Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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REUSABLE CREW MODULES
The concept of manned orbital crew modules has evolved from the Mercury capsules to
the Gemini and Apollo programs to the space shuttle, the first fully reusable crew
module. The shuttle also has a combined payload bay used for transporting satellites
into orbit and hardware for developing the International Space Station and for repairing
and refurbishing the Hubble telescope, among other uses. The reusable nature of the
shuttle crew module introduced a number of design and materials selection chaUenges.
Perhaps foremost among these is the requirement for a thermal protection system
(TPS} that would protect the crew during reentry and also minimize the intermission
refurbishment requirements of the spacecraft itself. During the early days of the shuttle
development program, there was much initial interest in a metallic TPS because it
appeared to better meet the program's needs. Ultimately, however, ceramic tiles were
used on the underbody and carbon-carbon composites (C-CCs) were used on the
leading edges of the wings. The shuttle design itself can therefore be characterized as a
"cold structure" with an insulating TPS. For example, much of the shuttle load-bearing
structure is made of the Al alloy 2219, in part because it is fusion weldable and in part
because it retains its strength at moderately elevated temperatures better than other
high-strength Al alloys can. The refurbishment needs of the ceramic shuttle tiles after
each flight reputedly are considerable and increase with vehicle age. The C-CC wing
leading edges are basically not repairable but require scrutiny. Hindsight shows that c-
CCs \'age1
' and lose much of their fracture toughness during repeated thermal exposure.
(This loss of toughness was a prime factor in the Columbia disaster. Had a metallic heat
shield that included the wing leading edges been used, this disaster arguably could
have been avoided.) If the discussion of a metallic TPS were held today, the outcome
might not be much different. Certainly any serious consideration of a reusable single-
stage-to-orbit vehicle today would need to reopen the discussion of a metallic TPS. The
challenges and opportunities associated with a metallic TPS are discussed later.
The design efficiency of an integrated TPS and load-bearing structure is extremely
attractive. Such a design requires availability of high-temperature alloys that also have
good strength at the moderate temperatures to withstand the aerodynamic and
vibrational loads encountered during launch and orbital insertion. Any attractive alloy
also must have reasonable intrinsic resistance to oxidation at the reentry temperatures
and should be capable of being fabricated into sheet gauges at reasonable cost.
Meeting these various requirements in combination becomes quite daunting. Earlier
programs such as the DynaSoar reusable reentry glider devoted considerable time and
resources to examining the use of refractory alloys such as Mo-0.5%Ti (Moly half Ti)
and several Columbia-based alloys for the TPS and some hot structure. All the
refractory metal alloys are solid solution strengthened and, consequently, have
relatively low ambient temperature strengths. They also are quite dense, making the
density-corrected strength even less attractive. Refractory metals and their alloys react
extensively when exposed to air at elevated temperatures. Therefore, even if the
mechanical property limitations could be overcome, any hot structure would require an
oxidation-resistant protective coating. In the case of the DynaSoar program, scientists
extensively investigated a surface conversion coating of MoSb formed by reacting the
Moly half-Ti alloy with Si powder in a high-temperature fluidized bed. With the benefit
of hindsight, it is now unclear how a large structure could have been successfully
coated in this manner. This is particularly true when the brittle nature of M0Si2 is
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