bug: fix b and c geometrical parameters calculation

Author: MateusStano

rocketpy/Flight.py

@@ -1356,16 +1356,15 @@ def uDot(self, t, u, postProcessing=False):
         # Geometry
         # b = -self.rocket.distanceRocketPropellant
         b = (
-            -(self.rocket.motorPosition - self.rocket.centerOfDryMassPosition)
+            -(
+                self.rocket.centerOfPropellantPosition(0)
+                - self.rocket.centerOfDryMassPosition
+            )
             * self.rocket._csys
         )
         # c = -self.rocket.distanceRocketNozzle
         c = (
-            -(
-                self.rocket.motor.nozzlePosition
-                + self.rocket.motorPosition
-                - self.rocket.centerOfDryMassPosition
-            )
+            -(self.rocket.motorPosition - self.rocket.centerOfDryMassPosition)
             * self.rocket._csys
         )
         a = b * Mt / M

rocketpy/Rocket.py

@@ -114,7 +114,7 @@ class Rocket:
         Rocket.motor : Motor
             Rocket's motor. See Motor class for more details.
         Rocket.motorPosition : float
-            Position, in m, of the motor's reference origin relative to the user defined
+            Position, in m, of the motor's nozzle exit area relative to the user defined
             rocket coordinate system. See `Rocket.coordinateSystemOrientation` for more
             information regarding the rocket's coordinate system.
         Rocket.centerOfPropellantPosition : Function