CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
FEBRUARY 2016 60 CompositesWorld FOCUS ON DESIGN By Johanna Knapschaefer / Contributing Writer NASA/Boeing composite launch vehicle fuel tank scores firsts Subscale 5.5m-diameter cryogenic tank demonstrator with innovative fluted-core skirt is formed via robotic AFP and cured out of the autoclave. » For more than 50 years, heavy metal cryogenic tanks have carried the liquid hydrogen (LH 2 ) and oxygen necessary to launch vehicles into space. But in a joint efort, NASA and Te Boeing Co. (Chicago, IL, US) have designed, fabricated and tested a composite cryotank that, if scaled up to current space launch system dimen- sions, would weigh 30% less and cost 25% less than the best aluminum-lithium cryotanks used today, and could warrant transport of as much as 1,400 kg of additional payload to low-Earth orbit and beyond. Te US$25 million Composite Cryotank Technologies and Demonstration (CCTD) project, part of the NASA Space Tech- nology Mission Directorate's Game Changing Development (GCD) program, involved a team of Boeing and NASA engineers. "Tis is the frst efort to successfully build and test a tank of this scale," says Douglas McCarville, Technical Fellow at Boeing Research & Technology (BR&T;) in Seattle, WA, US. "Te tank would work for liquid oxygen or liquid hydrogen on a variety of next-generation launch systems." Te CCTD program goal was an inner tank that could contain pressurized LH 2 at cryogenic temperatures, ftted with an outer skirt that could take the axial compression loads during launch- vehicle takeof (see drawing, p. 61). Two cryotank demonstra- tors were built by Boeing and tested at the NASA Marshall Space Flight Center (Huntsville, AL, US). Final assembly of the latter was completed at the Boeing Developmental Center (Tukwila, WA, US) in April 2014. Prior tank failure leads to futed-core Te frst tank, at 2.4m diameter, was ftted with a laminate skirt and tested in 2013. But the second, 5.5m-diameter CCTD tank, featured an innovative futed-core design — a key part of the NASA/Boeing team's response to lessons learned during previous composite cryotank projects. Following a pressure test conducted Nov. 3, 1999, a large composite LH 2 fuel tank built by NASA and Lockheed Martin (Bethesda, MD, US) for NASA's half-scale experimental X-33 Venture Star space vehicle developed localized delaminations. Te tank was flled with LH 2 at -253°C to simulate mission-level pres- sures and loads, and the outer skin and honeycomb core debonded from the tank's inner skin in one tank lobe. Although the inner skin was undamaged, several fractures were observed in the outer skin in the same lobe. After extensive study, NASA determined that the delami- nations were likely caused by a combination of factors. Te inner carbon fber/epoxy unidirectional tape facesheet, which was designed to prevent permeation of hydrogen into the carbon honeycomb core, was inadequate and had micro-cracked under the low temperature and load, allowing hydrogen to collect in the core cells. Te ultra- cold hydrogen caused air in the cells to liquefy, which created a vacuum that pulled in purge gas from outside the core (a phenomenon called cryopumping). Cryopumping coupled with a degraded bondline between the CCTD cryotank: Significant progress for composites Douglas McCarville (top left), Brice Johnson (top right) and manufac- turing R&D; engineer Juan Carlos Guzman, (lower, center) of the Boeing Research and Technology project team pose with the fnished subscale 5.5m composite cryotank demonstrator, completed in fulfll- ment of NASA's Composite Cryotank Technologies and Demonstration project. The design, materials and manufacturing technologies are unprecedented and, says NASA and Boeing, are scalable to full-size tanks used on today's launch vehicles. Source | Boeing R&T;