 In this concept, the interior of the fuel tank would be filled with living quarters and various equipment that did not take up a significant amount of volume.The S-IVB would then be filled with propellants as normal and used to accelerate the craft on its way to Venus.Once the burn was complete, any remaining propellant was vented to space, and then the larger fuel tank could be used as living space, while the smaller oxygen tank would be used for waste storage.