Another view of the oxidizer side of the Space Shuttle Main Engine (SSME). The High Pressure Oxidizer Turbopump (HPOTP) is clearly seen in the middle of the engine, with its double volute. The Pre-burner Pump (silver color), seen just to the right of the HPOTP, provides oxidizer to both the fuel and oxidizer pre-burner pumps, which in turn provide high pressure gas to the turbines, which drive the high-pressure pumps. It's an amazing, if not complicated and confusing, marvel of technology.