From 318793ca2e8e6eeb740ff61bdc35a7c6e8fc1eff Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 28 Nov 2025 00:43:12 +0000
Subject: [PATCH 1/4] Fix hard-coded radius value for parachute added mass
 calculation
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Calculate radius from cd_s using a typical hemispherical parachute drag
coefficient (1.4) when radius is not explicitly provided. This fixes
drift distance calculations for smaller parachutes like drogues.

Formula: R = sqrt(cd_s / (Cd * π))

Closes #860

Co-authored-by: Gui-FernandesBR <63590233+Gui-FernandesBR@users.noreply.github.com>
---
 rocketpy/rocket/parachute.py | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/rocketpy/rocket/parachute.py b/rocketpy/rocket/parachute.py
index f0bf86f67..574890cee 100644
--- a/rocketpy/rocket/parachute.py
+++ b/rocketpy/rocket/parachute.py
@@ -94,8 +94,8 @@ class Parachute:
         Function of clean_pressure_signal.
     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. Estimated from ``cd_s`` if not explicitly provided.
+    Parachute.height : float
         Length of the unique semi-axis (height) of the inflated hemispheroid
         parachute in meters.
     Parachute.porosity : float
@@ -108,6 +108,10 @@ class Parachute:
         calculated from the porosity of the parachute.
     """
 
+    # Typical drag coefficient for hemispherical parachute
+    # Used to estimate radius from cd_s when radius is not provided
+    HEMISPHERICAL_CD = 1.4
+
     def __init__(
         self,
         name,
@@ -116,7 +120,7 @@ def __init__(
         sampling_rate,
         lag=0,
         noise=(0, 0, 0),
-        radius=1.5,
+        radius=None,
         height=None,
         porosity=0.0432,
     ):
@@ -173,7 +177,9 @@ def __init__(
             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.
+            parachute. If not provided, it is estimated from ``cd_s`` assuming a
+            typical hemispherical parachute drag coefficient of 1.4, using the
+            formula: ``radius = sqrt(cd_s / (HEMISPHERICAL_CD * pi))``.
             Units are in meters.
         height : float, optional
             Length of the unique semi-axis (height) of the inflated hemispheroid
@@ -200,8 +206,13 @@ 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
+        # Estimate radius from cd_s if not provided
+        if radius is None:
+            # cd_s = Cd * S = Cd * π * R² => R = sqrt(cd_s / (Cd * π))
+            self.radius = np.sqrt(cd_s / (self.HEMISPHERICAL_CD * np.pi))
+        else:
+            self.radius = radius
+        self.height = height or self.radius
         self.porosity = porosity
         self.added_mass_coefficient = 1.068 * (
             1
@@ -340,8 +351,8 @@ def from_dict(cls, data):
             sampling_rate=data["sampling_rate"],
             lag=data["lag"],
             noise=data["noise"],
-            radius=data.get("radius", 1.5),
-            height=data.get("height", None),
+            radius=data.get("radius"),
+            height=data.get("height"),
             porosity=data.get("porosity", 0.0432),
         )
 

From 4a13b2b587da80bdb0a4d3ef79039569b7d5f4f4 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 28 Nov 2025 00:46:08 +0000
Subject: [PATCH 2/4] Address code review: improve docstrings and add explicit
 None defaults

Co-authored-by: Gui-FernandesBR <63590233+Gui-FernandesBR@users.noreply.github.com>
---
 rocketpy/rocket/parachute.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/rocketpy/rocket/parachute.py b/rocketpy/rocket/parachute.py
index 574890cee..90645c4fd 100644
--- a/rocketpy/rocket/parachute.py
+++ b/rocketpy/rocket/parachute.py
@@ -94,7 +94,8 @@ class Parachute:
         Function of clean_pressure_signal.
     Parachute.radius : float
         Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-        parachute in meters. Estimated from ``cd_s`` if not explicitly provided.
+        parachute in meters. Estimated from ``cd_s`` using the formula
+        ``R = sqrt(cd_s / (1.4 * π))`` if not explicitly provided.
     Parachute.height : float
         Length of the unique semi-axis (height) of the inflated hemispheroid
         parachute in meters.
@@ -351,8 +352,8 @@ def from_dict(cls, data):
             sampling_rate=data["sampling_rate"],
             lag=data["lag"],
             noise=data["noise"],
-            radius=data.get("radius"),
-            height=data.get("height"),
+            radius=data.get("radius", None),
+            height=data.get("height", None),
             porosity=data.get("porosity", 0.0432),
         )
 

From 95b51310f70a003cf8b7bbd450f11af8fd61f879 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 28 Nov 2025 01:01:40 +0000
Subject: [PATCH 3/4] Add CHANGELOG entry for PR #889

Co-authored-by: Gui-FernandesBR <63590233+Gui-FernandesBR@users.noreply.github.com>
---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a1e06c193..6b35e3d7d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,6 +44,7 @@ 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)
 - 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)
 

From 77903d8c7683a2388a20dbb21743c65acdb77b56 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 2 Dec 2025 01:58:37 +0000
Subject: [PATCH 4/4] Update rocket.add_parachute to use radius=None for
 consistency

Changed the default radius from 1.5 to None in the add_parachute method
to match the Parachute class behavior. This ensures consistent automatic
radius calculation from cd_s across both APIs.

Co-authored-by: Gui-FernandesBR <63590233+Gui-FernandesBR@users.noreply.github.com>
---
 rocketpy/rocket/rocket.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/rocketpy/rocket/rocket.py b/rocketpy/rocket/rocket.py
index c3b1f364d..8faadc8ac 100644
--- a/rocketpy/rocket/rocket.py
+++ b/rocketpy/rocket/rocket.py
@@ -1475,7 +1475,7 @@ def add_parachute(
         sampling_rate=100,
         lag=0,
         noise=(0, 0, 0),
-        radius=1.5,
+        radius=None,
         height=None,
         porosity=0.0432,
     ):
@@ -1538,7 +1538,9 @@ def add_parachute(
             are in pascal.
         radius : float, optional
             Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-            parachute. Default value is 1.5.
+            parachute. If not provided, it is estimated from ``cd_s`` assuming a
+            typical hemispherical parachute drag coefficient of 1.4, using the
+            formula: ``radius = sqrt(cd_s / (1.4 * pi))``.
             Units are in meters.
         height : float, optional
             Length of the unique semi-axis (height) of the inflated hemispheroid