Turbopump

A turbopump (portmanteau of turbine pump) is an assembly consisting of a liquid pump driven by a gas turbine, connected via a shaft (and occasionally gears as well). They were initially developed in the US and Germany in the 1930s and 1940s. While other use cases can exist, the primary purpose of turbopumps is to dramatically raise the pressure of liquid propellants and feed them to the combustion chamber of a rocket engine. While they have considerably higher design complexity, turbopump fed systems scale much more favorably in large rockets than pressure-fed systems, which require increasingly thick and heavy tanks to supply high chamber pressures in the engines.

Two types of pumps have been used in turbopumps: most common are centrifugal pumps, where the pumping is done by throwing fluid outward at high speed, while much rarer are axial-flow pumps, where alternating rotating and static blades progressively raise the pressure of a fluid. Centrifugal designs give high head (pressure increase) but modest flow rates, while axial flow rates give high flow rates (advantageous for low density fluids, i.e. liquid hydrogen) but modest head, and so usually need many stages in series.