In gas generator cycle small portion of the fuel and oxidzier is burned in the preburner to drive centrifugal pumps feeding the main combustion chamber. But how is the fuel/oxidizer fed into the preburner, surely it has to be done against very high pressure? Logically, you would need an infinite series of turbopumps. Also, how is the pressure turned into the rotational kimetic energy? Are turbines similiar to jet engine's used?