SolidMotor Class Usage#

Here we explore different features of the SolidMotor class.

Key Assumptions#

A few comments on the grain configuration:

  • Only BATES grain configuration is currently supported;

  • Exhaust velocity is assumed to be constant throughout the burn, meaning the specific impulse is constant.

Creating a Solid Motor#

To define a solid 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

  • Physical parameters, such as the dry mass, inertia and nozzle radius

  • Propellant parameters, such as the number of grains, their geometry, and their density

  • 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 instantiate a SolidMotor object:

from rocketpy import SolidMotor

example_solid = SolidMotor(
  dry_inertia=(0.125, 0.125, 0.002),
  nozzle_radius=33 / 1000,
  grain_outer_radius=33 / 1000,
  grain_initial_inner_radius=15 / 1000,
  grain_initial_height=120 / 1000,
  grain_separation=5 / 1000,
  throat_radius=11 / 1000,


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.

  • grains_center_of_mass_position, center_of_dry_mass_position and nozzle_position are defined in relation to the coordinate system origin, which is the nozzle outlet in this case.

See also

You can find details on each of these parameters in rocketpy.SolidMotor.__init__

We can now access all the information about our motor:

Nozzle Details
Nozzle Radius: 0.033 m
Nozzle Throat Radius: 0.011 m

Grain Details
Number of Grains: 5
Grain Spacing: 0.005 m
Grain Density: 1815 kg/m3
Grain Outer Radius: 0.033 m
Grain Inner Radius: 0.015 m
Grain Height: 0.12 m
Grain Volume: 0.000 m3
Grain Mass: 0.591 kg

Motor Details
Total Burning Time: 3.9 s
Total Propellant Mass: 2.956 kg
Average Propellant Exhaust Velocity: 2038.745 m/s
Average Thrust: 1545.218 N
Maximum Thrust: 2200.0 N at 0.15 s after ignition.
Total Impulse: 6026.350 Ns

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