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Linerless composite tanks, using continuous-graphite-fiber reinforcement and a polymeric-resin matrix will enable significant mass savings and increased payload capacity in future launch vehicles. The critical application for which linerless composite tanks show the greatest impact to system performance is in the upper stages of future expendable launch vehicles. Many of these tanks will operate at cryogenic temperatures. The key technical challenge in developing these tanks will be to choose and/or design the material to resist microcracks that may lead to leakage. Microcracks are known to form in the matrix of a composite due to thermal and mechanical stresses transverse to the reinforcing fiber direction. The toughness of the matrix material has the most pronounced impact of all factors on microcracking. This paper presents an approach for characterizing a resin’s “microcracking fracture toughness,” or the resistance of the resin to form microcracks, at cryogenic temperatures. It describes an effort to develop new, toughened materials for linerless composite tanks, a unique testing methodology for determining the microcracking fracture toughness of the materials at cryogenic temperatures, and some preliminary test results. The performance of some new toughened composite materials is compared to that of industry-standard materials used traditionally in cryogenic tanks. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR] more...