Choosing the Right Stuff: Material Selection for Liquid Hydrogen Aircraft Cryotanks

A primary focus of liquid hydrogen-powered aircraft design surrounds the configuration and manufacture of the fuel storage tanks. Cryotank technology developed for space launch vehicles is applicable, but the prolonged lifecycle and passenger safety raise critical issues. The purpose of this white paper is to identify the decisions required in selecting composite or metallic material construction for cryotanks on commercial transports and provide recommendations based on the state of the art. Traditionally, metallic materials exhibit high damage tolerance and are a good choice for long-term durability. In contrast, composite materials exhibit high specific properties and are a good choice for lightweight structures. Emerging hybrid construction could potentially exploit the benefits of both material classes, but unaddressed issues of coefficient of thermal expansion (CTE) mismatch due to thermomechanical cycling obviate its near-term candidacy. The contributing factors considered here include cryotank weight/shape, boil-off/leakage, cost/manufacturability, crashworthiness, durability, inspectability, and tank locations. Although the ultimate design will depend on a complicated set of trade-offs, some preliminary conclusions can be drawn. Based on the key factors presented, metallic materials are favored due to the mature manufacturability (large-scales and high rate), good durability, and low cost, especially should the tanks be externally-mounted (on the wings) due to high impact resistance. However, composite materials show great promise if the durability concerns are overcome and probably favored should the tanks be internally-mounted (within the fuselage) due to the ability to create complex, conformal shapes.