ENH: add structural mass ratio to rocket

Author: Lucas-Prates

rocketpy/motors/motor.py

@@ -510,6 +510,8 @@ def structural_mass_ratio(self):
             Initial structural mass ratio.
         """
         initial_total_mass = self.dry_mass + self.propellant_initial_mass
+        if initial_total_mass == 0:
+            raise ValueError("Motor total mass is zero!")
         return self.dry_mass / initial_total_mass
 
     @funcify_method("Time (s)", "Motor center of mass (m)")
@@ -1517,6 +1519,7 @@ def __init__(self):
         self.nozzle_radius = 0
         self.thrust = Function(0, "Time (s)", "Thrust (N)")
         self.propellant_mass = Function(0, "Time (s)", "Propellant Mass (kg)")
+        self.propellant_initial_mass = 0
         self.total_mass = Function(0, "Time (s)", "Total Mass (kg)")
         self.total_mass_flow_rate = Function(
             0, "Time (s)", "Mass Depletion Rate (kg/s)"

rocketpy/prints/rocket_prints.py

@@ -36,8 +36,7 @@ def inertia_details(self):
         print(f"Rocket Mass: {self.rocket.mass:.3f} kg (without motor)")
         print(f"Rocket Dry Mass: {self.rocket.dry_mass:.3f} kg (with unloaded motor)")
         print(f"Rocket Loaded Mass: {self.rocket.total_mass(0):.3f} kg")
-        mass_ratio = self.rocket.dry_mass / self.rocket.total_mass(0)
-        print(f"Rocket Structural to total mass ratio: {mass_ratio:.3f}")
+        print(f"Rocket Structural Mass Ratio: {self.rocket.structural_mass_ratio:.3f}")
         print(
             f"Rocket Inertia (with unloaded motor) 11: {self.rocket.dry_I_11:.3f} kg*m2"
         )

rocketpy/rocket/rocket.py

@@ -361,6 +361,7 @@ def __init__(  # pylint: disable=too-many-statements
 
         # calculate dynamic inertial quantities
         self.evaluate_dry_mass()
+        self.evaluate_structural_mass_ratio()
         self.evaluate_total_mass()
         self.evaluate_center_of_dry_mass()
         self.evaluate_center_of_mass()
@@ -433,6 +434,26 @@ def evaluate_dry_mass(self):
 
         return self.dry_mass
 
+    def evaluate_structural_mass_ratio(self):
+        """Calculates and returns the rocket's structural mass ratio.
+        It is defined as the ratio between of the dry mass
+        (Motor + Rocket) and the initial total mass
+        (Motor + Propellant + Rocket).
+
+        Returns
+        -------
+        self.structural_mass_ratio: float
+            Initial structural mass ratio dry mass (Rocket + Motor) (kg)
+            divided by total mass (Rocket + Motor + Propellant) (kg).
+        """
+        # Make sure there is a motor associated with the rocket
+
+        self.structural_mass_ratio = self.dry_mass / (
+            self.dry_mass + self.motor.propellant_initial_mass
+        )
+
+        return self.structural_mass_ratio
+
     def evaluate_center_of_mass(self):
         """Evaluates rocket center of mass position relative to user defined
         rocket reference system.
@@ -951,6 +972,7 @@ def add_motor(self, motor, position):  # pylint: disable=too-many-statements
         self.nozzle_position = self.motor.nozzle_position * _ + self.motor_position
         self.total_mass_flow_rate = self.motor.total_mass_flow_rate
         self.evaluate_dry_mass()
+        self.evaluate_structural_mass_ratio()
         self.evaluate_total_mass()
         self.evaluate_center_of_dry_mass()
         self.evaluate_nozzle_to_cdm()