Hēki: A superconducting magnet for space propulsion on the International space Station – Pathfinder design and experimental thermal testing

Abstract

Applied-field magnetoplasmadynamic (AF-MPD) thrusters are a high-power electric propulsion solution for satellites and spacecraft, offering high efficiency, high specific impulse and high thrust density.

The integration of high-temperature superconducting (HTS) electromagnets energised with flux pumps as the applied field module can significantly reduce the mass, power and volume of AF-MPD thrusters, making their deployment as practical spacecraft propulsion systems more feasible. To validate HTS magnet and flux pump technology, a New Zealand team led by the Paihau-Robinson Research Institute is collaborating with Nanoracks LLC to send an HTS magnet to the International Space Station (ISS). Dubbed the “Hēki Mission”, an HTS magnet and flux pump will be installed on the Nanoracks External Platform (NREP) for an in-space technology demonstration. This aims to gain space heritage for HTS magnets and flux pumps for the first time, a crucial step toward practical application and commercialisation of HTS-powered thrusters in space.

This paper details the preliminary design of the Hēki mission payload. An extension of work presented at the European Applied Superconductivity Conference in 2023, we provide more detail on the electromagnetic and thermal design of the “pathfinder” Hēki magnet, our first attempt at designing a realistic space payload that meets stringent size, weight and power requirements typical of a small satellite. Through the development of finite element models, we detail the electromagnetic design of the HTS magnet which features a large warm bore to accommodate future integration with a realistically sized AF-MPD thruster, and detail the design philosophy and mass optimisation tools developed to achieve a central field of 0.5 T while simultaneously magnetically shielding the magnet to comply with ISS safety requirements. We also detail the conduction cooled thermal design of the pathfinder Hēki magnet, showing how magnet temperatures below 75 K can be achieved with a cryogenic cooling system that consumes less than 100 W of electrical power. These thermal models were compared with thermal experiments in a simulated space environment for model validation purposes. Difficult to measure input variables such as the contact resistance between surfaces and the effective emissivity of the thermal radiation shielding were empirically determined to improve model predictive power.

Scheduled for launch in the first quarter of 2025, the Hēki pathfinder design outlined in this paper serves as a pivotal preliminary effort that has identified the major risks potentially impacting mission success. Consequently, this body of work represents a significant step forward in developing a flight-qualified system capable of achieving our space mission objectives.