner_radius = 0
+                height_derivative_wrt_height = 0
+                # Height is a constant, so all the derivatives with respect to it are set to zero
+
+                return [
+                    [
+                        inner_radius_derivative_wrt_inner_radius,
+                        inner_radius_derivative_wrt_height,
+                    ],
+                    [height_derivative_wrt_inner_radius, height_derivative_wrt_height],
+                ]
+
+            else:
+                factor = volume_diff / (
+                    2
+                    * np.pi
+                    * (
+                        grain_outer_radius**2
+                        - grain_inner_radius**2
+                        + grain_inner_radius * grain_height
+                    )
+                    ** 2
+                )
+
+                inner_radius_derivative_wrt_inner_radius = factor * (
+                    grain_height - 2 * grain_inner_radius
+                )
+                inner_radius_derivative_wrt_height = factor * grain_inner_radius
+                height_derivative_wrt_inner_radius = (
+                    -2 * inner_radius_derivative_wrt_inner_radius
+                )
+                height_derivative_wrt_height = -2 * inner_radius_derivative_wrt_height
+
+                return [
+                    [
+                        inner_radius_derivative_wrt_inner_radius,
+                        inner_radius_derivative_wrt_height,
+                    ],
+                    [height_derivative_wrt_inner_radius, height_derivative_wrt_height],
+                ]
 
         def terminate_burn(t, y):  # pylint: disable=unused-argument
             end_function = (self.grain_outer_radius - y[0]) * y[1]
@@ -597,16 +640,24 @@ def burn_area(self):
         burn_area : Function
             Function representing the burn area progression with the time.
         """
-        burn_area = (
-            2
-            * np.pi
-            * (
-                self.grain_outer_radius**2
-                - self.grain_inner_radius**2
-                + self.grain_inner_radius * self.grain_height
+        if self.only_radial_burn:
+            burn_area = (
+                2
+                * np.pi
+                * (self.grain_inner_radius * self.grain_height)
+                * self.grain_number
+            )
+        else:
+            burn_area = (
+                2
+                * np.pi
+                * (
+                    self.grain_outer_radius**2
+                    - self.grain_inner_radius**2
+                    + self.grain_inner_radius * self.grain_height
+                )
+                * self.grain_number
             )
-            * self.grain_number
-        )
         return burn_area
 
     @funcify_method("Time (s)", "burn rate (m/s)")
@@ -778,6 +829,7 @@ def to_dict(self, **kwargs):
                 "grain_initial_height": self.grain_initial_height,
                 "grain_separation": self.grain_separation,
                 "grains_center_of_mass_position": self.grains_center_of_mass_position,
+                "only_radial_burn": self.only_radial_burn,
             }
         )
 
@@ -827,4 +879,5 @@ def from_dict(cls, data):
             interpolation_method=data["interpolate"],
             coordinate_system_orientation=data["coordinate_system_orientation"],
             reference_pressure=data.get("reference_pressure"),
+            only_radial_burn=data.get("only_radial_burn", False),
         )

tests/fixtures/motor/hybrid_fixtures.py

@@ -4,7 +4,7 @@
 
 
 @pytest.fixture
-def hybrid_motor(spherical_oxidizer_tank):
+def hybrid_motor(oxidizer_tank):
     """An example of a hybrid motor with spherical oxidizer
     tank and fuel grains.
 
@@ -35,6 +35,6 @@ def hybrid_motor(spherical_oxidizer_tank):
         grains_center_of_mass_position=-0.1,
     )
 
-    motor.add_tank(spherical_oxidizer_tank, position=0.3)
+    motor.add_tank(oxidizer_tank, position=0.3)
 
     return motor

tests/integration/motors/test_hybridmotor.py

@@ -1,8 +1,9 @@
-# pylint: disable=unused-argument
 from unittest.mock import patch
 
+import numpy as np
 
-@patch("matplotlib.pyplot.show")
+
+@patch("matplotlib.pyplot.show")  # pylint: disable=unused-argument
 def test_hybrid_motor_info(mock_show, hybrid_motor):
     """Tests the HybridMotor.all_info() method.
 
@@ -15,3 +16,40 @@ def test_hybrid_motor_info(mock_show, hybrid_motor):
     """
     assert hybrid_motor.info() is None
     assert hybrid_motor.all_info() is None
+
+
+def test_hybrid_motor_only_radial_burn_behavior(hybrid_motor):
+    """
+    Test if only_radial_burn flag in HybridMotor propagates to its SolidMotor
+    and affects burn_area calculation.
+    """
+    motor = hybrid_motor
+
+    # Activates the radial burning
+    motor.solid.only_radial_burn = True
+    assert motor.solid.only_radial_burn is True
+
+    # Calculates the expected initial area
+    burn_area_radial = (
+        2
+        * np.pi
+        * (motor.solid.grain_inner_radius(0) * motor.solid.grain_height(0))
+        * motor.solid.grain_number
+    )
+
+    assert np.isclose(motor.solid.burn_area(0), burn_area_radial, atol=1e-12)
+
+    # Deactivates the radial burning and recalculate the geometry
+    motor.solid.only_radial_burn = False
+    motor.solid.evaluate_geometry()
+    assert motor.solid.only_radial_burn is False
+
+    # In this case the burning area also considers the bases of the grain
+    inner_radius = motor.solid.grain_inner_radius(0)
+    outer_radius = motor.solid.grain_outer_radius
+    burn_area_total = (
+        burn_area_radial
+        + 2 * np.pi * (outer_radius**2 - inner_radius**2) * motor.solid.grain_number
+    )
+    assert np.isclose(motor.solid.burn_area(0), burn_area_total, atol=1e-12)
+    assert motor.solid.burn_area(0) > burn_area_radial

tests/integration/motors/test_solid_motor.py

@@ -0,0 +1,58 @@
+import numpy as np
+
+
+def test_only_radial_burn_parameter_effect(cesaroni_m1670):
+    """Tests the effect of the only_radial_burn parameter on burn area
+    calculation. When enabled, the burn area should only account for
+    the radial surface of the grains (no axial regression).
+
+    Parameters
+    ----------
+    cesaroni_m1670 : rocketpy.SolidMotor
+        The SolidMotor object used in the test.
+    """
+    motor = cesaroni_m1670
+    motor.only_radial_burn = True
+    assert motor.only_radial_burn
+
+    # When only_radial_burn is active, burn_area should consider only radial area
+    burn_area_radial = (
+        2
+        * np.pi
+        * motor.grain_inner_radius(0)
+        * motor.grain_height(0)
+        * motor.grain_number
+    )
+    assert np.isclose(motor.burn_area(0), burn_area_radial, atol=1e-12)
+
+
+def test_evaluate_geometry_updates_properties(cesaroni_m1670):
+    """Tests if the grain geometry evaluation correctly updates SolidMotor
+    properties after instantiation. It ensures that grain geometry
+    functions are created and behave as expected.
+
+    Parameters
+    ----------
+    cesaroni_m1670 : rocketpy.SolidMotor
+        The SolidMotor object used in the test.
+    """
+    motor = cesaroni_m1670
+
+    assert hasattr(motor, "grain_inner_radius")
+    assert hasattr(motor, "grain_height")
+
+    # Checks if the domain of grain_inner_radius function is consistent
+    times = motor.grain_inner_radius.x_array
+    values = motor.grain_inner_radius.y_array
+
+    # expected initial time
+    assert times[0] == 0
+
+    # expected initial inner radius
+    assert values[0] == motor.grain_initial_inner_radius
+
+    # final inner radius should be less or equal than outer radius
+    assert values[-1] <= motor.grain_outer_radius
+
+    # evaluate at intermediate time
+    assert isinstance(motor.grain_inner_radius(0.5), float)