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The S-IV and S-IVB stages for the Saturn vehicles were designed to utilize the high specific impulse of the liquid hydrogen - liquid oxygen propulsion system. The use of liquid hydrogen presented special structural problems that led to the development of the reinforced foam internal insulation and the sandwiched honeycomb common bulkhead. The major design problem in each case was the high thermal stress resulting from the steep thermal gradient across the depth of the structure. For the extreme temperatures involved the thermal stresses were very high and were dominant factors in establishing the designs.