DOW-UAP-D120, AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

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UNCLASSIFIED/ /Pel\ OPPICIAL USE ONLY Materials for Advanced Aerospace Platforms Prepared by: Acquisition Support Division (DW0-3) Defense warning Office Directorate for Analysis Defense Intelligence Agency AAP Person 66 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, Defense Warning Office's Advanced Aerospace Weapon System Applications (AAWSA Program. Comments or questions pertaining to this document should be addressed to MP Person 1 AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DW0-3, Bldg 6000, Washington,

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Reusable Crew Modules...................................................................................... 9 Reusable Single-Stage-to-Orbit Vehicles ...................................................... 10 Advanced Al Alloys ....................................................................................... 11 Polymer Matrix Composites.......................................................................... 11 Al Matrix Composites.................................................................................... 12 Ti Matrix Composites.................................................................................... 14 Refractory Metal Alloys ................................................................................ 18 Ceramic Matrix Composites .......................................................................... 18 Propulsion Systems.......................................................................................... 21 Summary and Recommendations ..................................................................... 22 1. Schematic Diagram of Friction Stir Welding ....................................................... 2 2. Specially Modified 747 Transporter Unloading a Boeing 787 Composite Fuselage Barrel Section ..................................................................................... 4 3. Micrograph Showing Phase Formation at the Surface of a Ti Alloy That has Been Exposed to Air at Elevated Temperature ................................................. 13 4. Cross-Section Micrograph of a Ti Matrix Composite.......................................... 15 1. Potential Materials by Use Temperature Regime and Property ......................... 10 2. Example of Properties of Ti Matrix Composites ................................................ 14

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Materials for Advanced Aerospace Platforms "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 directly 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 inspection capabilities. 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

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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.

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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 (Li)-bearing alloys, such as Al alloy 2090. This alloy contains enough Li to reduce its density by 8