ENH: add structural to total mass ratio for motor and rocket (#713)

* ENH: add structural to total mass ratio for motor and rocket

* ENH: adding structural mass ratio as an attribute of Motor

* ENH: add structural mass ratio to rocket

* ENH: capitalize words on prints

* DOC: adding structural mass ratio attribute to docstrings

* DOC: fix incorrect documentation names of attributes

* Remove erroneous comment

Co-authored-by: Gui-FernandesBR <[email protected]>

* DOC: modify changelog

* ENH: properly testing division by zero when computing the structural mass ratio

* MNT: make black

---------

Co-authored-by: Gui-FernandesBR <[email protected]>

CHANGELOG.md

@@ -32,7 +32,7 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
-
+- ENH: add structural to total mass ratio for motor and rocket [#713](https://github.com/RocketPy-Team/RocketPy/pull/713)
 
 ### Changed
 

rocketpy/motors/hybrid_motor.py

@@ -72,6 +72,8 @@ class HybridMotor(Motor):
     HybridMotor.propellant_mass : Function
         Total propellant mass in kg as a function of time, this includes the
         mass of fluids in each tank and the mass of the solid grains.
+    HybridMotor.structural_mass_ratio: float
+        Initial ratio between the dry mass and the total mass.
     HybridMotor.total_mass_flow_rate : Function
         Time derivative of propellant total mass in kg/s as a function
         of time as obtained by the thrust source.

rocketpy/motors/liquid_motor.py

@@ -47,6 +47,8 @@ class LiquidMotor(Motor):
     LiquidMotor.propellant_mass : Function
         Total propellant mass in kg as a function of time, includes fuel
         and oxidizer.
+    LiquidMotor.structural_mass_ratio: float
+        Initial ratio between the dry mass and the total mass.
     LiquidMotor.total_mass_flow_rate : Function
         Time derivative of propellant total mass in kg/s as a function
         of time as obtained by the tanks mass flow.

rocketpy/motors/motor.py

@@ -49,6 +49,8 @@ class Motor(ABC):
     Motor.propellant_mass : Function
         Total propellant mass in kg as a function of time, including solid,
         liquid and gas phases.
+    Motor.structural_mass_ratio: float
+        Initial ratio between the dry mass and the total mass.
     Motor.total_mass_flow_rate : Function
         Time derivative of propellant total mass in kg/s as a function
         of time as obtained by the thrust source.
@@ -497,6 +499,24 @@ def propellant_initial_mass(self):
             Propellant initial mass in kg.
         """
 
+    @property
+    def structural_mass_ratio(self):
+        """Calculates the structural mass ratio. The ratio is defined as
+        the dry mass divided by the initial total mass.
+
+        Returns
+        -------
+        float
+            Initial structural mass ratio.
+        """
+        initial_total_mass = self.dry_mass + self.propellant_initial_mass
+        try:
+            return self.dry_mass / initial_total_mass
+        except ZeroDivisionError as e:
+            raise ValueError(
+                "Total motor mass (dry + propellant) cannot be zero"
+            ) from e
+
     @funcify_method("Time (s)", "Motor center of mass (m)")
     def center_of_mass(self):
         """Position of the center of mass as a function of time. The position
@@ -1502,6 +1522,7 @@ 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/motors/solid_motor.py

@@ -70,6 +70,8 @@ class SolidMotor(Motor):
         of propellant and dry mass.
     SolidMotor.propellant_mass : Function
         Total propellant mass in kg as a function of time.
+    SolidMotor.structural_mass_ratio: float
+        Initial ratio between the dry mass and the total mass.
     SolidMotor.total_mass_flow_rate : Function
         Time derivative of propellant total mass in kg/s as a function
         of time as obtained by the thrust source.

rocketpy/prints/hybrid_motor_prints.py

@@ -77,6 +77,7 @@ def motor_details(self):
         print(
             f"Total Propellant Mass: {self.hybrid_motor.propellant_initial_mass:.3f} kg"
         )
+        print(f"Structural Mass Ratio: {self.hybrid_motor.structural_mass_ratio:.3f}")
         avg = self.hybrid_motor.exhaust_velocity.average(*self.hybrid_motor.burn_time)
         print(f"Average Propellant Exhaust Velocity: {avg:.3f} m/s")
         print(f"Average Thrust: {self.hybrid_motor.average_thrust:.3f} N")

rocketpy/prints/liquid_motor_prints.py

@@ -47,6 +47,7 @@ def motor_details(self):
         print(
             f"Total Propellant Mass: {self.liquid_motor.propellant_initial_mass:.3f} kg"
         )
+        print(f"Structural Mass Ratio: {self.liquid_motor.structural_mass_ratio:.3f}")
         avg = self.liquid_motor.exhaust_velocity.average(*self.liquid_motor.burn_time)
         print(f"Average Propellant Exhaust Velocity: {avg:.3f} m/s")
         print(f"Average Thrust: {self.liquid_motor.average_thrust:.3f} N")

rocketpy/prints/motor_prints.py

@@ -35,6 +35,7 @@ def motor_details(self):
         print("Motor Details")
         print("Total Burning Time: " + str(self.motor.burn_out_time) + " s")
         print(f"Total Propellant Mass: {self.motor.propellant_initial_mass:.3f} kg")
+        print(f"Structural Mass Ratio: {self.motor.structural_mass_ratio:.3f}")
         print(
             "Average Propellant Exhaust Velocity: "
             f"{self.motor.exhaust_velocity.average(*self.motor.burn_time):.3f} m/s"

rocketpy/prints/rocket_prints.py

@@ -36,6 +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")
+        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/prints/solid_motor_prints.py

@@ -65,6 +65,7 @@ def motor_details(self):
         print(
             f"Total Propellant Mass: {self.solid_motor.propellant_initial_mass:.3f} kg"
         )
+        print(f"Structural Mass Ratio: {self.solid_motor.structural_mass_ratio:.3f}")
         average = self.solid_motor.exhaust_velocity.average(*self.solid_motor.burn_time)
         print(f"Average Propellant Exhaust Velocity: {average:.3f} m/s")
         print(f"Average Thrust: {self.solid_motor.average_thrust:.3f} N")

rocketpy/rocket/rocket.py

@@ -89,6 +89,8 @@ class Rocket:
         Function of time expressing the total mass of the rocket,
         defined as the sum of the propellant mass and the rocket
         mass without propellant.
+    Rocket.structural_mass_ratio: float
+        Initial ratio between the dry mass and the total mass.
     Rocket.total_mass_flow_rate : Function
         Time derivative of rocket's total mass in kg/s as a function
         of time as obtained by the thrust source of the added motor.
@@ -361,6 +363,7 @@ 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 +436,28 @@ 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).
+        """
+        try:
+            self.structural_mass_ratio = self.dry_mass / (
+                self.dry_mass + self.motor.propellant_initial_mass
+            )
+        except ZeroDivisionError as e:
+            raise ValueError(
+                "Total rocket mass (dry + propellant) cannot be zero"
+            ) from e
+        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 +976,7 @@ 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()