CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
61 CompositesWorld.com Cryotank's Fluted Core Skirt inner facesheet and the core, a piece of polytetrafuoroethylene (PTFE) tape found on the core in the debond area, and other defects, contributed to the failure. Signifcantly, the tank report noted that the results of the inves- tigation did not invalidate the use of composite materials for cryo- genic tanks. It was reportedly no surprise to the X-33 project's engineers that the composite LH 2 tank could fail during testing because there was little industry experience with large-scale composite structures. Despite the delamination incident, NASA and Lockheed Martin still believed in the promise of composites. To prevent such delaminations in future composite cryotanks, the CCTD team, leveraging Boeing's experience producing composites for aircraft, took several unusual steps. Instead of the gantry-based automated fber placement (AFP) equipment classically used to build large cylindrical parts, Boeing opted to use robotic AFP. Boeing also trialed new materials that don't require expensive autoclave curing, and it completely reconsidered its skirt design. Prior to the CCTD tank, AFP heads with self-contained material spools on relatively low-cost robotic platforms were generally considered "right-sized" equipment for small, special jobs. Te large fuselage and rocket component work went to traditional machines with an onboard creel. "Ironically, the robotic fber placement system enabled improved capabilities — better reach in the dome areas and lower shaft clearance," notes Brice Johnson, another Technical Fellow at BR&T.; And robotic AFP, he claims, also can be scaled up to build the full-size 8.4m cryotank and skirt combination that would be required for NASA's currently used space launch systems. C r y o t a n k ' s F l u t e d C o r e S k i r t Illustration / Karl Reque NASA/Boeing CCTD Cryotank with Fluted-Core Skirt › Robotic AFP technology proves more reliable during layup of complex curved surfaces in cryogenic tank dome areas. › Fluted core design features co-cured facesheet/ core construction, eliminating potential adhesive failure at cryogenic temperatures. › Open passageways created by trapezoidal fute design enables venting of skirt in the event of tank leakage. Tank dome Softening strip Skirt Fluted core Tank dome SKIRT BARREL Softening strip at dome/skirt tangent point Co-cured facesheet/ core interface eliminates adhesive bond lines FLUTED-CORE SKIRT CROSS SECTION Tank layup — combined thick (145 g/m 2 ) and thin (70 g/m 2 ) plies form crack-resistant laminate Trapezoidal design exhibits both axial and compressive strength and enables venting 5.5m DIAMETER CRYOTANK HALF-SECTION