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Overall view of the solid propellant gas generator (SPGG), sometimes called the "spinner".

During steady state operation, the liquid propellant gas generator uses propellant delivered from the turbopumps to generates the gasses necessary to power the turbine. The SPGG is used during engine start: The SPGG is ignited and the resulting high-velocity gasses spin the turbine, powering the turbopump. This bootstraps the engine, and before the SPGG is expended, sufficient propellants are being delivered to the LPGG for it to take over.