Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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~-Ti alloys would be an attractive option for a launch vehicle with a very heavy payload.
These alloys can be processed and heat-treated to ultimate tensile strengths greater
than 1,300 mega-Pascals (MPa), making them very efficient structural alloys. However,
at these strength levels, the fracture toughness is decreased to about 40 MPa-m0 •5,
making damage tolerance marginal. For example, using a design stress that is two-
thirds that of ultimate tensile strength, the critical crack size for an alloy with these
properties is about 4 millimeters. Such a small critical crack size poses a challenge to
any required field inspections associated with reusability requirements. p-Ti alloys'
stiffness also is as much as 10 percent lower than that of a+P alloys. Ten percent lower
stiffness generally is not an issue in a tension-loaded structure, but it can be an issue
for a compression-loaded structure because of the potential for buckling. Designs with a
higher section modulus can eliminate this concern, but the additional shape complexity
will almost certainly add cost. In extreme circumstances, Ti alloys can be reinforced
with ceramic fibers (typically silicon carbide) to increase their intrinsic stiffness, as will
be discussed later.
Ti alloys are reactive when exposed to air at temperatures of 550 °Celsius or above.
Consequently, any manufacturing operations that exceed this temperature limit must
be performed in a protective atmosphere of argon (Ar) or helium (He) gas. An
exception· occurs during forging if enough excess material is left on the raw forging to
contain the oxygen-contaminated layer so it can be machined away during the final
machining of the finished component. How to deal with this reactivity issue is well
understood, and it poses no concern other than the additional costs associated with the
excess material and additional machining. In other operations, such as welding, use of
specially designed fixtures incorporating inert shielding gas also effectively eliminates
concerns about oxygen contamination. In hot-forming applications, the as-formed part
is typically chemically milled to remove the oxygen-rich surface layer because this layer
typically has lower ductility and can cause fatigue cracking in service. The practice of
eliminating all oxygen-contaminated material has served the aerospace industry well
over the years but is quite conservative and restrictive. This matter is discussed in
greater detail in the Reusable Reentry Vehicle section of this document.
In summary, although the choices of materials for launch vehicles are in principle
numerous, 1n practice these choices are reduced by a variety of application-specific
considerations that include manufacturing capability for large components and
manufacturing cost. The foregoing discussion has attempted to examine the prospects
for advanced Al alloys, PMCs, and Ti alloys in light of these perceived practical
constraints. On a case-by-case basis, a variety of requirements are imposed by design
constraints, whfch some materials meet more readily than others. In all cases,
discussion of available material options was constrained by the assumption that the
maximum service temperatures would be relatively low. Consequently, the material
classes discussed here are all intended for relatively low-temperature use. The separate
case of a reusable single-stage-to-orbit vehicle, where operating temperature
requirements are considerably higher, is discussed in a later section of this document.
Ultimately, materials are selected to optimize structural performance, and a coordinated
approach of materials selection and geometric design is essential to this. Going forward,
a design using a synthesis process that treats form, fit, function, and materials
capability as equal constraints is needed to achieve true optimum structural efficiency.
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