In-orbit validation of a ferrofluidic Thermal Switch in ISS microgravity

Abstract Ferrofluids offer a wide range of applications by enabling wearless systems for future space missions. As part of the FARGO (Ferrofluid Application Research Goes Orbital) project, an active Thermal Switch using ferrofluid was developed and validated during a 26-day International Space Station (ISS) mission. This experiment was conducted as part of the Überflieger2 competition organized by the Space Agency within the German Aerospace Center (DLR). Thermal management of spacecraft (S/C) and satellites is a universal challenge. In the development of thermal management components, it is therefore essential to minimize or even eliminate stress/strain peaks and potential safety risks for S/C operation and payloads induced by thermal differences. The thermal management system shall be able to keep all components within their acceptable temperature ranges. This shall be achieved under all orbit conditions, for example, when solar irradiance heats one side of the S/C resulting in possibly unwanted temperature gradients toward the cold vacuum of space. An active Thermal Switch allows heat flows to be routed along different paths within the S/C depending on the prevailing conditions. Within project FARGO, an active Thermal Switch was tested during an ISS mission. Tests on the ground already verified a measurable difference in thermal conductivity depending on the state of the switch. Magnetically moving the ferrofluid allows the creation of a thermally isolating and conducting Thermal Switch state. The ferrofluid utilized for the FARGO experiment is EFH-1 with a thermal conductivity of $${0.19\,\mathrm{\text {W}\text {m}^{-1}\text {K}^{-1}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0.19</mml:mn> <mml:mspace/> <mml:mrow> <mml:msup> <mml:mtext>Wm</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mtext>K</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . The Thermal Switch consists of two silver rods in line, acting as heat conductors, separated by an airgap. One side of the switch is heated, and the other one cooled by one thermoelectric cooler (TEC) each. The gap between the conductors can be bridged by ferrofluid moved via magnetic fields. As the EFH-1 has a higher thermal conductivity k than air, thermal conductive change is achieved by switching. The temperature difference between hot and cold side are measured in the ON state as well as the OFF state. From these measurements, the switching ratio as a performance characteristic is calculated. The switch was tested under various heat loads and switching times. Electropermanent magnets (EPMs) are magnets that can be magnetically switched on and off by a current pulse. Compared to conventional electromagnets, heat generation and power generation are significantly reduced. In addition, the switch remains reliably in the selected state, even if a failure of the control electronics occurs, making it bi-stable. Moreover, electric power is only required to switch the state of the EPM, but not to maintain it.