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Source: Cobham Life Support focus on design Source: Cobham Life Support the stiffness of the external part of the tank. This would have been much more difficult to achieve in aluminum or titanium than with a tailored composite layup, especially with mass constraints. Also, the ability to lay the skirt up in pieces enabled almost any shape." A conventional tank skirt would be a straight barrel of a constant diameter, but Cobham devised a novel tapered section. Pemberton says, "Although it complicated design and fabrication, it provided an elegant solution to the mass, space and volume requirements. Because we could control the geometry of the skirt, it made the geometry of the spacecraft simpler, which is definitely what you want in terms of cost and schedule for the program as a whole." Unidirectional prepreg was used to meet stiffness-to-weight requirements, but it was difficult to drape and form to the tapered section without wrinkling. The skirt layup was symmetrical and quasi-isotropic, varying from 48 plies at the bolt inserts to 24 plies at the bonded interface with the tank and required precise placement of roughly 1,200 individual pieces. "The pieces had to bend around the circumference and lean into the taper," Pemberton explains. "We used hand calculations and experience to develop the necessary ply shapes that prevented wrinkling and minimized distortion." Vacuum debulks, used after the first and third plies and a minimum of once every 12 plies thereafter, also helped. "Composites also enabled us to tailor the modulus of the skirt in the hoop direction," says Pemberton, "so that it was enough to handle the tank pressurization loads and act like a belt but not so much that it crushed the tank." Tailored to optimize performance The winding was tailored to optimize stresses and achieve required burst strength without adding unnecessary material. Here, winding begins with low-angle helical courses. 64 | Complex skirt carefully designed The carbon fiber/epoxy skirt layup required roughly 1200 individual prepreg pieces to achieve its final complex shape. The photo at right shows templates used to cut plies for the skirt. Grande adds, "That flexibility to constrain the modulus in one direction but not in the other helped us achieve the design we needed." Tailorability also ensured that the tank met a key launch requirement: the natural frequencies of the assemblies must not contribute to spacecraft vibration. "This became an area for composites to shine," notes Estes, "because there was a change in the vibration requirements late in the game, which significantly increased the minimum natural frequency required. Such a design change with aluminum or titanium would have required scrapping the already developed tooling and starting over. But with composites, the change was met by adjusting thickness and layup." Thus, the massive change made a minimal impact. "We were able to beat the program schedule and meet cost and weight requirements as well," Pemberton recalls. here in order to achieve overall efficiency in the design process." Cobham also streamlines the statistical approach to developing material properties, putting strain gauges on pieces as early as possible and throughout development so computer models are updated with real properties and are continuously verified as the design progresses. Grande summarizes, "In the end, we cut the number of required destructive tests by almost half, saving months of time and roughly $500,000." Estes credits one more factor: "Cobham is kind of unique in that the guy doing analysis one day will hand lay a skirt part the next day and apply a strain gauge and run a stress/strain test the next. Thus, the folks doing the design ... see the connection between manufacturing, material properties and structural performance," he says. "This improves the analysis and keeps them focused on what's critical." Intelligent analysis According to Grande, one of Cobham's major achievements was that there was never a huge redesign or breakdown in progress. This was due, in part, to their development philosophy. "We chose to introduce novel solutions in the design, not the materials and processes," he explains, "but we are also very efficient in computer-aided engineering." Cobham starts with closed-form computer-aided calculations, or "hand calcs," because, says Grande, "Finite element analysis [FEA] is only as good as the data fed into it. This avoids easy mistakes and enables us to start with sound fundamentals in our loads, geometries and boundary conditions." Pemberton adds, "We then compare the hand calcs to the FEA. This is a key step that many companies overlook but there must be good correlation high-performance composites Future fuel tanks Validated via vibration and nondestructive burst-proof testing, the tank has been integrated into the GPM Core for launch in 2014. "By using this ... demiseable design," says Grande, "we increased the life from five to seven and maybe even 10 years, possibly doubling the amount of data collected per investment into the vehicle." Learn More @ www.compositesworld.com Read this article online | http://short.compositesworld.com/Mkb30XL9. Read more about the GPM mission online at http://short.compositesworld.com/lweJFnsD.