ENH: changing ma calculation and porosity documentation from previous comments

Author: ArthurJWH

rocketpy/rocket/parachute.py

@@ -99,8 +99,10 @@ class Parachute:
         Length of the unique semi-axis (height) of the inflated hemispheroid
         parachute in meters.
     Parachute.porosity : float
-        Porosity of the parachute is the ratio of open space in the canopy to total
-        canopy area.
+        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).
     """
 
     def __init__(
@@ -175,8 +177,10 @@ def __init__(
             parachute. Default value is the radius of the parachute.
             Units are in meters.
         porosity : float, optional
-            Porosity of the parachute is the ratio of open space in the canopy to total
-            canopy area. Default value is 0.0432 (for consistency with previous versions).
+            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).
         """
         self.name = name
         self.cd_s = cd_s

rocketpy/rocket/rocket.py

@@ -1510,8 +1510,10 @@ def add_parachute(
             parachute. Default value is the radius of the parachute.
             Units are in meters.
         porosity : float, optional
-            Porosity of the parachute is the ratio of open space in the canopy to total
-            canopy area. Default value is 0.0432 (for consistency with previous versions).
+            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).
 
         Returns
         -------

rocketpy/simulation/flight.py

@@ -2014,7 +2014,7 @@ def u_dot_parachute(self, t, u, post_processing=False):
         ma = (
             self.ka
             * rho
-            * (4 / 3)
+            * (2 / 3)
             * np.pi
             * self.parachute_radius**2
             * self.parachute_height