from functools import cached_property
import numpy as np
from rocketpy.mathutils.function import funcify_method, reset_funcified_methods
from rocketpy.tools import parallel_axis_theorem_from_com
from ..plots.liquid_motor_plots import _LiquidMotorPlots
from ..prints.liquid_motor_prints import _LiquidMotorPrints
from .motor import Motor
[docs]
class LiquidMotor(Motor):
"""Class to specify characteristics and useful operations for Liquid
motors. This class inherits from the Motor class.
See Also
--------
Motor
Attributes
----------
LiquidMotor.coordinate_system_orientation : str
Orientation of the motor's coordinate system. The coordinate system
is defined by the motor's axis of symmetry. The origin of the
coordinate system may be placed anywhere along such axis, such as
at the nozzle area, and must be kept the same for all other
positions specified. Options are "nozzle_to_combustion_chamber" and
"combustion_chamber_to_nozzle".
LiquidMotor.nozzle_radius : float
Radius of motor nozzle outlet in meters.
LiquidMotor.nozzle_area : float
Area of motor nozzle outlet in square meters.
LiquidMotor.nozzle_position : float
Motor's nozzle outlet position in meters, specified in the motor's
coordinate system. See
:doc:`Positions and Coordinate Systems </user/positions>` for more
information.
LiquidMotor.positioned_tanks : list
List containing the motor's added tanks and their respective
positions.
LiquidMotor.dry_mass : float
Same as in Motor class. See the :class:`Motor <rocketpy.Motor>` docs.
LiquidMotor.propellant_initial_mass : float
Total propellant initial mass in kg, includes fuel and oxidizer.
LiquidMotor.total_mass : Function
Total motor mass in kg as a function of time, defined as the sum
of propellant mass and the motor's dry mass (i.e. structure mass).
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.
LiquidMotor.center_of_mass : Function
Position of the motor center of mass in
meters as a function of time.
See :doc:`Positions and Coordinate Systems </user/positions>`
for more information regarding the motor's coordinate system.
LiquidMotor.center_of_propellant_mass : Function
Position of the motor propellant center of mass in meters as a
function of time.
See :doc:`Positions and Coordinate Systems </user/positions>`
for more information regarding the motor's coordinate system.
LiquidMotor.I_11 : Function
Component of the motor's inertia tensor relative to the e_1 axis
in kg*m^2, as a function of time. The e_1 axis is the direction
perpendicular to the motor body axis of symmetry, centered at
the instantaneous motor center of mass.
LiquidMotor.I_22 : Function
Component of the motor's inertia tensor relative to the e_2 axis
in kg*m^2, as a function of time. The e_2 axis is the direction
perpendicular to the motor body axis of symmetry, centered at
the instantaneous motor center of mass.
Numerically equivalent to I_11 due to symmetry.
LiquidMotor.I_33 : Function
Component of the motor's inertia tensor relative to the e_3 axis
in kg*m^2, as a function of time. The e_3 axis is the direction of
the motor body axis of symmetry, centered at the instantaneous
motor center of mass.
LiquidMotor.I_12 : Function
Component of the motor's inertia tensor relative to the e_1 and
e_2 axes in kg*m^2, as a function of time. See LiquidMotor.I_11 and
LiquidMotor.I_22 for more information.
LiquidMotor.I_13 : Function
Component of the motor's inertia tensor relative to the e_1 and
e_3 axes in kg*m^2, as a function of time. See LiquidMotor.I_11 and
LiquidMotor.I_33 for more information.
LiquidMotor.I_23 : Function
Component of the motor's inertia tensor relative to the e_2 and
e_3 axes in kg*m^2, as a function of time. See LiquidMotor.I_22 and
LiquidMotor.I_33 for more information.
LiquidMotor.propellant_I_11 : Function
Component of the propellant inertia tensor relative to the e_1
axis in kg*m^2, as a function of time. The e_1 axis is the
direction perpendicular to the motor body axis of symmetry,
centered at the instantaneous propellant center of mass.
LiquidMotor.propellant_I_22 : Function
Component of the propellant inertia tensor relative to the e_2
axis in kg*m^2, as a function of time. The e_2 axis is the
direction perpendicular to the motor body axis of symmetry,
centered at the instantaneous propellant center of mass.
Numerically equivalent to propellant_I_11 due to symmetry.
LiquidMotor.propellant_I_33 : Function
Component of the propellant inertia tensor relative to the e_3
axis in kg*m^2, as a function of time. The e_3 axis is the
direction of the motor body axis of symmetry, centered at the
instantaneous propellant center of mass.
LiquidMotor.propellant_I_12 : Function
Component of the propellant inertia tensor relative to the e_1 and
e_2 axes in kg*m^2, as a function of time. See
LiquidMotor.propellant_I_11 and LiquidMotor.propellant_I_22 for
more information.
LiquidMotor.propellant_I_13 : Function
Component of the propellant inertia tensor relative to the e_1 and
e_3 axes in kg*m^2, as a function of time. See
LiquidMotor.propellant_I_11 and LiquidMotor.propellant_I_33 for
more information.
LiquidMotor.propellant_I_23 : Function
Component of the propellant inertia tensor relative to the e_2 and
e_3 axes in kg*m^2, as a function of time. See
LiquidMotor.propellant_I_22 and LiquidMotor.propellant_I_33 for
more information.
LiquidMotor.thrust : Function
Motor thrust force obtained from thrust source, in Newtons, as a
function of time.
LiquidMotor.vacuum_thrust : Function
Motor thrust force when the rocket is in a vacuum. In Newtons, as a
function of time.
LiquidMotor.total_impulse : float
Total impulse of the thrust curve in N*s.
LiquidMotor.max_thrust : float
Maximum thrust value of the given thrust curve, in N.
LiquidMotor.max_thrust_time : float
Time, in seconds, in which the maximum thrust value is achieved.
LiquidMotor.average_thrust : float
Average thrust of the motor, given in N.
LiquidMotor.burn_time : tuple of float
Tuple containing the initial and final time of the motor's burn time
in seconds.
LiquidMotor.burn_start_time : float
Motor burn start time, in seconds.
LiquidMotor.burn_out_time : float
Motor burn out time, in seconds.
LiquidMotor.burn_duration : float
Total motor burn duration, in seconds. It is the difference between the
burn_out_time and the burn_start_time.
LiquidMotor.exhaust_velocity : Function
Effective exhaust velocity of the propulsion gases in m/s. Computed
as the thrust divided by the mass flow rate. This corresponds to the
actual exhaust velocity only when the nozzle exit pressure equals the
atmospheric pressure.
LiquidMotor.reference_pressure : int, float
Atmospheric pressure in Pa at which the thrust data was recorded.
It will allow to obtain the net thrust in the Flight class.
"""
[docs]
def __init__(
self,
thrust_source,
dry_mass,
dry_inertia,
nozzle_radius,
center_of_dry_mass_position,
nozzle_position=0,
burn_time=None,
reshape_thrust_curve=False,
interpolation_method="linear",
coordinate_system_orientation="nozzle_to_combustion_chamber",
reference_pressure=None,
):
"""Initialize LiquidMotor class, process thrust curve and geometrical
parameters and store results.
Parameters
----------
thrust_source : int, float, callable, string, array, Function
Motor's thrust curve. Can be given as an int or float, in which
case the thrust will be considered constant in time. It can
also be given as a callable function, whose argument is time in
seconds and returns the thrust supplied by the motor in the
instant. If a string is given, it must point to a .csv or .eng file.
The .csv file can contain a single line header and the first column
must specify time in seconds, while the second column specifies
thrust. Arrays may also be specified, following rules set by the
class Function. Thrust units are Newtons.
.. seealso:: :doc:`Thrust Source Details </user/motors/thrust>`
dry_mass : int, float
Same as in Motor class. See the :class:`Motor <rocketpy.Motor>` docs.
dry_inertia : tuple, list
Tuple or list containing the motor's dry mass inertia tensor
components, in kg*m^2. This inertia is defined with respect to the
the ``center_of_dry_mass_position`` position.
Assuming e_3 is the rocket's axis of symmetry, e_1 and e_2 are
orthogonal and form a plane perpendicular to e_3, the dry mass
inertia tensor components must be given in the following order:
(I_11, I_22, I_33, I_12, I_13, I_23), where I_ij is the
component of the inertia tensor in the direction of e_i x e_j.
Alternatively, the inertia tensor can be given as
(I_11, I_22, I_33), where I_12 = I_13 = I_23 = 0.
nozzle_radius : int, float
Motor's nozzle outlet radius in meters.
center_of_dry_mass_position : int, float
The position, in meters, of the motor's center of mass with respect
to the motor's coordinate system when it is devoid of propellant.
See :doc:`Positions and Coordinate Systems </user/positions>`
nozzle_position : float
Motor's nozzle outlet position in meters, specified in the motor's
coordinate system. See
:doc:`Positions and Coordinate Systems </user/positions>` for
more information.
burn_time: float, tuple of float, optional
Motor's burn time.
If a float is given, the burn time is assumed to be between 0 and
the given float, in seconds.
If a tuple of float is given, the burn time is assumed to be between
the first and second elements of the tuple, in seconds.
If not specified, automatically sourced as the range between the
first and last-time step of the motor's thrust curve. This can only
be used if the motor's thrust is defined by a list of points, such
as a .csv file, a .eng file or a Function instance whose source is
a list.
reshape_thrust_curve : boolean, tuple, optional
If False, the original thrust curve supplied is not altered. If a
tuple is given, whose first parameter is a new burn out time and
whose second parameter is a new total impulse in Ns, the thrust
curve is reshaped to match the new specifications. May be useful
for motors whose thrust curve shape is expected to remain similar
in case the impulse and burn time varies slightly. Default is
False.
interpolation_method : string, optional
Method of interpolation to be used in case thrust curve is given
by data set in .csv or .eng, or as an array. Options are 'spline'
'akima' and 'linear'. Default is "linear".
coordinate_system_orientation : string, optional
Orientation of the motor's coordinate system. The coordinate system
is defined by the motor's axis of symmetry. The origin of the
coordinate system may be placed anywhere along such axis, such as
at the nozzle area, and must be kept the same for all other
positions specified. Options are "nozzle_to_combustion_chamber"
and "combustion_chamber_to_nozzle". Default is
"nozzle_to_combustion_chamber".
reference_pressure : int, float, optional
Atmospheric pressure in Pa at which the thrust data was recorded.
"""
super().__init__(
thrust_source=thrust_source,
dry_inertia=dry_inertia,
nozzle_radius=nozzle_radius,
center_of_dry_mass_position=center_of_dry_mass_position,
dry_mass=dry_mass,
nozzle_position=nozzle_position,
burn_time=burn_time,
reshape_thrust_curve=reshape_thrust_curve,
interpolation_method=interpolation_method,
coordinate_system_orientation=coordinate_system_orientation,
reference_pressure=reference_pressure,
)
self.positioned_tanks = []
# Initialize plots and prints object
self.prints = _LiquidMotorPrints(self)
self.plots = _LiquidMotorPlots(self)
@funcify_method("Time (s)", "Exhaust Velocity (m/s)")
def exhaust_velocity(self):
"""Computes the exhaust velocity of the motor from its mass flow
rate and thrust.
Returns
-------
self.exhaust_velocity : Function
Gas exhaust velocity of the motor.
Notes
-----
The exhaust velocity is computed as the ratio of the thrust and the
mass flow rate. Therefore, this will vary with time if the mass flow
rate varies with time. This corresponds to the actual exhaust velocity
only when the nozzle exit pressure equals the atmospheric pressure.
"""
times, thrusts = self.thrust.source[:, 0], self.thrust.source[:, 1]
mass_flow_rates = self.mass_flow_rate(times)
exhaust_velocity = np.zeros_like(mass_flow_rates)
# Compute exhaust velocity only for non-zero mass flow rates
valid_indices = mass_flow_rates != 0
exhaust_velocity[valid_indices] = (
-thrusts[valid_indices] / mass_flow_rates[valid_indices]
)
return np.column_stack([times, exhaust_velocity])
@funcify_method("Time (s)", "Propellant Mass (kg)")
def propellant_mass(self):
"""Evaluates the total propellant mass of the motor as the sum of fluids
mass in each tank, which may include fuel and oxidizer and usually vary
with time.
Returns
-------
Function
Mass of the motor, in kg.
"""
propellant_mass = 0
for positioned_tank in self.positioned_tanks:
propellant_mass += positioned_tank.get("tank").fluid_mass
return propellant_mass
@cached_property
def propellant_initial_mass(self):
"""Property to store the initial mass of the propellant, this includes
fuel and oxidizer.
Returns
-------
float
Initial mass of the propellant, in kg.
"""
return self.propellant_mass(self.burn_start_time)
@funcify_method("Time (s)", "Mass flow rate (kg/s)", extrapolation="zero")
def mass_flow_rate(self):
"""Evaluates the mass flow rate of the motor as the sum of mass flow
rate from each tank, which may include fuel and oxidizer and usually
vary with time.
Returns
-------
Function
Mass flow rate of the motor, in kg/s.
See Also
--------
Motor.total_mass_flow_rate :
Calculates the total mass flow rate of the motor assuming
constant exhaust velocity.
"""
mass_flow_rate = 0
for positioned_tank in self.positioned_tanks:
mass_flow_rate += positioned_tank.get("tank").net_mass_flow_rate
return mass_flow_rate
@funcify_method("Time (s)", "Center of mass (m)")
def center_of_propellant_mass(self):
"""Evaluates the center of mass of the motor from each tank center of
mass and positioning. The center of mass height is measured relative to
the origin of the motor's coordinate system.
Returns
-------
Function
Position of the propellant center of mass, in meters.
"""
total_mass = 0
mass_balance = 0
for positioned_tank in self.positioned_tanks:
tank = positioned_tank.get("tank")
tank_position = positioned_tank.get("position")
total_mass += tank.fluid_mass
mass_balance += tank.fluid_mass * (tank_position + tank.center_of_mass)
return mass_balance / total_mass
@funcify_method("Time (s)", "Inertia I_11 (kg m²)")
def propellant_I_11(self):
"""Inertia tensor 11 component of the propellant, the inertia is
relative to the e_1 axis, centered at the instantaneous propellant
center of mass.
Returns
-------
Function
Propellant inertia tensor 11 component at time t.
Notes
-----
The e_1 direction is assumed to be the direction perpendicular to the
motor body axis.
References
----------
https://en.wikipedia.org/wiki/Moment_of_inertia#Inertia_tensor
"""
I_11 = 0
center_of_mass = self.center_of_propellant_mass
for positioned_tank in self.positioned_tanks:
tank = positioned_tank.get("tank")
tank_position = positioned_tank.get("position")
distance = tank_position + tank.center_of_mass - center_of_mass
I_11 += parallel_axis_theorem_from_com(
tank.inertia, tank.fluid_mass, distance
)
return I_11
@funcify_method("Time (s)", "Inertia I_22 (kg m²)")
def propellant_I_22(self):
"""Inertia tensor 22 component of the propellant, the inertia is
relative to the e_2 axis, centered at the instantaneous propellant
center of mass.
Returns
-------
Function
Propellant inertia tensor 22 component at time t.
Notes
-----
The e_2 direction is assumed to be the direction perpendicular to the
motor body axis, and perpendicular to e_1.
References
----------
https://en.wikipedia.org/wiki/Moment_of_inertia#Inertia_tensor
"""
return self.propellant_I_11
@funcify_method("Time (s)", "Inertia I_33 (kg m²)")
def propellant_I_33(self):
"""Inertia tensor 33 component of the propellant, the inertia is
relative to the e_3 axis, centered at the instantaneous propellant
center of mass.
Returns
-------
Function
Propellant inertia tensor 33 component at time t.
Notes
-----
The e_3 direction is assumed to be the axial direction of the rocket
motor.
References
----------
https://en.wikipedia.org/wiki/Moment_of_inertia#Inertia_tensor
"""
return 0
@funcify_method("Time (s)", "Inertia I_12 (kg m²)")
def propellant_I_12(self):
return 0
@funcify_method("Time (s)", "Inertia I_13 (kg m²)")
def propellant_I_13(self):
return 0
@funcify_method("Time (s)", "Inertia I_23 (kg m²)")
def propellant_I_23(self):
return 0
[docs]
def add_tank(self, tank, position):
"""Adds a tank to the rocket motor.
Parameters
----------
tank : Tank
Tank object to be added to the rocket motor.
position : float
Position of the tank relative to the origin of the motor
coordinate system. The tank reference point is its
geometry zero reference point.
See Also
--------
:ref:`Adding Tanks`
"""
self.positioned_tanks.append({"tank": tank, "position": position})
reset_funcified_methods(self)
[docs]
def draw(self, *, filename=None):
"""Draw a representation of the LiquidMotor.
Parameters
----------
filename : str | None, optional
The path the plot should be saved to. By default None, in which case
the plot will be shown instead of saved. Supported file endings are:
eps, jpg, jpeg, pdf, pgf, png, ps, raw, rgba, svg, svgz, tif, tiff
and webp (these are the formats supported by matplotlib).
Returns
-------
None
"""
self.plots.draw(filename=filename)
def to_dict(self, **kwargs):
data = super().to_dict(**kwargs)
data.update(
{
"positioned_tanks": [
{"tank": tank["tank"], "position": tank["position"]}
for tank in self.positioned_tanks
],
}
)
return data
@classmethod
def from_dict(cls, data):
motor = cls(
thrust_source=data["thrust_source"],
burn_time=data["burn_time"],
nozzle_radius=data["nozzle_radius"],
dry_mass=data["dry_mass"],
center_of_dry_mass_position=data["center_of_dry_mass_position"],
dry_inertia=(
data["dry_I_11"],
data["dry_I_22"],
data["dry_I_33"],
data["dry_I_12"],
data["dry_I_13"],
data["dry_I_23"],
),
nozzle_position=data["nozzle_position"],
interpolation_method=data["interpolate"],
coordinate_system_orientation=data["coordinate_system_orientation"],
reference_pressure=data.get("reference_pressure"),
)
for tank in data["positioned_tanks"]:
motor.add_tank(tank["tank"], tank["position"])
return motor
