>>7617912A lot of the little things are sensors and valve actuators.
As for the big things, a liquid-fueled rocket engine will normally have pumps for fuel and oxidizer, a shaft-power engine or two to drive the pumps, possibly additional pre-pumps and shaft-power engines, a starter system for the shaft-power engines, an ignition system for the main combustion chamber, a flush system to blow the remaining fuel and oxidizer out of the system on shutdown, and usually a curtain cooling system in addition to the regenerative cooling.
Because they're pumping large volumes of fuel from a low-pressure supply to a high-pressure combustion chamber, the pumps need to be extremely powerful.
This is the space shuttle main engine, so it's probably the most complicated rocket engine ever built. It was started with an external high-pressure helium supply, so it couldn't be restarted in orbit.
