Fall Voltage as a Key Parameter in MPD Thruster Performance Modeling

Magnetoplasmadynamic (MPD) thrusters are a class of electric propulsion devices capable of producing high exhaust velocities and thrust through the control of magnetic fields and electric currents. These thrusters are of particular interest for deep-space missions due to their ability to operate effectively for extended durations. A key aspect of MPD thruster performance is the fall voltage, a voltage drop associated with plasma sheaths at the cathode and anode interfaces that represents energy lost as electrons cross electrode surfaces. In many system-level models, fall voltage is treated as a constant, but this simplification may introduce inaccuracies in predicting performance, particularly at lower discharge currents. This study investigates the sensitivity of MPD thruster performance to fall voltage using a simplified self-field MPD thruster model implemented numerically in Python. By varying fall voltage across a range of 20–60 V while holding other parameters constant, we evaluate impacts on thrust, discharge voltage, exhaust velocity, and efficiency over a discharge current range of 0–1000 A. Results demonstrate that while fall voltage does not influence thrust or exhaust velocity in the electromagnetic framework, it significantly reduces thruster efficiency. A 40 V increase in fall voltage (20 V to 60 V) produced approximately 65–66% reduction in predicted efficiency at both 200 A and 800 A. Efficiency trends predicted by the model are qualitatively consistent with experimental data from the Princeton University full-scale benchmark thruster reported by Nada [8], supporting the physical relevance of the sensitivity structure identified here. These findings underscore the importance of accurately modeling sheath behavior in MPD performance predictions and suggest that constant fall voltage assumptions may lead to substantial efficiency overestimation, particularly in low-current operational regimes.