Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SHUTTLE/THERMAL.TXT

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ORBITER THERMAL PROTECTION SYSTEM

     When a manned space vehicle re-enters the Earth's atmosphere,
air friction can produce external surface temperatures as high as
3,000 degrees Fahrenheit -- well above the melting point of steel.
Special thermal barriers are required to protect the vehicle and its
occupants.

     Earlier manned spacecraft, such as Mercury, Gemini and Apollo,
were not maneuverable and followed ballistic re-entry trajectories,
parachuting to a watery landing in the ocean. The space capsules were
protected during re-entry by shedding layers of a heavy, resinous
heat shield through a process called ablation. The spacecraft were
not reusable.

     For the Space Shuttle orbiter, a different kind of heat
protection system was needed. With a design life of about 100
missions, this revolutionary new space vehicle required a
lightweight, reusable Thermal Protection System composed of entirely
new materials.

     NASA selected four basic materials for the original design used
on Columbia, the first operational orbiter. Each was designed to
insulate the orbiter's aluminum and/or graphite epoxy skin against a
wide range of extreme temperatures, including a low of minus 250 F.
The basic materials were Reinforced Carbon-Carbon, Low- and High-
Temperature Reusable Surface Insulation tiles, and Felt Reusable
Surface Insulation blankets.

     Subsequent design improvements included use of advanced
materials in certain areas. These materials are Flexible Insulation
Blankets and Fibrous Refractory Composite Insulation.

     There are approximately 24,300 tiles and 2,300 Flexible
Insulation Blankets on the outside of each orbiter.

     The orbiter's nose cone (including the chin panel) and the
leading edge of its wings are the hottest areas during re-entry. When
maximum heating occurs, about 20 minutes before touchdown,
temperatures on these surfaces reach as high as 3,000 F.

     Reinforced Carbon-Carbon (RCC) is a light gray, all-carbon
composite. RCC, along with inconel foil (metal) insulators and quartz
blankets, protect the orbiter's nose, chin, and wing leading edges
from the highest expected temperatures and aerodynamic forces. It
also is used in the arrowhead area at the forward section of the
orbiter where the external tank is attached. RCC is used there for
shock protection during pyrotechnic separation of the external tank
from the orbiter.

     Fabrication of the RCC begins with graphite cloth which is
saturated with a special resin. Layers of the cloth are then
laminated and cured, after which they are heat-treated to convert the
resin into carbon.

     After further processing, the material is treated with a mixture
of alumina, silicon and silicon carbide to give it a grayish,
oxidation-resistant coating, and then heated in a furnace. The
orbiter's nose cap is fabricated as one piece while each of the wings
has 22 separate RCC panels and T-seals on the leading edge. Each
panel is affixed to the orbiter's skin by mechanical attachments.

     About 70 percent of an orbiter's external surface is shielded
from heat by a network of more than 24,000 tiles formed from a silica
fiber compound. More advanced materials such as Flexible Insulation
Blankets have replaced tiles on some of the upper surfaces of the
orbiter.

     Coated black tiles--known as High Temperature Reusable Surface
Insulation (HRSI)--cover the lower surface of the orbiter, areas
around the forward windows, upper body flap, the base heat shield,
the "eyeballs" on the front of the Orbital Maneuvering Syste&
pods, and the leading and trailing edge of the vertical stabilizer
and rudder speed brake. The black tiles are located where
temperatures can reach as high as 2,300 F.

     Coated white tiles--known as Low Temperature Reusable Surface
Insulation (LRSI)--are designed to insulate the spacecraft from
temperatures up to 1,200 F. LRSI tiles were originally used
extensively, but are now replaced in most areas by thermal insulation
blankets. LRSI is still used on the upper surface of the forward
fuselage above the crew windows and on some parts of the OMS pods.

     Tiles vary in size, thickness and density. HRSI tiles are
generally 6 inches square; thickness varies from 1 to 5 inches. They
come in different densities: 9- and 22-pound-per-cubic-foot tiles.
LRSI tiles are larger and thinner, generally 8 inches square and from
0.2 to 1 inch thick. LRSI tiles come in 9- and 12-pound-per-cubic-
foot densities.

     The thermal properties of the tiles are dependent on their very
high purity. The manufacture of both types of tiles begins with
fibers of pure white silica refined from common sand. The fibers are
mixed with deionized water and other chemicals and poured into a
plastic mold where excess liquid is squeezed out of the mixture.

     The damp blocks are dried in the nation's largest microwave oven
at the Sunnyvale, Calif., plant of Lockheed Space Operations Co.
Then, they are sintered in a 2,350 F oven. Sintering fuses the fibers
without melting them.

     Rough cutting and precision sizing of the tiles are done with
saws. Final shaping of the surface is accomplished with 3- and 5-axis
numerically controlled milling machines using diamond-tipped cutters.
The tiles are then spray-coated, glazed and waterproofed. The
processing and inspection of each tile is documented, and individual
tiles are traceable back to material lots.

     The two types of tiles are the same except for their coating,
which is primarily borosilicate glass. Chemicals are added to give
the tiles different colors and heat rejection capabilities.

     Surface heat dissipates so quickly that a tile can be held by
its corners with a bare hand only seconds after removal from a 2,300
degrees oven, while the center of the tile still glows red with heat.

     Improvements to the Thermal Protection System have reduced the
amount of maintenance required after each mission. In many cases,
scratches and gouges on the tiles can be repaired. A new assembly and
refurbishment facility for thermal protection materials opened in
1988 at Kennedy Space Center. Two other tile assembly and
refurbishment facilities are at Lockheed's Sunnyvale plant, and at
Rockwell International's Palmdale, Calif., plant.

     The tiles are delicate and have to be protected from the
stresses on the orbiter's structure during flight. Launch blasts,
aerodynamic pressures, steering forces, vibration and acceleration
cause the vehicle body to bend and shift slightly during launch. In
the cold soak of space, the vehicle shrinks slightly, only to expand
again during re-entry.

     To prevent damage to the tiles, Strain Isolation Pads -- a layer
of nylon felt Nomex (flame-retardant material) -- are used between
the tiles and the orbiter's surface. The pads are bonded to the
tiles, as well as to the skin of the Shuttle, with RTV, a
room-temperature vulcanizing silicone adhesive. The tile surface
bonded to the pads is densified with silica-type solutions for added
strength.

      Another type of protective blanket material is Felt Reusable
Surface Insulation (FRSI) blankets. These blankets protect the
orbiter surfaces from temperatures between 350 and 700 F. The
insulation is coated with a white silicone rubber paint. FRSI once
covered about 25 percent of the vehicle. Now, the material is used
only on the upper section of the payload bay doors and the inboard
sections of the wing upper surface.

      Most of the LRSI tiles and FRSI pads have been replaced by
Flexible Insulation Blankets (FIBs), composed of a waterproofed,
quilted fabric with silica felt between two layers of glass cloth
sewn together with silica thread. Each FIB weighs 4.9 kilograms or 11
pounds per cubic foot.

      The blankets have better durability, and cost less to make and
install than the tiles. They are used on the upper sidewalls of the
orbiter's fuselage, sections of the payload bay doors, most of the
vertical stabilizer and rudder speed brake areas, the outboard and
aft sections of the upper wing, parts of the elevons, and around the
observation windows.

     Some of the HRSI tiles have been replaced by Fibrous Refractory
Composite Insulation (FRCI-12), which are less dense than the
22-pound-per-cubic-foot HRSI tiles but comparable in strength. They
are used around penetrations and leading edge areas.

      Other thermal materials used are the filler bar and gap fillers
which seal gaps between tiles and between the tiles and t&
structure. The seals protect the aluminum and/or graphite epoxy outer
skin of the orbiter by preventing the influx of hot plasma gas. The
gap fillers are envelopes of ceramic fiber cloth stuffed with a
resilient ceramic filler batt, and sometimes with a metal foil. The
filler bar consists of strips of Nomex felt coated with RTV, and are
part of the assembly method used for tiles.

      A combination of white and black pigmented silica cloth make up
thermal barriers, and some gap fillers are installed around
penetrable areas such as main and nose landing gear doors, the
orbiter's side hatch, umbilical doors, elevons, forward Reaction
Control System module and thrusters, the OMS pods, and gaps between
tiles in high differential pressure areas.

     Fused silica is used for the outer windows in the orbiter. Metal
is used for the forward reaction control system fairings and elevon
seal panels on the upper wing elevon interface.

     All of the major ingredients in the Shuttle's external Thermal
Protection System--tiles, Flexible Insulation Blankets and Felt
Reusable Surface Insulation--are bonded to the orbiter with the RTV
adhesive. The cement will withstand temperatures as high as 550 F,
and as low as minus 250 F without losing its bond strength.

     After each flight, the orbiter's external Thermal Protection
System is rewaterproofed. Dimethylethoxysilane is injected into the
tiles through an existing hole in the surface coating with a
needleless gun, and the blankets are injected by a needle gun. The
procedure must be done each time because the waterproofing material
burns out at 1,100 F., thus exposing the outer surface of the thermal
system to water absorption.

     There are numerous and far-ranging possibilities for spinoffs or
commercial applications of Thermal Protection System materials. For
example, tiles can be ideal as a jeweler's soldering base because
they absorb so little heat from a torch, do not contaminate precious
metals and are soft enough to hold items to be soldered. Because of
their purity, tiles can be an excellent high-temperature filter for
liquid metals. Carbon-carbon pistons have been shown to be lighter
than aluminum pistons and increase the mechanical and thermal
efficiencies of internal combustion engines.

     High costs at this time are a deterrent to widespread
application of the techniques and materials of the Thermal Protection
System. A single coated tile can cost as much as $2,000. But
technological advances may make these pure, lightweight thermal
materials the new insulators of the future.

                              The End