Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

UNCLASSIFIED, /POR OPPICIAE USE JIit I
Table 1 shows there are essentially 10 distinct classes of candidate materials for use in
a reusable SSO vehicle. Their usefulness for specific applications and components
depends on the operating temperature regime and the design-limiting material
property. Some background and characteristics of each of these materials are described
in this section. Applications of some of these materials have already been addressed, so
the discussion here is limited to applications for a reusable SSO vehicle.
Adva need Al Alloys
This class of materials is limited by its temperature capability because, even if used in
an embedded structure, the thermal soakback from the hot external structure can lead
to softening in real time or averaging during extended exposure (that is, after several
mfssions). Scientists have worked to develop high-temperature powder-metallurgy Al
alloys, mainly alloys containing Fe, cobalt (Co) and cesium (Ce) or Mn and Si. These
alloys are not routinely produced today owing to a lack of demand stemming in part
from their cost and their relative lack of maturity as judged by the high variability in
mechanical properties between lots of material. Consequently, the main application for
Al alloys is in the crew compartment itself1 where temperatures must be maintained at
levels that are tolerable for the human occupants. Al alloys have marginal stiffness,
even on a density-corrected basis. However, using fabricated panels with Al face sheets
and a lightweight core can minimize this limitation. Such panels have a high section
modulus, which increases the structural stiffness without adding much weight. Earlier
uses of phenolic honeycomb cores experienced only limited success because of the
tendency of the core to absorb water from the environment if a breach of the face sheet
or the face sheet core bond occurred. Today, Al phenolic honeycomb sandwich
construction is unpopular among aircraft and spacecraft designers because of this
earlier, unfavorable experience. However, a new possibility for lightweight stiff
structures-Al face sheets with a foamed Al alloy core-is worthy of consideration.
Considerable progress in making uniform-density Al and other metallic foams has been
realized in the past 10 years. This new class of sandwich materials is sufficiently
different from the earlier versions to merit a careful evaluation.
Polymer Matrix Composites
As described in the Launch Vehicle section, polymer matrix composites have excellent
strength, stiffness, fatigue resistance, and fracture toughness. All polymeric materials
are to some degree susceptible to degradation when exposed to ultraviolet (UV)
radiation. For an orbiting vehicle, a major limitation is the effects of UV radiation on the
polymer matrix. Given their limited time at altitude, this vulnerability is not an issue for
launch vehicles; however, it is a concern for structures that remain in orbit for an
extended time at altitudes where the UV intensity is much greater. Although there are
coatings that protect substrates (PMCs in this case) from UV radiation, the risk of these
coatings being breached and the uncertainty about their effectiveness in orbit make UV
exposure an ongoing concern. Any consideration of using PMCs for exterior applications
would need to include an evaluation of their effectiveness. As discussed for Al alloys,
PMCs also could be considered for interior applications where temperatures are within
the material capabilities. As discussed earlier, some polyimide resins are usable up to
about 300 °Celsius; therefore, the low density and excellent specific stiffness of PMCs
make them viable candidates for some components. As also discussed, some
polyimides, such as PMR-15, contain methylenedianiline, and care must be taken to
11
UNCLASSIFIED;"/ P8ft err1e1A1s l!t&i tHllr,lf