ENH: implement a linear varying center of mass for Hybrid motors

data/motors/Hypertek_835CC125J-K240.eng

@@ -0,0 +1,27 @@
+K240-835CC125J 54 819 0 0.570 1.859 HyperTek 
+0.131 278.007
+0.396 329.552
+0.66 338.024
+0.925 334.092
+1.191 326.199
+1.456 319.745
+1.721 315.195
+1.985 311.182
+2.25 302.916
+2.516 305.943
+2.781 289.975
+3.046 281.781
+3.31 273.33
+3.575 268.852
+3.841 255.702
+4.106 251.068
+4.371 234.82
+4.635 159.972
+4.9 96.543
+5.166 73.367
+5.431 55.477
+5.696 40.928
+5.96 29.542
+6.225 21.25
+6.491 14.787
+6.756 0

rocketpy/Motor.py

@@ -188,7 +188,7 @@ def __init__(
                 # mass = float(desc[4])
                 # nozzleRadius = diameter/4
                 # throatRadius = diameter/8
-                # grainNumber = grainnumber
+                # grainNumber = grainNumber
                 # grainVolume = height*np.pi*((diameter/2)**2 -(diameter/4)**2)
                 # grainDensity = mass/grainVolume
                 # grainOuterRadius = diameter/2
@@ -355,15 +355,15 @@ def evaluateMassDot(self):
     @abstractmethod
     def evaluateCenterOfMass(self):
         """Calculates and returns the time derivative of motor center of mass.
-        The result is a function of time, object of the Function class, which is stored in self.yCM.
+        The result is a function of time, object of the Function class, which is stored in self.zCM.
 
         Parameters
         ----------
         None
 
         Returns
         -------
-        yCM : Function
+        zCM : Function
             Position of the center of mass as a function
             of time.
         """
@@ -829,7 +829,7 @@ def __init__(
                 # mass = float(desc[4])
                 # nozzleRadius = diameter/4
                 # throatRadius = diameter/8
-                # grainNumber = grainnumber
+                # grainNumber = grainNumber
                 # grainVolume = height*np.pi*((diameter/2)**2 -(diameter/4)**2)
                 # grainDensity = mass/grainVolume
                 # grainOuterRadius = diameter/2
@@ -943,22 +943,22 @@ def evaluateMassDot(self):
 
     def evaluateCenterOfMass(self):
         """Calculates and returns the time derivative of motor center of mass.
-        The result is a function of time, object of the Function class, which is stored in self.yCM.
+        The result is a function of time, object of the Function class, which is stored in self.zCM.
 
         Parameters
         ----------
         None
 
         Returns
         -------
-        yCM : Function
+        zCM : Function
             Position of the center of mass as a function
             of time.
         """
 
-        self.yCM = 0
+        self.zCM = 0
 
-        return self.yCM
+        return self.zCM
 
     def evaluateGeometry(self):
         """Calculates grain inner radius and grain height as a
@@ -1531,7 +1531,7 @@ def __init__(
                 # mass = float(desc[4])
                 # nozzleRadius = diameter/4
                 # throatRadius = diameter/8
-                # grainNumber = grainnumber
+                # grainNumber = grainNumber
                 # grainVolume = height*np.pi*((diameter/2)**2 -(diameter/4)**2)
                 # grainDensity = mass/grainVolume
                 # grainOuterRadius = diameter/2
@@ -1574,8 +1574,8 @@ def __init__(
         self.injectorArea = injectorArea
         # Other quantities that will be computed
         self.massDot = None
-        self.yCM = None
-        self.oxidizerInitialMass = None
+        self.zCM = None
+        self.liquidInitialMass = None
         self.mass = None
         self.grainInnerRadius = None
         self.grainHeight = None
@@ -1659,23 +1659,42 @@ def evaluateMassDot(self):
 
     def evaluateCenterOfMass(self):
         """Calculates and returns the time derivative of motor center of mass.
-        The formulas used are the Bernoulli equation, law of the ideal gases and Boyle's law.
-        The result is a function of time, object of the Function class, which is stored in self.yCM.
+        The final mass of the propellant is assumed to be zero,
+        so a linear extrapolation is used to calculate the position of the center of mass.
+        The result is a function of time, object of the Function class, which is stored in self.zCM.
 
         Parameters
         ----------
         None
 
         Returns
         -------
-        yCM : Function
+        zCM : Function
             Position of the center of mass as a function
             of time.
         """
+        self.solidInitialMass = self.grainInitialMass * self.grainNumber
+        self.liquidInitialMass = self.oxidizerInitialVolume * self.oxidizerDensity
 
-        self.yCM = 0
+        self.solidPropellantInitialCM = 0
 
-        return self.yCM
+        self.liquidPropellantInitialCM = (
+            self.oxidizerInitialVolume / (np.pi * (self.oxidizerTankRadius**2))
+        ) / 2 + self.distanceGrainToTank
+
+        zCM0 = (
+            self.solidInitialMass * self.solidPropellantInitialCM
+            + self.liquidInitialMass * self.liquidPropellantInitialCM
+        ) / (self.solidInitialMass + self.liquidInitialMass)
+
+        self.zCM = Function(
+            [(0, zCM0), (self.burnOutTime, 0)],
+            interpolation="linear",
+            inputs="Time (s)",
+            outputs="Propellant center of mass position (m)",
+        )
+
+        return self.zCM
 
     def evaluateGeometry(self):
         """Calculates grain inner radius and grain height as a
@@ -1971,11 +1990,96 @@ def info(self):
 
         # Show plots
         print("\nPlots")
-        # self.thrust()
-
-        self.yCM
+        self.thrust()
+        self.zCM()
 
         return None
 
     def allInfo(self):
-        pass
+        """Prints out all data and graphs available about the Motor.
+
+        Parameters
+        ----------
+        None
+
+        Return
+        ------
+        None
+        """
+        # Print nozzle details
+        print("Nozzle Details")
+        print("Nozzle Radius: " + str(self.nozzleRadius) + " m")
+        print("Nozzle Throat Radius: " + str(self.throatRadius) + " m")
+        print(
+            "Distance Nozzle - Motor reference point: "
+            + str(self.distanceNozzleMotorReference)
+            + " m"
+        )
+
+        # Print grain details
+        print("\nGrain Details")
+        print("Number of Grains: " + str(self.grainNumber))
+        print("Grain Spacing: " + str(self.grainSeparation) + " m")
+        print("Grain Density: " + str(self.grainDensity) + " kg/m3")
+        print("Grain Outer Radius: " + str(self.grainOuterRadius) + " m")
+        print("Grain Inner Radius: " + str(self.grainInitialInnerRadius) + " m")
+        print("Grain Height: " + str(self.grainInitialHeight) + " m")
+        print("Grain Volume: " + "{:.3f}".format(self.grainInitialVolume) + " m3")
+        print("Grain Mass: " + "{:.3f}".format(self.grainInitialMass) + " kg")
+
+        # Print oxidizer details
+        print("\nOxidizer Details")
+        print("Oxidizer Tank Radius: " + str(self.oxidizerTankRadius) + " m")
+        print("Oxidizer Tank Height: " + str(self.oxidizerTankHeight) + " m")
+        print(
+            "Oxidizer Initial Pressure: " + str(self.oxidizerInitialPressure) + " atm"
+        )
+        print("Oxidizer Initial Mass: " + str(self.liquidInitialMass) + " kg")
+        print("Oxidizer Density: " + str(self.oxidizerDensity) + " kg/m3")
+        print("Oxidizer Molar Mass: " + str(self.oxidizerMolarMass) + " g/mol")
+        print(
+            "Oxidizer Initial Volume: "
+            + "{:.3f}".format(self.oxidizerInitialVolume)
+            + " m3"
+        )
+
+        # Print motor details
+        print("\nMotor Details")
+        print("Total Burning Time: " + str(self.burnOutTime) + " s")
+        print(
+            "Total Propellant Mass: "
+            + "{:.3f}".format(self.propellantInitialMass)
+            + " kg"
+        )
+        print(
+            "Propellant Exhaust Velocity: "
+            + "{:.3f}".format(self.exhaustVelocity)
+            + " m/s"
+        )
+        print("Average Thrust: " + "{:.3f}".format(self.averageThrust) + " N")
+        print(
+            "Maximum Thrust: "
+            + str(self.maxThrust)
+            + " N at "
+            + str(self.maxThrustTime)
+            + " s after ignition."
+        )
+        print("Total Impulse: " + "{:.3f}".format(self.totalImpulse) + " Ns")
+
+        # Show plots
+        print("\nPlots")
+        self.thrust()
+        self.mass()
+        self.massDot()
+        self.zCM()
+        self.grainInnerRadius()
+        self.grainHeight()
+        self.burnRate()
+        self.burnArea()
+        self.Kn()
+        self.inertiaI()
+        self.inertiaIDot()
+        self.inertiaZ()
+        self.inertiaZDot()
+
+        return None

rocketpy/Rocket.py

@@ -177,7 +177,7 @@ def __init__(
         self.inertiaZ = inertiaZ
 
         self.centerOfMass = (
-            (self.distanceRocketMotorReference - self.motor.yCM)
+            (self.distanceRocketMotorReference - self.motor.zCM)
             * motor.mass
             / (mass + motor.mass)
         )

tests/test_hybridmotor.py

@@ -0,0 +1,30 @@
+from unittest.mock import patch
+import os
+
+import numpy as np
+import pytest
+
+from rocketpy import HybridMotor
+
+
+def test_Initial_Center_Of_Mass_Position_correct():
+    example_motor = HybridMotor(
+        thrustSource="data/motors/Hypertek_835CC125J-K240.eng",
+        burnOut=5.4,
+        distanceNozzleMotorReference=1,
+        grainNumber=6,
+        grainDensity=1707,
+        grainOuterRadius=21.40 / 1000,
+        grainInitialInnerRadius=9.65 / 1000,
+        grainInitialHeight=120 / 1000,
+        oxidizerTankRadius=62.5 / 1000,
+        oxidizerTankHeight=600 / 1000,
+        oxidizerInitialPressure=51.03,
+        oxidizerDensity=1.98,
+        oxidizerMolarMass=44.01,
+        oxidizerInitialVolume=62.5 / 1000 * 62.5 / 1000 * np.pi * 600 / 1000,
+        distanceGrainToTank=200 / 1000,
+        injectorArea=3e-05,
+    )
+
+    assert abs(example_motor.zCM(0)) - abs(0.005121644685784456) < 1e-6