LiquidMotor Class Usage#

Here we explore different features of the LiquidMotor class.

Creating a Liquid Motor#

To define a liquid motor, we will need a few information about our motor:

The thrust source file, which is a file containing the thrust curve of the motor. This file can be a .eng file, a .rse file, or a .csv file. See more details in Thrust Source Details
Tank objects, which will define propellant tanks. See more details in Tank Usage
Position related parameters, such as the position of the center of mass of the dry mass, the position of the center of mass of the grains, and the position of the nozzle. See more details in Motor Coordinate Systems

Let’s first import the necessary modules:

from math import exp
from rocketpy import Fluid, LiquidMotor, CylindricalTank, MassFlowRateBasedTank

Then we must first define the tanks:

See also

Tank Usage

# Define fluids
oxidizer_liq = Fluid(name="N2O_l", density=1220)
oxidizer_gas = Fluid(name="N2O_g", density=1.9277)
fuel_liq = Fluid(name="ethanol_l", density=789)
fuel_gas = Fluid(name="ethanol_g", density=1.59)

# Define tanks geometry
tanks_shape = CylindricalTank(radius = 0.1, height = 1.2, spherical_caps = True)

# Define tanks
oxidizer_tank = MassFlowRateBasedTank(
    name="oxidizer tank",
    geometry=tanks_shape,
    flux_time=5,
    initial_liquid_mass=32,
    initial_gas_mass=0.01,
    liquid_mass_flow_rate_in=0,
    liquid_mass_flow_rate_out=lambda t: 32 / 3 * exp(-0.25 * t),
    gas_mass_flow_rate_in=0,
    gas_mass_flow_rate_out=0,
    liquid=oxidizer_liq,
    gas=oxidizer_gas,
)

fuel_tank = MassFlowRateBasedTank(
    name="fuel tank",
    geometry=tanks_shape,
    flux_time=5,
    initial_liquid_mass=21,
    initial_gas_mass=0.01,
    liquid_mass_flow_rate_in=0,
    liquid_mass_flow_rate_out=lambda t: 21 / 3 * exp(-0.25 * t),
    gas_mass_flow_rate_in=0,
    gas_mass_flow_rate_out=lambda t: 0.01 / 3 * exp(-0.25 * t),
    liquid=fuel_liq,
    gas=fuel_gas,
)

Warning: Adding spherical caps to the tank will not modify the total height of the tank 1.2 m. Its cylindrical portion height will be reduced to 1.0 m.

Note

Here we define two tanks, one for the oxidizer and one for the fuel. We use the MassFlowRateBasedTank, which means that the mass flow rates of the liquid and gas are defined by the user.

In this case, we are using a lambda functions to define the mass flow rates, but .csv files can also be used. See more details in rocketpy.motors.Tank.MassFlowRateBasedTank.__init__

Now we can define our liquid motor and add the tanks. We are using a lambda function as the thrust curve, but keep in mind that you can use different formats here.

example_liquid = LiquidMotor(
    thrust_source=lambda t: 4000 - 100 * t**2,
    dry_mass=2,
    dry_inertia=(0.125, 0.125, 0.002),
    nozzle_radius=0.075,
    center_of_dry_mass_position=1.75,
    nozzle_position=0,
    burn_time=5,
    coordinate_system_orientation="nozzle_to_combustion_chamber",
)
example_liquid.add_tank(tank=oxidizer_tank, position=1.0)
example_liquid.add_tank(tank=fuel_tank, position=2.5)

Caution

Pay special attention to:

dry_inertia is defined as a tuple of the form (I11, I22, I33). Where I11 and I22 are the inertia of the dry mass around the perpendicular axes to the motor, and I33 is the inertia around the motor center axis.
dry inertia is defined in relation to the center of dry mass, and not in relation to the coordinate system origin.
center_of_dry_mass_position, nozzle_position and the tanks position are defined in relation to the coordinate system origin, which is the nozzle outlet in this case.
Both dry_mass and center_of_dry_mass_position must consider the mass of the tanks.

See also

You can find details on each of the initialization parameters in rocketpy.LiquidMotor.__init__

And you can find details on adding tanks in Adding Tanks

After defining the motor, we can plot basic attributes using the info() method.

example_liquid.info()

Nozzle Details
Nozzle Radius: 0.075 m

Motor Details
Total Burning Time: 5 s
Total Propellant Mass: 53.020 kg
Average Propellant Exhaust Velocity: 327.234 m/s
Average Thrust: 3166.493 N
Maximum Thrust: 4000.0 N at 0.0 s after ignition.
Total Impulse: 15832.466 Ns

Other plots can also be done, in order to check if the motor is behaving as expected. For example:

Propellant mass
Mass flow rate
Motor center of mass
Inertial moment
Exhaust velocity

example_liquid.propellant_mass.plot(0, 5)

example_liquid.mass_flow_rate.plot(0, 5)

example_liquid.center_of_mass.plot(0, 5)

example_liquid.I_11.plot(0, 5)

example_liquid.exhaust_velocity.plot(0, 5)

Alternatively, you can plot all the information at once:

example_liquid.all_info()

Nozzle Details
Nozzle Radius: 0.075 m

Motor Details
Total Burning Time: 5 s
Total Propellant Mass: 53.020 kg
Average Propellant Exhaust Velocity: 327.234 m/s
Average Thrust: 3166.493 N
Maximum Thrust: 4000.0 N at 0.0 s after ignition.
Total Impulse: 15832.466 Ns