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View of the auxiliary tunnel and the S-IVB forward skirt.

Picture 2 of 2.

The primary function of the auxiliary tunnel was to provide an aerodynamic fairing to the line connecting the cold helium spheres on mounted on the interior of this side of the tank to the helium spheres mounted on the other side of the tank, along the main tunnel (the line ran up the auxiliary tunnel, across the interior of the forward skirt, and down the main tunnel). Helium from these spheres was routed to the J-2 engine's heat exchanger, expanded, and used to pressurize the stage's LOX tank during flight.

This photo was taken during the Saturn V's restoration, while the auxiliary tunnel segments were removed. As this is the dynamic test stage, flight helium bottles were not installed, but rather mass simulators (see page 12 in the PDF), so there is no associated plumbing. However, it seems reasonable to believe that the mount points which penetrated the tank wall were substantially similar to a flight stage.