Hydrogen tanks made of polymer-matrix composite material have been proposed as an enabling technology for reducing the weight of reusable launch vehicles and increasing their payload. A key development issue for these lightweight structures is the leakage of hydrogen through the composite material, which is generally a function of the tank material, manufacturing method, mechanical load, any internal damage states in the material, and the temperatures at which the tank must operate. A method for measuring leakage through a geometrically complex structure at cryogenic temperatures and under mechanical load that has been developed, calibrated, and used to measure helium and hydrogen leakage through the X-33 liquid hydrogen tank structure is presented. In particular, results from the calibration tests are presented that indicate that the measurement errors are less than 10 for leak rates ranging from 0.3 to 200 cm(3)/min at standard atmospheric conditions. In addition, both hydrogen and helium leak tests that were performed on two specimens taken from a discarded segment of the X-33 tank structure and results are compared. For both the hydrogen and helium tests leak rates varied with the applied mechanical load. The level of hydrogen leakage is shown to be significantly higher than the helium leakage and to exceed the acceptable leak rate for a vehicle like the X-33 liquid hydrogen tank by an order of magnitude. References: 8
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