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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 12 January 2010, is one of a series of advanced technology reports produced in FY 2009 under the Advanced Aerospace Weapon System Applications (AAWSA) program. It reviews materials for launch vehicles, space vehicles and reusable rocket engines, including aluminum alloys, polymer and metal matrix composites, titanium and nickel alloys, ceramics and titanium aluminides. It concludes that newer materials and design methods offer many ways to improve structural efficiency and cost compared with the space shuttle.
UNCLASSIFIED/,fli'OA: Oli'Fl6l111L W&C 8'1Ua>C leaving voids. This limitation makes it difficult to use many thermoplastic resins that are otherwise attractive because they are recyclable and have much longer shelf lives than thermosets (epoxies}. Another, more obvious limitation of RTM is component size. This is in part because weaving of very large fiber preforms is challenging and requires a very large weaving machine. It also requires a large injection machine capable of multiple injection sites to ensure the complete infiltration necessary to avoid formation of voids. In practice, there are several significant challenges associated with manufacturing large PMC structures today. Among these, perhaps the most significant are manufacturing cost and the difficulty associated with making large, nonaxisymmetric components. Large axisymmetric components that can be produced by winding tows of pre-preg are clearly possible today, as demonstrated by the Boeing 787 fuselage whose section is shown in Figure 2. For other shapes, because the load-bearing capability of PMCs depends on the transfer of externally imposed loads to the strong, stiff fibers, joints that intersect the principal load path become problematic. This is because the fibers and, as a direct consequence1 the load path are discontinuous, requiring local section size adjustments to offset this local structural inefficiency. Thus 1 achieving the most structurally efficient use of PMCs requires monolithic structures with continuous fibers. In large structures, this is at best a challenge. Even in the Boeing 787's PMC fuselage, mechanically fastened joints are used to connect adjacent barrel sections. In the fuselage, as in any cylindrical pressure vessel, the principal stresses are hoop stresses stemming from pressurization during flight. However, the compromise in structural efficiency is minimized by the circumferential orientation of the joints. Although these bolted joints add weight, the overall structural efficiency of the PMC structure is still better, albeit considerably more expensive, than a longitudinally and circumferentially stiffened Al structure. The circumferential joints also create discontinuous longitudinal crack paths that improve the structure's damage tolerance. Some of the added expense of the PMC fuselage stems from the use of Ti alloy fasteners because of the galvanic coupling issues that would accompany Al or steel fasteners. For other applications, which are limited by different material properties, the PMC system can be tailored to optimize structural performance. This is possible because both the matrix and the fiber can be independently selected. Moreover, the fiber \\architecture" (fiber orientation, weave geometry, and fiber volume fraction) can be varied spatially to optimize load-bearing capability under complex stress states. For example, again drawing on recent applications in commercial aircraft, the fan blades of the large, high-bypass-ratio turbofan engine (GE90) produced by General Electric for the Boeing 777 are made from PMCs. The limiting design consideration for these fan blades is resistance to bird strikes. To optimize the PMCs' impact resistance, a medium- modulus, high-tensile-strength carbon fiber was selected in combination with a thermoplastic toughened epoxy matrix. Furthermore, the fiber architecture was set to optimize the bending strength under the impact of a bird. The GE90 fan blades are produced by hand layup and are quite costly to produce. In the 10-plus years that these fan blades have been in service, not a single unscheduled engine removal related to the PMC fan blades has occurred. The competitive fan blade technology for B-777- class engines is hollow Ti, which is used by both Pratt & Whitney and Rolls Royce. By all informal accounts, these blades are cost intensive. This example supports the unwritten rule that the pathways leading to high-performance, high-value structures typically are 5 UNCLASSIFIED//FOlil GFFIGI Pk Wfi& tUUA..
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 27 pages are in the text index: search them above, or from the library's search.