Liquid hydrogen pump impellor assembly

An impellor assembly for a liquid hydrogen pump, the assembly comprising:

• a shaft;
• one or more impellors mounted to the shaft;
• a fastener configured to engage with the shaft at a first engagement point and with the one or more impellor at a second engagement point to axially clamp the one or more impellor to the shaft, an axially unconstrained length of the fastener being defined by an axial distance between the first and second engagement points;
• wherein
• an axial extent of a clamped portion of the one or more impellors corresponds to the axially unconstrained length of the fastener;
• and
• the fastener and the impellor comprise a material having a first coefficient of thermal expansion and the shaft comprises a material having a second coefficient of thermal expansion, wherein the first and second coefficients of thermal expansion are different, and wherein the axial distance is within 20% of the axial extent.