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The cylindrical solid propellant gas generator.

The SPGG is used during engine start: The SPGG is ignited and the resulting high-velocity gases spin the turbine, powering the turbopump. This bootstraps the engine, and before the SPGG is expended, sufficient propellants are being delivered to the liquid propellant gas generator for the LPGG to take over.

Picture 2 of 2.

It's difficult to read much on the ID plate on the SPGG, but the "Part Name" is listed as "[unintelligible] Gas Gen. Solid Prop."