Design, Analysis, and Testing of the HEMM Cryocooler Linear Motor

Electrified aircraft propulsion systems will require powertrain components that are both lightweight and highly efficient. NASA's 1.4 MW High Efficiency Megawatt Motor (HEMM) has been designed to run at 6800 RPM with >98% efficiency and 16 kW/kg, achieving low losses by incorporating a cryocooler within its rotor shaft to cool superconducting DC windings. This pulse-tube cryocooler contains an oscillating piston that drives acoustic waves, where the piston is driven by a linear motor. This linear motor is four times more powerful and 2.5 times smaller in diameter than a typical commercial cryocooler motor. Its small diameter affects both the electromagnetic design as well as the mechanical design of the flexure bearings. As part of a risk reduction effort for HEMM, the linear motor preliminary design was fabricated and tested. The design, analysis, and 0 RPM dynamic and fatigue testing is described. Testing was successful, but shows that improvements can be made to both electromagnetic and mechanical components. The linear motor design has been updated for subsequent fabrication and testing at 6800 RPM.