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The nature of the environment in outer space and its significance
for materials of construction of astronautic vehicles are considered.
The most advanced experience with heat-resisting engineering
materials has been gained in gas turbine applications. The potential
developments towards higher operating temperatures of alloys based
on iron, nickel and cobalt are approaching exhaustion. The next stage
may use the higher melting point metals, especially molybdenum,
columbium and tungsten, non-metallics such as carbon and ceramics,
or combinations of metals and ceramics. The s refractory metals are
capable of stressed service at 2500[degrees]F. (1370[degrees]C.) and higher, if means
of protection against oxidation can be found. On the same condition
graphite can be used for much higher temperatures.
For the ballistic missile, ablation of surface layers on the nose
cone offers the best prospect of successful heat-dissipation. The
ablating material may be an organic material, e.g. synthetic resin, or
a ceramic compound. For longer spells at high temperatures, as in
satellites on re-entry, the alternatives are thermal insulation by nonmetallic
surface coatings, and skins of metals having very high melting
points. Coatings which provide insulation and protection from oxidation
are provided as flame-sprayed ceramic oxides, especially alumina and
zirconia, or ceramics reinforced by a refractory metal grid attached to
the base metal.
The major technical difficulties in applying the refractory metals to
service at very high temperatures arise from their reactivity with ambient
gases, especially oxygen, and their tendency to brittleness at low and
moderate temperatures.
Characteristics of materials which acquire special importance in
astronautic applications are: thermal conductivity, specific heat, latent
heat of fusion and evaporation, coefficient of thermal expansion, reactivity
at high temperatures, sensitivity to irradiation, creep strength and
resistance to high fatigue stresses at high temperatures and mechanical
properties at low temperatures. |
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