The paper describes the pre-development of the compact turbo-inachinery forming the heart of a reverse- Brayton-cycle refrigeration unit for application in a microgravity-compatible freezer. The machine has been configured to provide refrigeration at -80°C in the face of continuous loads of 10 W and transient loads of up to 30 W without disturbing the microgravity environment. Other important requirements were: operational life-time without maintenance of at least 10 years, high coefficient of performance to minimise power consumption, low mass and small size.In the interests of reliability and microgravity compatibility, the compressor wheel and the expansion turbine have been mounted on the same shaft, driven by a brushless DC electric motor. This approach assures the most compact design, however, it gave rise to complex design iterations. The need to minimise thermal losses resulting from conduction through the shaft and to avoid mechanical resonance effects at the very high (up to 200000 Rpm) shaft speeds, also presented interesting design challenges.Results are presented covering tests on each of the individual components together with performance projection of the complete machine.