Fix hard-coded radius value for parachute added mass calculation

CHANGELOG.md

@@ -55,8 +55,10 @@ Attention: The newest changes should be on top -->
 
 ### Fixed
 
+- BUG: Fix hard-coded radius value for parachute added mass calculation [#889](https://github.com/RocketPy-Team/RocketPy/pull/889)
 - DOC: Fix documentation build [#908](https://github.com/RocketPy-Team/RocketPy/pull/908)
 - BUG: energy_data plot not working for 3 dof sims [[#906](https://github.com/RocketPy-Team/RocketPy/issues/906)]
+- BUG: Fix parallel Monte Carlo simulation showing incorrect iteration count [#806](https://github.com/RocketPy-Team/RocketPy/pull/806)
 - BUG: Fix CSV column header spacing in FlightDataExporter [#864](https://github.com/RocketPy-Team/RocketPy/issues/864)
 - BUG: Fix parallel Monte Carlo simulation showing incorrect iteration count [#806](https://github.com/RocketPy-Team/RocketPy/pull/806)
 - BUG: Duplicate _controllers in Flight.TimeNodes.merge() [#931](https://github.com/RocketPy-Team/RocketPy/pull/931)

rocketpy/rocket/parachute.py

@@ -92,17 +92,25 @@ class Parachute:
         Function of noisy_pressure_signal.
     Parachute.clean_pressure_signal_function : Function
         Function of clean_pressure_signal.
+    Parachute.drag_coefficient : float
+        Drag coefficient of the inflated canopy shape, used only when
+        ``radius`` is not provided to estimate the parachute radius from
+        ``cd_s``: ``R = sqrt(cd_s / (drag_coefficient * pi))``. Typical
+        values: 1.4 for hemispherical canopies (default), 0.75 for flat
+        circular canopies, 1.5 for extended-skirt canopies.
     Parachute.radius : float
         Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-        parachute in meters.
-    Parachute.height : float, None
+        parachute in meters. If not provided at construction time, it is
+        estimated from ``cd_s`` and ``drag_coefficient``.
+    Parachute.height : float
         Length of the unique semi-axis (height) of the inflated hemispheroid
         parachute in meters.
     Parachute.porosity : float
-        Geometric porosity of the canopy (ratio of open area to total canopy area),
-        in [0, 1]. Affects only the added-mass scaling during descent; it does
-        not change ``cd_s`` (drag). The default, 0.0432, yields an added-mass
-        of 1.0 (“neutral” behavior).
+        Geometric porosity of the canopy (ratio of open area to total canopy
+        area), in [0, 1]. Affects only the added-mass scaling during descent;
+        it does not change ``cd_s`` (drag). The default value of 0.0432 is
+        chosen so that the resulting ``added_mass_coefficient`` equals
+        approximately 1.0 ("neutral" added-mass behavior).
     Parachute.added_mass_coefficient : float
         Coefficient used to calculate the added-mass due to dragged air. It is
         calculated from the porosity of the parachute.
@@ -116,7 +124,8 @@ def __init__(
         sampling_rate,
         lag=0,
         noise=(0, 0, 0),
-        radius=1.5,
+        radius=None,
+        drag_coefficient=1.4,
         height=None,
         porosity=0.0432,
     ):
@@ -172,18 +181,33 @@ def __init__(
             passed to the trigger function. Default value is ``(0, 0, 0)``.
             Units are in Pa.
         radius : float, optional
-            Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-            parachute. Default value is 1.5.
+            Length of the non-unique semi-axis (radius) of the inflated
+            hemispheroid parachute. If not provided, it is estimated from
+            ``cd_s`` and ``drag_coefficient`` using:
+            ``radius = sqrt(cd_s / (drag_coefficient * pi))``.
             Units are in meters.
+        drag_coefficient : float, optional
+            Drag coefficient of the inflated canopy shape, used only when
+            ``radius`` is not provided. It relates the aerodynamic ``cd_s``
+            to the physical canopy area via
+            ``cd_s = drag_coefficient * pi * radius**2``. Typical values:
+
+            - **1.4** — hemispherical canopy (default, NASA SP-8066)
+            - **0.75** — flat circular canopy
+            - **1.5** — extended-skirt canopy
+
+            Has no effect when ``radius`` is explicitly provided.
         height : float, optional
             Length of the unique semi-axis (height) of the inflated hemispheroid
             parachute. Default value is the radius of the parachute.
             Units are in meters.
         porosity : float, optional
-            Geometric porosity of the canopy (ratio of open area to total canopy area),
-            in [0, 1]. Affects only the added-mass scaling during descent; it does
-            not change ``cd_s`` (drag). The default, 0.0432, yields an added-mass
-            of 1.0 (“neutral” behavior).
+            Geometric porosity of the canopy (ratio of open area to total
+            canopy area), in [0, 1]. Affects only the added-mass scaling
+            during descent; it does not change ``cd_s`` (drag). The default
+            value of 0.0432 is chosen so that the resulting
+            ``added_mass_coefficient`` equals approximately 1.0 ("neutral"
+            added-mass behavior).
         """
         self.name = name
         self.cd_s = cd_s
@@ -200,8 +224,14 @@ def __init__(
         self.clean_pressure_signal_function = Function(0)
         self.noisy_pressure_signal_function = Function(0)
         self.noise_signal_function = Function(0)
-        self.radius = radius
-        self.height = height or radius
+        self.drag_coefficient = drag_coefficient
+        # Estimate radius from cd_s if not provided.
+        # cd_s = Cd * S = Cd * π * R²  =>  R = sqrt(cd_s / (Cd * π))
+        if radius is None:
+            self.radius = np.sqrt(cd_s / (drag_coefficient * np.pi))
+        else:
+            self.radius = radius
+        self.height = height or self.radius
         self.porosity = porosity
         self.added_mass_coefficient = 1.068 * (
             1
@@ -309,6 +339,7 @@ def to_dict(self, **kwargs):
             "lag": self.lag,
             "noise": self.noise,
             "radius": self.radius,
+            "drag_coefficient": self.drag_coefficient,
             "height": self.height,
             "porosity": self.porosity,
         }
@@ -341,7 +372,8 @@ def from_dict(cls, data):
             sampling_rate=data["sampling_rate"],
             lag=data["lag"],
             noise=data["noise"],
-            radius=data.get("radius", 1.5),
+            radius=data.get("radius", None),
+            drag_coefficient=data.get("drag_coefficient", 1.4),
             height=data.get("height", None),
             porosity=data.get("porosity", 0.0432),
         )

rocketpy/rocket/rocket.py

@@ -1488,7 +1488,8 @@ def add_parachute(
         sampling_rate=100,
         lag=0,
         noise=(0, 0, 0),
-        radius=1.5,
+        radius=None,
+        drag_coefficient=1.4,
         height=None,
         porosity=0.0432,
     ):
@@ -1550,26 +1551,34 @@ def add_parachute(
             passed to the trigger function. Default value is (0, 0, 0). Units
             are in pascal.
         radius : float, optional
-            Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-            parachute. Default value is 1.5.
+            Length of the non-unique semi-axis (radius) of the inflated
+            hemispheroid parachute. If not provided, it is estimated from
+            `cd_s` and `drag_coefficient` using:
+            `radius = sqrt(cd_s / (drag_coefficient * pi))`.
             Units are in meters.
+        drag_coefficient : float, optional
+            Drag coefficient of the inflated canopy shape, used only when
+            `radius` is not provided. Typical values: 1.4 for hemispherical
+            canopies (default), 0.75 for flat circular canopies, 1.5 for
+            extended-skirt canopies. Has no effect when `radius` is given.
         height : float, optional
             Length of the unique semi-axis (height) of the inflated hemispheroid
             parachute. Default value is the radius of the parachute.
             Units are in meters.
         porosity : float, optional
-            Geometric porosity of the canopy (ratio of open area to total canopy area),
-            in [0, 1]. Affects only the added-mass scaling during descent; it does
-            not change ``cd_s`` (drag). The default, 0.0432, yields an added-mass
-            of 1.0 (“neutral” behavior).
+            Geometric porosity of the canopy (ratio of open area to total
+            canopy area), in [0, 1]. Affects only the added-mass scaling
+            during descent; it does not change `cd_s` (drag). The default
+            value of 0.0432 yields an `added_mass_coefficient` of
+            approximately 1.0.
 
         Returns
         -------
         parachute : Parachute
-            Parachute containing trigger, sampling_rate, lag, cd_s, noise, radius,
-            height, porosity and name. Furthermore, it stores clean_pressure_signal,
-            noise_signal and noisyPressureSignal which are filled in during
-            Flight simulation.
+            Parachute containing trigger, sampling_rate, lag, cd_s, noise,
+            radius, drag_coefficient, height, porosity and name. Furthermore,
+            it stores clean_pressure_signal, noise_signal and
+            noisyPressureSignal which are filled in during Flight simulation.
         """
         parachute = Parachute(
             name,
@@ -1579,6 +1588,7 @@ def add_parachute(
             lag,
             noise,
             radius,
+            drag_coefficient,
             height,
             porosity,
         )

rocketpy/stochastic/stochastic_parachute.py

@@ -31,6 +31,9 @@ class StochasticParachute(StochasticModel):
         List with the name of the parachute object. This cannot be randomized.
     radius : tuple, list, int, float
         Radius of the parachute in meters.
+    drag_coefficient : tuple, list, int, float
+        Drag coefficient of the inflated canopy shape, used only when
+        ``radius`` is not provided.
     height : tuple, list, int, float
         Height of the parachute in meters.
     porosity : tuple, list, int, float
@@ -46,6 +49,7 @@ def __init__(
         lag=None,
         noise=None,
         radius=None,
+        drag_coefficient=None,
         height=None,
         porosity=None,
     ):
@@ -74,6 +78,9 @@ def __init__(
             time-correlation).
         radius : tuple, list, int, float
             Radius of the parachute in meters.
+        drag_coefficient : tuple, list, int, float
+            Drag coefficient of the inflated canopy shape, used only when
+            ``radius`` is not provided.
         height : tuple, list, int, float
             Height of the parachute in meters.
         porosity : tuple, list, int, float
@@ -86,6 +93,7 @@ def __init__(
         self.lag = lag
         self.noise = noise
         self.radius = radius
+        self.drag_coefficient = drag_coefficient
         self.height = height
         self.porosity = porosity
 
@@ -100,6 +108,7 @@ def __init__(
             noise=noise,
             name=None,
             radius=radius,
+            drag_coefficient=drag_coefficient,
             height=height,
             porosity=porosity,
         )

tests/unit/rocket/test_parachute.py

@@ -0,0 +1,111 @@
+"""Unit tests for the Parachute class, focusing on the radius and
+drag_coefficient parameters introduced in PR #889."""
+
+import numpy as np
+import pytest
+
+from rocketpy import Parachute
+
+
+def _make_parachute(**kwargs):
+    defaults = dict(
+        name="test",
+        cd_s=10.0,
+        trigger="apogee",
+        sampling_rate=100,
+    )
+    defaults.update(kwargs)
+    return Parachute(**defaults)
+
+
+class TestParachuteRadiusEstimation:
+    """Tests for auto-computed radius from cd_s and drag_coefficient."""
+
+    def test_radius_auto_computed_from_cd_s_default_drag_coefficient(self):
+        """When radius is not provided the radius is estimated using the
+        default drag_coefficient of 1.4 and the formula R = sqrt(cd_s / (Cd * pi))."""
+        cd_s = 10.0
+        parachute = _make_parachute(cd_s=cd_s)
+        expected_radius = np.sqrt(cd_s / (1.4 * np.pi))
+        assert parachute.radius == pytest.approx(expected_radius, rel=1e-9)
+
+    def test_radius_auto_computed_uses_custom_drag_coefficient(self):
+        """When drag_coefficient is provided and radius is not, the radius
+        must be estimated using the given drag_coefficient."""
+        cd_s = 10.0
+        custom_cd = 0.75
+        parachute = _make_parachute(cd_s=cd_s, drag_coefficient=custom_cd)
+        expected_radius = np.sqrt(cd_s / (custom_cd * np.pi))
+        assert parachute.radius == pytest.approx(expected_radius, rel=1e-9)
+
+    def test_explicit_radius_overrides_estimation(self):
+        """When radius is explicitly provided, it must be used directly and
+        drag_coefficient must be ignored for the radius calculation."""
+        explicit_radius = 2.5
+        parachute = _make_parachute(radius=explicit_radius, drag_coefficient=0.5)
+        assert parachute.radius == explicit_radius
+
+    def test_drag_coefficient_stored_on_instance(self):
+        """drag_coefficient must be stored as an attribute regardless of
+        whether radius is provided or not."""
+        parachute = _make_parachute(drag_coefficient=0.75)
+        assert parachute.drag_coefficient == 0.75
+
+    def test_drag_coefficient_default_is_1_4(self):
+        """Default drag_coefficient must be 1.4 for backward compatibility."""
+        parachute = _make_parachute()
+        assert parachute.drag_coefficient == pytest.approx(1.4)
+
+    def test_drogue_radius_smaller_than_main(self):
+        """A drogue (cd_s=1.0) must have a smaller radius than a main (cd_s=10.0)
+        when using the same drag_coefficient."""
+        main = _make_parachute(cd_s=10.0)
+        drogue = _make_parachute(cd_s=1.0)
+        assert drogue.radius < main.radius
+
+    def test_drogue_radius_approximately_0_48(self):
+        """For cd_s=1.0 and drag_coefficient=1.4, the estimated radius
+        must be approximately 0.48 m (fixes the previous hard-coded 1.5 m)."""
+        drogue = _make_parachute(cd_s=1.0)
+        assert drogue.radius == pytest.approx(0.476, abs=1e-3)
+
+    def test_main_radius_approximately_1_51(self):
+        """For cd_s=10.0 and drag_coefficient=1.4, the estimated radius
+        must be approximately 1.51 m, matching the old hard-coded value."""
+        main = _make_parachute(cd_s=10.0)
+        assert main.radius == pytest.approx(1.508, abs=1e-3)
+
+
+class TestParachuteSerialization:
+    """Tests for to_dict / from_dict round-trip including drag_coefficient."""
+
+    def test_to_dict_includes_drag_coefficient(self):
+        """to_dict must include the drag_coefficient key."""
+        parachute = _make_parachute(drag_coefficient=0.75)
+        data = parachute.to_dict()
+        assert "drag_coefficient" in data
+        assert data["drag_coefficient"] == 0.75
+
+    def test_from_dict_round_trip_preserves_drag_coefficient(self):
+        """A Parachute serialized to dict and restored must have the same
+        drag_coefficient."""
+        original = _make_parachute(cd_s=5.0, drag_coefficient=0.75)
+        data = original.to_dict()
+        restored = Parachute.from_dict(data)
+        assert restored.drag_coefficient == pytest.approx(0.75)
+        assert restored.radius == pytest.approx(original.radius, rel=1e-9)
+
+    def test_from_dict_defaults_drag_coefficient_to_1_4_when_absent(self):
+        """Dicts serialized before drag_coefficient was added (no key) must
+        fall back to 1.4 for backward compatibility."""
+        data = dict(
+            name="legacy",
+            cd_s=10.0,
+            trigger="apogee",
+            sampling_rate=100,
+            lag=0,
+            noise=(0, 0, 0),
+            # no drag_coefficient key — simulates old serialized data
+        )
+        parachute = Parachute.from_dict(data)
+        assert parachute.drag_coefficient == pytest.approx(1.4)