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
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
degrees 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 degrees 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
Fabrication of 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 seperate 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 System (OMS) pods, and the leading and trailing edges of the
vertical stabilizer and the rudder speed brake. The black tiles are
located where temperatures can reach as high as 2,300 degrees F.
Coated white tiles-known as Low-Temperature
Reusable Surface Insulation (LRSI)-are designed to insulate the spacecraft
from temperatures up to 1,200 degrees F. LRSI tiles were originally used
extensively, but are now replaced in most areas by Flexible 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
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 degrees 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 the orginal sand lots. No two
tiles on an orbiter are exactly alike. The curvature of each tile's
underside is matched to the contour of the Shuttle's skin at the exact
point the tile is to be bonded.
The two types of tiles are the same except
for their coating, which is primarily borosilicate glass. Chemicals are
added to the coating to give the tiles different colors and heat rejection
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 F 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 manufacturing facility for thermal protection materials
opened in 1988 at Kennedy Space Center. Two other tile assembly and
manufacturing 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 tensile
Another type of protective blanket material
is Felt Reusable Surface Insulation (FRSI) blankets. These blankets
protect the orbiter surfaces from temperatures between 350 degrees and 700
degrees 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 blankets
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. The average 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
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 the orbiter structure. The seals protect the aluminum and/or graphite
epoxy 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 is part of the
assembly method used for tiles.
A combination of white and black pigmented
silica cloth make up thermal barriers, and 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, OMS pods, and gaps between tiles in high differential pressure
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 degrees F, and as low as minus 250 degrees F without losing its bond
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 degrees 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.