Ion Thruster Power Model

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Description

The Ion Thruster Power Model is a Power Node Model that attaches to an Ion Thruster to model the electrical interface with the spacecraft power system. It implements a Constant Power Load (CPL) pattern where the Power Processing Unit (PPU) adjusts its effective input impedance to maintain constant power draw from the bus.

The model includes PPU efficiency losses, standby power consumption, ignition thresholds, and brownout detection to provide realistic power-limited thruster behavior.

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Example Use Cases

• Power-Constrained Operations: Simulate thruster behavior when available bus power is insufficient for full thrust.
• Brownout Response: Model thruster shutdown when bus voltage drops below safe operating thresholds.
• PPU Efficiency Analysis: Evaluate total DC power draw including PPU losses for power budget analysis.
• Ignition Sequencing: Model the minimum power threshold required to ignite and sustain thruster operation.

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Module Implementation

Power Consumption

The DC power draw depends on whether the thruster is ignited:

Standby Mode (when $P_{\text{desired}}<P_{\text{min,ignition}}$):

$$P_{\text{DC}}=P_{\text{standby}}$$

Ignited (when $P_{\text{desired}}\ge P_{\text{min,ignition}}$):

$$P_{\text{DC}}=P_{\text{standby}}+\frac{P_{\text{beam}}}{{\eta }_{\text{PPU}}}$$

where $P_{\text{beam}}$ is the beam power clamped to MaxOperatingPower and ${\eta }_{\text{PPU}}$ is the PPU efficiency.

Available Power

The beam power actually available to the thruster is computed from the absorbed electrical power:

$$P_{\text{available}}=(P_{\text{absorbed}}-P_{\text{standby}})\cdot {\eta }_{\text{PPU}}$$

This value is clamped between zero and the desired beam power. If the thruster is in brownout or below the ignition threshold, AvailablePower is zero and no thrust is produced.

Constant Power Load Model

The model uses quasi-static iteration to achieve CPL behavior with a linear SPICE resistor. The resistance is computed from the previous tick’s solved voltage:

$$R=\frac{V_{\text{solved}}^2}{P_{\text{DC}}}$$

On the first tick or after power loss, the resistance seeds from NominalOperationalVoltage. The voltage is floored at MinOperationalVoltageRatio × NominalOperationalVoltage to prevent resistance collapse during brownout.

Brownout Detection

The thruster enters brownout and produces no thrust when either condition is met:

1. Voltage Threshold: $|V_{\text{node}}|<V_{\text{nominal}}\times r_{\text{min,voltage}}$
2. Power Threshold: $P_{\text{absorbed}}<P_{\text{DC}}\times r_{\text{min,power}}$

Set MinPowerRatio to 0 to disable the power threshold and rely on voltage alone.

Parameters

Parameter	Description	Default
NominalOperationalVoltage	Nominal PPU input voltage (V)	28.0
PPUEfficiency	Power Processing Unit efficiency (0–1)	0.92
StandbyPower	Standby power for cathode/electronics (W)	8.0
MinIgnitionPower	Minimum beam power to ignite (W)	40.0
MaxOperatingPower	Maximum beam power capacity (W)	150.0
MinOperationalVoltageRatio	Brownout voltage threshold (0–1)	0.8
MinPowerRatio	Brownout power threshold (0–1)	0.13

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Assumptions/Limitations

• The PPU efficiency is assumed constant across the operating range.
• Standby power is drawn even when the thruster is not producing thrust.
• The quasi-static CPL model converges over successive time steps; instantaneous load changes may take 1–2 ticks to stabilize.
• Thermal effects on PPU efficiency are not modeled.
• The model does not simulate PPU startup transients or soft-start behavior.