Detection of Micro-leaks through Complex Geometries under Mechanical Load and at Cryogenic Temperature

Polymer Matrix Composite (PMC) hydrogen tanks have been proposed as an enabling technology for reducing the weight of Single-Stage-to-Orbit reusable launch vehicles where structural mass has a large impact on vehicle performance. A key development issue of these lightweight structures is the leakage of hydrogen through the composite material. The rate of hydrogen leakage can be a function of the material used, method of fabrication used to manufacture the tank, mechanical load the tank must react, internal damage-state of the material, and the temperatures at which the tank must operate. A method for measuring leakage through a geometrically complex structure at cryogenic temperature and under mechanical load was developed, calibrated and used to measure hydrogen leakage through complex X-33 liquid-hydrogen tank structure sections. * NASA Langley Research Center, Hampton, VA † Analytical Services &Materials, Inc., Hampton, VA, currently with Lockheed-Martin Engineering and Sciences 1 Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U. S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes all other rights are reserved by the copyright owner. Introduction Current research in reusable launch vehicles has focused on reducing the cost of delivering payloads to orbit [1]. An important aspect of reducing the cost of access to space is the reduction of launch vehicle weight. Liquid hydrogen (LH2) tanks can be the largest structural component of a launch vehicle and the design of lightweight hydrogen tanks is important to reducing the cost of space access. Polymer Matrix Composite (PMC) hydrogen tanks have been proposed as an enabling technology for reducing the weight of launch vehicles. A significant development issue of these composite structures is the leakage of hydrogen through the tank wall. Hydrogen is difficult to contain due to its small molecular size. Containment is critical due to its chemical reactivity. Concentrations of hydrogen in air above 4 percent by volume are flammable and hydrogen can detonate in air when concentrations reach 18.3 percent by volume [2]. Since the open cavities that may be filled with hydrogen are dependent on launch vehicle concepts, the acceptable hydrogen leak rate varies with each vehicle concept definition. For the National Aerospace Plane (NASP) and Single-Stage-toOrbit (SSTO) vehicle definitions [3], acceptable minimum leak rates for the hydrogen tanks were based on the total level of leakage expected through fittings and valves and was calculated to be 10 to 10 SCC/sec.-in. The rate of hydrogen leakage can be a function of the material, the method of fabrication used, the internal damage-state of the material, mechanical load the tank must react, and operational temperature. Typical permeability tests are performed on small coupon specimens, without the complexities of mechanical or thermal loads, using helium or hydrogen as a test gas [4]. Although these tests are useful for screening materials and fabrication processes, they do not address the important issue of determining the in-situ rate of hydrogen leakage in built-up structural components exposed to temperature and mechanical loads. Figure 1. The X-33 structural arrangement showing quad-lobed LH2 tanks and dual-lobed liquid oxygen (LOX) tank.