Development of techniques for directly measuring thrust from a superconducting applied field magnetoplasmadynamic thruster

Abstract

Superconducting magnet technology can significantly improve the system-level performance of electromagnetic thrusters by reducing the size, weight and power of the magnet module. However, a superconducting magnet poses some unique challenges to accurate and direct thrust measurement, a key performance metric of any thruster. The Robinson Research Institute (Robinson) in New Zealand and the DLR Institute of Aerodynamics and Flow Technology (DLR) in Göttingen, Germany, are working together on a project to directly measure thrust from a kW-class applied-field magnetoplasmadynamic thruster (AFMPDT) with a conduction-cooled superconducting magnet for its applied field module. This paper presents results of the characterization and modification of the thrust stands at DLR and Robinson to meet the additional challenges of stray magnetic fields, cryocooler vibrations and thermal gradients associated with the superconducting magnet system. We find that after some modifications to thrust stand parts and data analysis methods, the stray magnetic field and vibrations of a cryocooler, although large at approximately 1g peak acceleration, should have negligible impact on accuracy of thrust measurement. Moreover, the vibrations may improve the quality of the measurement by reducing hysteresis arising from bearing stiction. A variety of methods will need to be carefully employed to reduce sensitivity to temperature, particularly to an uneven temperature distribution across the pivot axis. We deem it feasible to make direct thrust measurement in our facilities of a flight-like superconducting AFMPDT accurate to within mN for thrust up to 100 mN. The key next stage of this project involves comparative measurements of a superconducting AFMPDT planned for 2025.