SEP 2018


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SEPTEMBER 2018 86 CompositesWorld FOCUS ON DESIGN Two-layer, 3D-weave "We decided to differentiate the top layer from the bottom layer," Venkatapathy explains. "e top layer, exposed to high heat flux, would be pure carbon fiber and very dense." It would re-radiate heat and act as an ablative layer, burning away in a controlled manner, carrying heat away from the spacecraft, while leaving char to provide insulation. e bottom layer would be less dense and mix CF with lower-density phenolic yarn to act as an addi- tional insulating layer. "We looked at various weaves as well as types of carbon fiber and other yarns," he adds. Six-inch samples were tested over two years for aerothermal performance, using an arc jet (hot plasma) wind tunnel that mimics spacecraft-entry conditions. AS4 carbon fiber (Hexcel, Stamford, CT, US) was chosen due to its wide availability. For the CF/phenolic hybrid, a commingled yarn was produced by carding and stretch-breaking the two fiber types, then blending the filaments. "e phenolic pyrolizes and goes away so that only char is left, basically the same as carbon fiber," Ellerby explains. e selected weave was a 0°/90° fabric, interconnected layer- to-layer, says Curt Wilkinson, program manager for NASA woven TPS at BRM. "e char we achieve is similar to the older style of carbon fiber/phenolic material, even though the density of our system is 40-50% less," says Venkatapathy. "is is what the 3D weave achieves — strength from layer to layer [interlam- inar strength]." e weave pattern wasn't difficult, says Wilkinson, but achieving the density NASA required was daunting. e number of yarn ends that might typically be introduced into a loom for an entire fabric — 1,400 — was the count per inch for the HEEET fabric. And NASA wanted a 24-inch width. "We had to design and build a new loom," says Wilkinson. Resin-infused tiles and gap-fillers e next steps involved fine-tuning the woven fabric's layers and designing the composite parts. e two were interdependent. Because the issues confronted in designing the fabric layers require an understanding of the heatshield construction, the latter is described first. Shield geometry is fairly standard — a flat top (nose) and curved sections comprising a 45° cone. However, shield size can vary with that of the spacecraft, determined by mission scope and distance. Today, that can range from the 1m-diameter Saturn probe to a probable 3m Venus lander. "is drove a tiled design for the heatshield construction, which had to be scalable," says Ellerby. e team used the Saturn probe as the design basis for an engineering test unit (ETU), completed in February 2018 and now in final testing and post-test analysis. e ETU comprises 22 cone sections, each a composite tile made by infusing cut and formed 3D woven fabric with phenolic resin. However, Ellerby explains, "the composite is not completely filled with resin, because we don't need a fully dense, resin- infused material. at's just extra weight." Developed by the HEEET team, the resin infusion process was transferred to Fiber Materials Inc. (FMI, Biddeford, ME, US), which has decades of experience building NASA heatshields, and fabricated the ETU components. "One of our objectives was to transfer the HEEET manufacturing processes into industry to establish the industrial base for future missions," Ellerby explains. Completed tiles were then bonded to the spacecraft structure, but there remained the issue of joining the tile edges. If hot gases can penetrate tile joints, the heatshield will disintegrate. "What you put between those tiles became the biggest challenge for both structural and aerothermal requirements," Ellerby confirms. Read this article online | Read more online about NASA's 3D-MAT program | Cutting, tiling and bonding 3D woven fabric is cut, molded into tiles (above) using resin infusion and then bonded to the spacecraft with film adhesive (right). Source | NASA Ames

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