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The white, spherical start tank.

Picture 1 of 2.

The start tank contains pressurized hydrogen gas (initially from a ground source, but is refilled during S-IVB engine operation to allow for engine restart) which is used to provide the initial spin on the turbines, bootstrapping the turbopumps.

Note the large start tank discharge valve at upper center of the start tank and the braided start tank discharge hose assembly running from it to the fuel turbopump.

The grey cables partially obscuring the right-hand side of the start tank are part of the customer connect electrical system.