Electrification of aircraft propulsion systems is one of the key solutions for realizing Net Zero of CO2 emission by 2050, given by ATAG (Air Transport Action Group). It is discussed that the installation of electric motors and power converters into the aircraft propulsion systems can be effective for the improvement of system efficiency. However, the weight of the electric equipment should be lighter than that of state-of-the-art motors and power converters. One of the demanded values is estimated over 16 kW/kg .
Superconducting technologies have a potential for achieving or excessing the upper values. The current density of the superconducting (SC) wires cooled at less than -200 ℃ for the rotating machines can be several ten or hundred times higher as much as copper windings. Also, stator iron teeth can be reduced thanks to high magnetic field generated by the SC windings; therefore, the weight of motors can be lighter than that of state-of-the-art motors.
There are two kinds of SC rotating machines such as partial SC motors (PSCMs) and fully SC motors (FSCMs). While the PSCMs have SC field windings and copper armature windings, both field and armature windings of the FSCMs are made of SC wires. The two SC windings of the FSCMs tend to put into the same cryostat and then the mechanical gap can be reduced in comparison with the PSCM structures; finally, power-density of the FSCMs can be higher than that of the PSCM structures, which have both SC and copper windings.
The authors have been studying the FSCMs using MgB2 armature windings and REBCO field windings cooled at 20 K via liquid hydrogen . The MgB2 SC wires have multifilament structures, which have a potential to reduce AC losses. On the other hand, the REBCO wire have good Jc - B characteristics over 20 K. However, this FSCM structure demands two coolant paths for SC field/armature windings and results in complicated cooling structure. Therefore, the authors employed rotor cooling structures via dilute gas, while the stator side is still cooled at 20 K with liquid hydrogen. The REBCO field windings are operated below 50 K with heat conduction and convective heat transfer via dilute gas; this structure can simplify the rotor cooling structure. The authors conducted thermal and fluid analysis via finite element method to investigate the effect of the dilute gas cooling for SC field windings. By giving thermal load as electrical losses at the rotor part and changing parameters of pressure, rotation speed and so on, the cooling effect of this method is discussed.
 B. Łukasik, “Turboelectric Distributed Propulsion System as a Future Replacement for Turbofan Engines,” the ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition GT2017, GT2017-63834, 2017.
 Y. Terao, Y. Ishida, H. Ohsaki and H. Oyori, “Electromagnetic Characteristic Comparison of Superconducting Synchronous Motor Characteristics for Electric Aircraft Propulsion Systems,” SAE International Journal of Advances and Current Practices in Mobility, Vol. 2, pp. 828-837, 2019.
 M. Kato, Y. Terao, and H. Ohsaki, "Analytical and experimental study on the feasibility of rotor cooling with thin gas for fully superconducting rotating machines," Journal of Physics: Conference Series, Vol. 1857, No. 1, p. 012016, 2021.
Keywords: Electric aircraft, Dilute gas, Fully superconducting motors, Liquid hydrogen