| Reinforced
Carbon-Carbon
RCC fabrication begins
with a rayon cloth graphitized and impregnated with a phenolic
resin. This impregnated cloth is layed up as a laminate and cured in
an autoclave. After being cured, the laminate is pyrolized to
convert the resin to carbon. This is then impregnated with furfural
alcohol in a vacuum chamber, then cured and pyrolized again to
convert the furfural alcohol to carbon. This process is repeated
three times until the desired carbon-carbon properties are achieved.
To provide oxidation
resistance for reuse capability, the outer layers of the RCC are
converted to silicon carbide. The RCC is packed in a retort with a
dry pack material made up of a mixture of alumina, silicon and
silicon carbide. The retort is placed in a furnace, and the coating
conversion process takes place in argon with a
stepped-time-temperature cycle up to 3,200º F. A diffusion reaction
occurs between the dry pack and carbon-carbon in which the outer
layers of the carbon-carbon are converted to silicon carbide
(whitish-gray color) with no thickness
increase. It is this
silicon-carbide coating that protects the carbon-carbon from
oxidation. The silicon-carbide coating develops surface
cracks caused by differential thermal expansion mismatch, requiring
further oxidation resistance. That is provided by impregnation of a
coated RCC part with tetraethyl orthosilicate. The part is then
sealed with a glossy overcoat. The RCC laminate is superior to a
sandwich design because it is light in weight and rugged; and it
promotes internal cross-radiation from the hot stagnation region to
cooler areas, thus reducing stagnation temperatures and thermal
gradients around the leading edge. The operating range of RCC is
from minus 250º F to about 3,000º F. The RCC is
highly resistant to fatigue loading
that is experienced during ascent and entry.
| But is it (or was it ever designed to be)
impact
resistant? Or will it just shatter on impact, remain in situ
BUT then be very vulnerable to rapid oxidization because it's
lost the integrity of its protective silicon-carbide coating? |
The RCC panels are
mechanically attached to the wing with a series of floating joints
to reduce loading on the panels caused by wing deflections. The seal
between each wing leading edge panel is referred to as a T-seal. The
T-seals allow for lateral motion and thermal expansion differences
between the RCC and the orbiter wing. In addition, they prevent the
direct flow of hot boundary layer gases into the wing leading edge
cavity during entry. The T-seals are also
constructed of RCC.
Since carbon is a good
thermal conductor, the adjacent aluminum and the metallic
attachments must be protected from exceeding temperature limits by
internal insulation. Inconel 718 and A-286 fittings are bolted to
flanges on the RCC components and are attached to the aluminum wing
spars and nose bulkhead. Inconel-covered cerachrome insulation
protects the metallic attach fittings and spar from the heat
radiated from the inside surface of the RCC wing panels.
The nose cap thermal
insulation uses a blanket made from ceramic fibers and filled with
silica fibers. HRSI or FRCI tiles are used to protect the forward
fuselage from the heat radiated from the hot inside surface of the
RCC.
During flight operations,
damage has occurred in the area between the RCC nose cap and the
nose landing gear doors from impact during
ascent and excess heat during entry. The HRSI tiles in
this area are to be replaced with RCC.
In the immediate area
surrounding the forward orbiter/ET attach point, an AB312 ceramic
cloth blanket is placed on the forward fuselage. RCC is placed over
the blanket and is attached by metal standoffs for additional
protection from the forward orbiter/ET attach point pyrotechnics.
|