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SEP 2018

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87 CompositesWorld.com HEEET: A Broader Mission "e material we use to fill the gaps is essentially the same as for the tiles and woven on the same loom," says Venkatapathy. When FMI cut the BRM fabric into 22 tiles, it also cut 30-plus gap fillers. However, the gap fillers received an extra step to make them more compliant. All parts were then machined and sent to NASA for assembly. Tiles were placed and bonded to the underlying spacecraft structure using Solvay Composite Materials' (Alpharetta, GA, US) high-temperature (resistant to 500°F/260°C) HT 424 film adhesive. e assembly was taped to hold the parts in place, then vacuum- bagged and oven-cured. "We install the tiles oversize and then route out channels at the edges into which we place the gap fillers, using a nitrile phenolic film adhesive," Ellerby describes. e bonded gap fillers also were taped in place, vacuum bagged and oven-cured. HEEET's final dual-layer design was driven by two key constraints. e first was to maintain the temperature at the TPS- structure bondline below the film adhesive's 500°F maximum. "We had an added constraint where we didn't want to expose the insulation layer during entry," Ellerby explains. "In other words, we didn't want the outer layer to recede all the way through. ese two constraints drove the sizing of the ablative vs. insulating layer." For the ETU, final thicknesses were as follows: recession layer, 1.5 cm; insulating layer, 3.8 cm. Fine-tuning layers for future performance Although HEEET's interlocked layers improve aerothermal performance, they were challenging to model. But, Ellerby says, "We have developed both structural and aerothermal models of the system, providing the tools future missions will need to design with. We can determine the thickness of the layers for each mission and spacecraft, accounting for factors, such as the atmosphere of the planet we are entering, vehicle shape and entry speed." "Having one system, which can be tailored, drives down quali- fication and certification costs," says Venkatapathy. Another cost savings is in testing. "In the baseline design, the insulation layer is never exposed, thus, it didn't need to be tested alone," Venkata- pathy explains. "It added another design constraint that was a bit more difficult to model, but the testing was simpler, almost halving the number of tests required." Venkatapathy concedes there are limits to HEEET tailorability. "Right now, we are only varying thickness of the two layers. Our computer models don't currently allow changing the fabric layer densities." But Ellerby points out that there are so many parame- ters that potential combinations are almost infinite. "We screened a lot of weaves and densities and chose a combination that addressed the higher priority missions. While there are certainly missions that could benefit from increased tailorability, we had project milestones and a limited budget." He says the computa- tional material science tools to vary all of the parameters simulta- neously are still in development. "Ultimately, our vision is to have this complete tailorability," says Venkatapathy. CW senior editor Ginger Gardiner has an engineering/ materials background and more than 20 years of experience in the composites industry. ginger@compositesworld.com Secondary bonding to secure joints After oven cure, joints between the tiles are routed (right), resin infused gap fillers (left) are placed into these grooves and bonded with film adhesive during a second cure cycle. Source | NASA Ames

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