Reusable Surface Insulation Tiles
The HRSI tiles are made
of a low-density, high-purity silica 99.8-percent amorphous fiber
(fibers derived from common sand, 1 to 2 mils thick) insulation
that is made rigid by ceramic bonding. Because 90 percent of the
tile is void and the remaining 10 percent is material, the tile
weighs approximately 9 pounds per cubic foot. A slurry containing
fibers mixed with water is frame-cast to form soft, porous blocks
to which a collodial silica binder solution is added. When it
is sintered, a rigid block is produced that is cut into quarters
and then machined to the precise dimensions required for individual
HRSI tiles vary in thickness from 1 inch to 5 inches. The variable
thickness is determined by the heat load encountered during entry.
Generally, the HRSI tiles are thicker at the forward areas of
the orbiter and thinner toward the aft end. Except for closeout
areas, the HRSI tiles are nominally 6- by 6-inch squares. The
HRSI tiles vary in sizes and shapes in the closeout areas on the
orbiter. The HRSI tiles withstand on-orbit cold soak conditions,
repeated heating and cooling thermal shock and extreme acoustic
environments (165 decibels) at launch.
For example, an HRSI tile taken from a 2,300º F oven can be immersed
in cold water without damage. Surface heat dissipates so quickly
that an uncoated tile can be held by its edges with an ungloved
hand seconds after removal from the oven while its interior still
The HRSI tiles are coated
on the top and sides with a mixture of powdered tetrasilicide and
borosilicate glass with a liquid carrier. This material is sprayed
on the tile to coating thicknesses of 16 to 18 mils. The coated
tiles then are placed in an oven and heated to a temperature of
2,300º F. This results in a black, waterproof glossy coating
that has a surface emittance of 0.85 and a solar absorptance of
about 0.85. After the ceramic coating heating process, the remaining
silica fibers are treated with a silicon resin to provide bulk waterproofing.
Note that the tiles cannot withstand airframe load deformation;
therefore, stress isolation is necessary between the tiles and
the orbiter structure. This isolation is provided by a strain
isolation pad. SIPs isolate the tiles from the orbiter's structural
deflections, expansions and acoustic excitation, thereby preventing
stress failure in the tiles. The SIPs are thermal isolators made
of Nomex felt material supplied in thicknesses of 0.090, 0.115
or 0.160 inch. SIPs are bonded to the tiles, and the SIP and tile
assembly is bonded to the orbiter structure by an RTV process.
Nomex felt is a basic aramid fiber. The fibers are 2 deniers
in fineness, 3 inches long and crimped. They are loaded into a
carding machine that untangles the clumps of fibers and combs
them to make a tenuous mass of lengthwise-oriented, relatively
parallel fibers called a web. The cross-lapped web is fed into
a loom, where it is lightly needled into a batt. Generally, two
such batts are placed face-to-face and needled together to form
felt. The felt then is subjected to a multineedle pass process
until the desired strength is reached. The needled felt is calendered
to stabilize at a thickness of 0.16 inch to 0.40 inch by passing
through heated rollers at selected pressures. The calendered material
is heat-set at approximately 500º F to thermally stabilize the
The RTV silicon adhesive is applied to the orbiter surface in
a layer approximately 0.008 inch thick. The very thin bond line
reduces weight and minimizes the thermal expansion at temperatures
of 500º F during entry and temperatures below minus 170º F on orbit.
The tile/SIP bond is cured at room temperature under pressure
applied by vacuum bags.
Since the tiles thermally expand or contract very little compared
to the orbiter structure, it is necessary to leave gaps of 25
to 65 mils between them to prevent tile-to-tile contact. Nomex
felt material insulation is required in the bottom of the gap
between tiles. It is referred to as a filler bar. The material,
supplied in thicknesses corresponding to the SIPs', is cut into
strips 0.75 inch wide and is bonded to the structure. The filler
bar is waterproof and temperature-resistant up to approximately
800º F, topside exposure.
SIP introduces stress concentrations at the needled fiber bundles.
This results in localized failure in the tile just above the RTV
bond line. To solve this problem, the inner surface of the tile
is densified to distribute the load more uniformly. The densification
process was developed from a Ludox ammonia-stabilized binder.
When mixed with silica slip particles, it becomes a cement. When
mixed with water, it dries to a finished hard surface. A silica-tetraboride
coloring agent is mixed with the compound for penetration identification.
Several coats of the pigmented Ludox slip slurry are brush-painted
on the SIP/tile bond interface and allowed to air-dry for 24 hours.
A heat treatment and other processing are done before installation.
The densification coating penetrates the tile to a depth of 0.125
inch, and the strength and stiffness of the tile and SIP system
are increased by a factor of two.
There are two different densities of HRSI tiles. The first weighs
22 pounds per cubic foot and is used in all areas around the nose
and main landing gears, nose cap interface, wing leading edge,
RCC/HRSI interface, external tank/orbiter umbilical doors, vent
doors and vertical stabilizer leading edge. The remaining areas
use tiles that weigh 9 pounds per cubic foot.