12 January 2010 ICOD: 1 December 2009 UNCLASSIFIED// PIR 8FFIGIC P P !SF AN! Intelligence Acquisition Threat Support Materials for Advanced Aerospace Platforms UNCLASSIFIED//FGA: QFFHilt tL W:81 8HLY UNCLASSIFIED ( (FOR OFFICJOP Materials for Advanced Aerospace Platforms Prepared by: Defense Intelligence Agency Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, !(b)(3):10 USC 424 ~dvanced Aerospace ........,,.c'-'---Jt'luestions pertaining to this document should be addressed to (b)(3):10 use 424;(b)(6) , AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b){S):10 use 424 Bldg 6000, Washington, UNCLASSIFIED,f,EF1iHI 81ifii&l1'1k H81i &Fdb\ UNCLASSIFIED/f Pelt 8PPleIAL 1:191 fJHLY 2. Specially Modified 747 Transporter Unloading a Boeing 787 Composite 3. Micrograph Showing Phase Formation at the Surface of a Ti Alloy That has 4. Cross-Section Micrograph of a Ti Matrix Composite 15 1. Potential Materials by Use Temperature Regime and Property 10 UNCLASSIFIEDJ,CFQA 8FFI&llal1k 1111 8tU1\' UNCLASSIFIED,S;F8R UFFIIIAI: W81 8Hbl/ Materials for Advanced Aerospace Platforms Introduction "Advanced aerospace platforms" is a broad topic that can be divided into several narrower subtopics to enable a more concise discussion of materials advances, challenges, and opportunities. Consequently, this document discusses the areas of launch vehicles, space vehicles, and space propulsion systems separately because their key requirements are often application specific, which affects materials selection decisions. In addition, single-use and reusable boosters have different durability requirements that dlrectly impinge on design and materials selection. Furthermore, current engineering practice has evolved to the point that design synthesis must integrate the structure and construction materials to achieve optimum product performance. For example, the space shuttle was designed to meet customer-imposed mission requirements (range, payload, empty weight, landing capability, and so forth) without significant real-time consideration of materials capability. This approach led to significant compromises at later stages in the shuttle's development and maturation. (Arguably, the shuttle could be designed as a more efficient vehicle today.) In the extreme, a spectacular engineering failure was the National Aerospace Plane (also dubbed the Orient Express), which was launched as a military project and was intended to be a mach 12 reusable strike vehicle. This project rapidly became materials limited and was canceled in 1993, after about $750 million in federal R&D expenditures and a substantial private sector investment. The point is that any "clean sheet of paper design" must start with an assessment of the requirements for construction materials and be accompanied by a realistic assessment of the capability of currently available materials to meet these needs. If these two assessments indicate a gap between requirements and existing materials capability, a risk assessment and a risk-mitigation plan must be developed before expending engineering hours and funds. Since the inception of manned space flight, the approach to design has changed to include the concept of damage tolerance. This shift in design philosophy was prompted by the (eventual) recognition that complex structures cannot be designed and produced with zero defects. With the maturation of fracture mechanics and means of reducing these concepts to practice, the transition from zero defects to defect tolerance became the norm. This new approach in turn led to recognition that high-performance materials required not only high specific strength and stiffness but pacing increases in strength with simultaneous improvements in fracture toughness and fatigue crack growth resistance. The introduction of damage tolerance was accompanied by a renewed emphasis on nondestructive inspectJon capabilltles. This latter thrust was driven by the need to demonstrate the capability to reproducibly locate small flaws that could become failure initiation sites, either because of static or because of cyclic loading conditions. In the case of atmospheric flight, the U.S. Air Force has introduced standards for airframes (the Aircraft Structural Integrity Program) and propulsion systems (the Engine Structural Integrity Program) that tie structural life and reliability to UNCLASSIFIED/ P8N: fJPPU!IIIII! U!II! 8HLif UNCLASSIFIED,C,f P8R: IPPllltllL "!IC 9HLI this demonstrated inspections capability. Implementation of these standards, starting with the B-1 bomber and the F-100 and F-110 engines, has dramatically reduced {but not eliminated) the incidence of catastrophic failures of critical components that endanger crews, vehicles, or both. Taking these changes into account, this document outlines the current situation regarding the design and production of high-performance structures for aerospace platforms, including launch vehicles, space vehicles, and propulsion systems for transporting space vehicles (and payloads) into orbit. UNCLASSIFIED//5OB OFFJCI0P 1 !SE ON! X UNCLASSIFIED//FOA 8FFlll,at tt91! CHEI Materials for Advanced Aerospace Platforms LAUNCH VEHICLES For the purposes of this document, launch vehicles are defined as the structure that supports and/or encloses the propulsion system, the fuel supply, and the crew or payload module. Launch vehicles today are either single use or multiple use after recovery and extensive refurbishment. This approach adds considerably to the cost of transporting a pound of payload into earth orbit, regardless of whether an unmanned satellite or a manned orbiting crew module that must withstand the temperatures and loads associated with safe reentry to earth. Furthermore, the larger the payloads are, the greater are the reaction forces the launch vehicle must withstand during launch. With the total weight of the payload, the empty weight of the launch vehicle, and fuel all needing to be lifted initially, fuel-efficient propulsion and lightweight launch vehicles are essential to maximizing the payload. Except in the area around the propulsion system exhaust, the temperatures experienced by launch vehicles during launch are not demanding. Therefore, advanced, high-strength aluminum (Al) alloys and polymer matrix carbon fiber composites (PMCs) are prime candidates for the parts of the structure that experience aerodynamic loads and where aerodynamic heating does not exceed about 125 Celsius. One class of advanced Al alloys is the lithium (U)-bearing alloys, such as Al alloy 2090. This alloy contains enough Li to reduce its density by 8 percent while increasing the elastic modulus (E) by 10 percent. Other, newer advanced Al alloys, such as 7050 and 2050, have been developed to have improved damage tolerance. These alloys have excellent specific strength at or near room temperature and experience no major loss of ductility at cryogenic temperatures. The newer variants of the 2000 and 7000 Al alloys also have substantially improved resistance to most types of corrosion, including exfoliation and stress corrosion cracking. This can be important in a reusable vehicle. Perhaps the most important aspect of the improved Al alloys is their higher fracture toughness, accomplished through a combination of alloy composition control and improved processing. In alloy composition control, the concentrations of the residual elements iron (Fe), chromium (Cr), manganese (Mn), and silicon (Si) are reduced at the ingot stage. These elements combine with Al to form hard, brittle intermetallic compounds known as constituent phases. The advanced alloys contain fewer, smaller constituent phases, leading to improved fracture resistance and higher fracture toughness values. In applications such as body skins for commercial aircraft improved toughness has enabled an increase in the spacing of the circumferential fuselage frames, or '\hat sections/ that serve both as stiffeners and as crack stoppers to prevent a catastrophic failure during pressurization. For any given operating stress- in this case the pressurization stress-the spacing of the frames is directly related to the critical crack size of the body skin. Higher toughness alloys have larger critical crack sizes, and the frames can be spaced further apart without increasing the risk of catastrophic failure. The increased spacing ultimately allows a fuselage design that requires fewer frames. Consequently, the airplane benefits from a commensurate reduction both in weight and in manufacturing cost. Similar possibilities exist for the design of a fail-safe launch vehicle that has a lower empty weight. Clearly advanced Al alloys offer intrinsic improvements over the alloys used in the Saturn launch vehicle and introduce the prospect of new, more efficient launch vehicle designs. UNCLASSIFIED/ FQA: QFFl&l O L WGliii 9.abac UNCLASSIFIED ,C,CFQA OFl=l&IAI:: t.J!II! 8Hl::Y The newer Al alloys also can be specially processed to render them superplastically formable. This capability opens a realm of possibilities to replace structures that, in the absence of this capability, are machined from thick plate. Very large structures are produced in sections that must be joined. Conventional fusion welding techniques do not work for high-strength Al alloys such as 7075, 7050, 2024, or 2050 because either the welds lead to cracks or the welds made under conditions that avoid cracking have greatly reduced tensile properties. Because these alloys are not amenable to welding, heavier, fatigue-prone mechanically fastened joints must be used. Recently, scientists developed a joining process that permits joining of Al alloys such as 7050. This process, called friction stir welding (FSW), allows joint designs in a variety of configurations that were not considered possible when fusion welding was the only alternative. In FSW, a rotating steel tool is inserted into the seam between the two Al alloy pieces to be joined. As the rotating tool is driven forward, the friction between the tool and the work piece generates enough heat to soften the Al alloy without melting it. A schematic of this process is shown in Figure 1. Figure 1, Schematic Diagram of Friction Stir Welding retreating The combined action of the rotation and the traversing of the tool essentially kneads the two pieces together, leaving a mechanically sound joint. Although the weld properties may be somewhat inferior to those of the base metal, they are good enough that a relatively small increase in thickness at the joint position can compensate. Although substantial development of the FSW process is ongoing, FSW already has been put into practice. For example, the current external propellant tank on the space shuttle is made from an Al-Li alloy fabricated through FSW. The weight advantage of using welded as opposed to bolted joints in a large structure such as a launch vehicle is considerable. With earlier high-strength alloys such as 7075, concerns about fracture toughness in conjunction with monolithic structures would have caused a welded UNCLASSIFIED},CFGA 8FFIGil1k U&i 8ftL\f UNCLASSIFIED//F&II &FFl&IAL . Hili aLY construction to be considered too risky. Today, the combination of higher toughness alloys and FSW opens up the possibility of greater design flexibility resulting in lighter large structures with equal or greater reliability than earlier ones. In sum, metallic, nonreusable (at least nominally so) launch vehicles made from advanced Al alloys and fabricated through FSW constitute an incremental but significant improvement over earlier versions. In recent years, PMCs have matured significantly. For many components that are not exposed to elevated temperatures, PMCs provide a degree of design flexibility not readily available in metals. Consequently, PMC materials have begun to supplant Al alloys in the construction of commercial subsonic aircraft. The use of PMCs in the empennage of the Boeing 777 was one of the first examples of Al alloys being displaced. Subsequently, the new Boeing 787 has more structure made from composites than from metallic materials. Once PMCs are introduced into a structure in significant quantities, a constraint related to galvanic incompatibility between the PMC structure and any adjoining Al alloys also is introduced. When a PMC structure is in direct contact with an Al alloy structure, catastrophic corrosion of the Al alloy components can occur. In the Boeing 787, the remedy for this concern is the use of titanium (Ti) alloys in areas where there is direct contact between the metallic and the PMC structures. This is directly analogous to the plastic bushing a plumber puts in the joint between copper and iron piping. Notwithstanding this constraint, the specific strength and stiffness of PMC structures make a compelling argument for their application in high-performance structures, such as launch vehicles. Composite structures can be manufactured using one of three methods: hand layup of pre-preg, automated tow placement, and resin transfer molding. The most rudimentary of these, but also the most flexible, is hand layup of pre-preg. This method uses sheets of material that contain both the fiber and the polymeric matrix (called pre-preg). The polymeric matrix can be either a thermoset (for example, epoxy) or a thermoplastic. Individual plies are cut from the pre-preg typically using a numerfcally controlled laser or mechanical cutting device and are laid up to form the desired shape. Areas that have heavier loads contain more plies locally, and the plies are cut in an orientation with respect to the fiber direction in the pre-preg to achieve the desired strength relative to the principal load path. These plies are carefully placed according to a drawing (blueprint), making hand layup a labor-intensive process and, therefore, making parts made using this method expensive. During ply placement, it is critical that no ply wrinkles are introduced, as these create severe reductions in the local load-bearing capability of the final component. Once all the plies are in their proper places, the article is placed in a vacuum-tight bag that is evacuated and placed in an autoclave for curing of the epoxy matrix or fusing of the thermoplastic. A disadvantage of a pre-preg whose matrix is a thermoset is limited shelf life. In practice, this is managed to a degree by storing the pre-preg in a freezer to slow the rate of chemical reaction that sets the epoxy. However, this does not completely halt the reaction, causing these materials to have a shelf life beyond which they are not easily manipulated during layup and do not develop full strength after curing in the autoclave. An additional issue is out time-the time the pre-preg can be out of the freezer during layup before the reaction proceeds at an accelerated rate and reaches a point at which the UNCLASSIFIED l;GFQa 8FFiifilJ11L lalOli ta.t UNCLASSIFIED,'/POft. 8PPIOIIIIL tj!IE BHl!.Y material is not suitable for the reasons stated earlier. Clearly, the time required for layup places practical limitations on component size. In automated tow placement, thin ribbons of a pre-preg are fed off a drum or rolled into a computer numerically controlled machine that places them in the desired position. In principle, this process trades recurring labor cost for up-front capital investment (the tow placement machine) and programming time. If the anticipated volume of identical parts is high enough to amortize the capital investment and, particularly, the programming cost, this can be an attractive means of reducing manufacturing costs. For axisymmetric shapes, such as cylinders, this essentially becomes a winding process and is quite efficient. An example of a finished composite fuselage barrel section for the Boeing 787 is shown in Figure 2. For more irregular three-dimensional shapes, such as a spar or a strut, placing the tows becomes much more difficult and presents a fundamental limitation. Consequently, PMC structures with complex shapes are still for the most part made using the hand layup process. A variant of automated tow placement is compression, whereby a preform, made by automated tow placement, is forced by a press into a preshaped die. This process allows fabrication of more complex shapes, but the rigidity of the fiber and the extreme anisotropy of the tows can lead to wrinkles, which are not acceptable because of the reductions in properties these cause. Figure 2. Specially Modified 747 Transporter Unloading a Boeing 787 Composite Fuselage Barrel The third main composite fabrication method, resin transfer molding (RTM), begins with a woven fiber mat or preform. The polymeric matrix is injected into this mat to create a full