Structural Considerations for Larger Upper Stage Development

The selection of design configurations and materials for large vehicles such as those proposed for manned missions to Mars in the early 1980's will require the resolving of major technological questions during the next few years. The structure required to contain the large volume of liquid propellants, particularly hydrogen, must be of efficient design so that payload capability can be maximized. In a typical study recently conducted, design criteria were established for a multimodule upper stage. Each stage module consists of a basic shell structure, an insulated, internally mounted LH2 propellant tank, and the thrust engine with its associated support system and hardware. Meteoroid protection is incorporated in the shell structure. The largest potential for structural weight-saving appears to be in the propellant tank design. Aluminum alloys are currently favored as tank material because of successful experience and the high level of technological development. Titanium alloys, however, offer sizeable potential weight savings because of their superior biaxial strength properties at cryogenic temperatures. The biaxial strengths of titanium alloys range from 30% to 70% greater than their uniaxial strength, compared to less than 15% for the aluminum alloys. However, there are certain requisite programs that must be conducted before titanium can be introduced as a qualified structural material for large cryogenic tankage. This study investigated the following areas: texture strengthening of titanium alloys, nondestructive inspection techniques, critical crack size, proof load levels, compatibility with LH;? under long-term storage conditions, stress corrosion, creep, and low-cycle fatigue. Only with positive results in these areas could the materials be used. Efficient means of attachment of structural components to thin-gage tanks were studied, as were manufacturing considerations for the vehicles. These included material size, development of welding methods and equipment, and techniques for handling the large, thin-gage upper stage structural components before, during, and after fabrication.