AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

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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 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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