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Detail of an ambient helium sphere on the S-IVB's thrust structure.

The ambient helium spheres on the Saturn V's S-IVB were used to repressurize the S-IVB's propellant tanks after the orbital coast, prior to trans-lunar injection. This particular S-IVB was the dynamic test stage, and so differs from a flight S-IVB in a number of ways, including the number, positioning, and attachment method of the ambient helium spheres.