Cold Gas Thruster

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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.

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Module Implementation

Thrust Generation

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

$$F=\dot{m}\cdot v_e$$

Where:

• $F$ = thrust N
• $\dot{m}$ = mass flow rate kg/s
• $v_e$​ = exhaust velocity m/s

The exhaust velocity is derived from the specific impulse $I_{sp}$​ as:

$$v_e=I_{sp}\cdot g_0$$

Thus, the thrust equation becomes:

$$F=\dot{m}\cdot I_{sp}\cdot g_0$$

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

$$F_{effective}=F\cdot (1-D_f)$$

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:

$${\dot{m}}_{desired}=\frac{F}{I_{sp}\cdot g_0}$$

Body Frame Force

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

$${\mathbf{F}}_B={\hat{\mathbf{d}}}_B\cdot F_{effective}$$

Where:

• ${\mathbf{F}}_B$ = thrust vector in the spacecraft body frame N
• ${\hat{\mathbf{d}}}_B$​ = 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.

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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: $I_{sp}$​ 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.

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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.