Celine Jane

The Robinson Research Institute is developing a type of electric propulsion for spacecraft called an applied-field magneto-plasma-dynamic thruster (AF-MPD) that is operating in the 500–1500 W range using argon as the propellant. The applied field module of the thruster is a cryocooler-cooled high-temperature-superconducting (HTS) electromagnet generating a central field exceeding 1 T. Ground-based characterisation of this type of electric thruster is challenging due to the millinewton-range thrust forces from the applied field module and numerous facility effects. This paper reports on the development of a purpose-built pendulum thrust-stand able to measure millinewton thrust whilst subjected to Tesla level magnetic flux densities and the vibrational loads generated by a mechanical cryocooler—required to maintain the HTS magnet at cryogenic temperatures between 55 and 75 K. Measurement data, postprocessing methods and uncertainty calculations are presented for direct AF-MPD thrust-measurements. These demonstrate accuracies of between ±1–2 mN (1 σ ) are achievable in the 100 mN calibration range when operating an AF-MPD thruster weighting 32 kg, with a 1 T central magnetic field and with the thruster exposed to 60 Hz oscillatory accelerations peaking at ±1.47 m s ⁻² , generated by a mechanical cryo-cooler.

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 Gottingen, 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 negligable 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. As such, we deem it feasible to make direct thrust measurement in our facilities of a flight-like superconducting AFMPDT accurate to within a few mN. The key next stage of this project involves comparative measurements of a superconducting AFMPDT planned for 2025.