ENH: use mass_flow_rate instead of differentiate

Author: MateusStano

rocketpy/simulation/flight.py

@@ -1645,8 +1645,8 @@ def u_dot_generalized(self, t, u, post_processing=False):
         # Retrieve necessary quantities
         rho = self.env.density.get_value_opt(z)
         total_mass = self.rocket.total_mass.get_value_opt(t)
-        total_mass_dot = self.rocket.total_mass.differentiate(t)
-        total_mass_ddot = self.rocket.total_mass.differentiate(t, order=2)
+        total_mass_dot = self.rocket.motor.total_mass_flow_rate(t)
+        total_mass_ddot = self.rocket.motor.total_mass_flow_rate.differentiate(t)
         ## CM position vector and time derivatives relative to CDM in body frame
         r_CM_z = (
             -1