The Saturn S-IVB stage has a requirement for orbiting around the earth for up to 4.5 hours with approximately 60 percent of its initial propellant remaining at the end of the coast (prior to restart) . Extensive analyses must be performed to insure that this requirement is met. Both the maximum and minimum heat transfer rates are important because the maximum rates affect the hydrogen boiloff losses and thus the initial propellant loading requirements. The minimum rates are important because the boil off gases are used to maintain a minimum axial thrust level by venting the gases continuously through aft facing nozzles. This provides for a settling of the propellant throughout the orbital coast and alleviates the need for periodically venting the tank under zero gravity.
