Cold Gas Thruster

Description

The Cold Gas Thruster class represents a cold gas propulsion system, a type of reaction control thruster that expels stored inert gas through a nozzle to produce thrust. It inherits from the abstract Thruster base class and extends it with functionality for fuel consumption modelling via the Fuel Consumer Model.

Unlike electrically driven systems, a cold gas thruster produces thrust directly from momentum exchange of high-pressure stored gas without combustion or ionization.

The class provides logic to:

Compute thrust based on fuel flow rate and specific impulse.
Determine the required fuel flow rate from desired thrust.
Model connection and disconnection of a fuel source.

Module Implementation

Thrust Generation

Cold gas thrusters generate thrust according to the rocket equation, simplified for steady-state flow:

F = \overset{m}{˙} \cdot v_{e}

Where:

$F$ = thrust N
$\overset{m}{˙}$ = mass flow rate kg/s
$v_{e}$ = exhaust velocity m/s

The exhaust velocity is derived from the specific impulse $I_{s p}$ as:

v_{e} = I_{s p} \cdot g_{0}

Thus, the thrust equation becomes:

F = \overset{m}{˙} \cdot I_{s p} \cdot g_{0}

The thrust is then scaled by a dispersion factor to account for imperfect nozzle alignment or gas expansion:

F_{e ff ec t i v e} = F \cdot (1 - D_{f})

Where $D_{f}$ is the dispersed factor

Required Fuel Flow

To determine how much propellant is needed for a desired thrust, the above equation is inverted:

\overset{m}{˙}_{d es i re d} = \frac{F}{I _{s p} \cdot g _{0}}

Body Frame Force

The resulting body-frame force vector applied to the spacecraft is:

F_{B} = \hat{d}_{B} \cdot F_{e ff ec t i v e}

Where:

$F_{B}$ = thrust vector in the spacecraft body frame N
$\hat{d}_{B}$ = unit direction vector of thrust This ensures the force is correctly oriented relative to the spacecraft’s local axes.

Fuel Source Connectivity

The thruster connects to and disconnects from a Fuel Source via the Fuel Consumer Model. This linkage allows for resource-aware simulation, ensuring mass depletion and flow continuity from tanks to thrusters.

ConnectFuelSource(source): Connect the thruster to a Fuel Source
DisconnectFuelSource(): Disconnect from the current fuel source

Thermodynamic Parameters

The cold gas thruster includes parameters for modeling the thermodynamic state of the propellant:

Parameter	Units	Description	Default
`SpecificHeatRatio`	—	Ratio of specific heats ( $γ = c_{p} / c_{v}$ ) for the propellant gas	1.4
`TotalTemperature`	K	Stagnation (total) temperature of the gas	250
`TotalPressure`	Pa	Stagnation (total) pressure of the gas	2750

Nozzle Geometry

The thruster nozzle is characterized by two area parameters:

Parameter	Units	Description
`ThroatArea`	m²	Cross-sectional area at the nozzle throat (minimum area)
`ExitArea`	m²	Cross-sectional area at the nozzle exit plane

The expansion ratio $ϵ$ is defined as:

ϵ = \frac{A _{e}}{A _{t}}

where $A_{e}$ is the exit area and $A_{t}$ is the throat area. The throat area is constrained to be no larger than the exit area.

Assumptions/Limitations

Steady-State Flow: Gas flow and pressure are constant during operation; transient valve or regulator effects are ignored.
Isentropic Expansion: The gas expansion through the nozzle is assumed ideal with no energy losses except those modelled via Dispersed Factor.
Constant Specific Impulse: $I_{s p}$ remains constant throughout the burn.
Negligible Heat Effects: Thermal effects, freezing, or condensation in nozzles are not modelled.
Single Gas Species: The model assumes one inert gas (typically nitrogen or argon).
No Tank Pressure Model: Variations in tank pressure, temperature, and density over time are not represented.
No Transient Valve Dynamics: Opening and closing delays or flow oscillations are excluded.
No Multi-Nozzle Coupling: Each thruster acts independently; coupled plumes or manifold effects are not simulated.
Constant Efficiency: No consideration of nozzle efficiency degradation or flow slip.
Ideal Gas Assumption: Flow behaviour assumes perfect gas dynamics without viscous or boundary-layer effects.

References

[1] Sutton, G. P., & Biblarz, O. (2017). Rocket Propulsion Elements (9th ed.). Wiley.

[2] Wertz, J. R., Everett, D. F., & Puschell, J. J. (2011). Space Mission Engineering: The New SMAD (2nd ed.). Microcosm Press.

[3] Hauser, D. M., & Quinn, F. D. (2012). Simulation of a Cold Gas Thruster System and Test Data Correlation (NASA/TM-2012-217271). National Aeronautics and Space Administration, Glenn Research Center.