Space Shuttle thermal protection system



The Space Shuttle thermal protection system (TPS) is the barrier that protects the Space Shuttle Orbiter during the searing 1,650 °C (3,000 °F) heat of atmospheric reentry. A secondary goal is to protect from the heat and cold of space while on orbit.

Materials
The TPS covers essentially the entire orbiter surface, and consists of seven different materials in varying locations based on amount of required heat protection: Each type of TPS has specific heat protection, impact resistance, and weight characteristics, which determine the locations where it is used and the amount used.
 * Reinforced carbon-carbon (RCC), used in the nose cap and wing leading edges. Used where reentry temperature exceeds 1,260 °C (2,300 °F).
 * High-temperature reusable surface insulation (HRSI) tiles, used on the orbiter underside. Made of coated LI-900 Silica ceramics. Used where reentry temperature is below 1260 °C.
 * Fibrous refractory composite insulation (FRCI) tiles, used to provide improved strength, durability, resistance to coating cracking and weight reduction. Some HRSI tiles were replaced by this type.
 * Flexible Insulation Blankets (FIB), a quilted, flexible blanket-like surface insulation. Used where reentry temperature is below 649 °C (1,200 °F).
 * Low-temperature Reusable Surface Insulation (LRSI) tiles, formerly used on the upper fuselage, but now mostly replaced by FIB. Used in temperature ranges roughly similar to FIB.
 * Toughened unipiece fibrous insulation (TUFI) tiles, a stronger, tougher tile which came into use in 1996. Used in high and low temperature areas.
 * Felt reusable surface insulation (FRSI). White Nomex felt blankets on the upper payload bay doors, portions of the midfuselage and aft fuselage sides, portions of the upper wing surface and a portion of the OMS/RCS pods. Used where temperatures stay below 371 °C (700 °F).

The shuttle TPS has three key characteristics that distinguish it from the TPS used on previous spacecraft:
 * Reusable. Previous spacecraft generally used ablative heat shields which burned off during reentry and so couldn't be reused. This insulation was robust and reliable, and the single-use nature was appropriate for a single-use vehicle. By contrast, the reusable shuttle required a reusable thermal protection system.
 * Lightweight. Previous ablative heat shields were very heavy. For example the ablative heat shield on the Apollo Command Module comprised about 1/3 of the vehicle weight. The winged shuttle had much more surface area than previous spacecraft, so a lightweight TPS was crucial.
 * Fragile. The only known technology in the early 1970s with the required thermal and weight characteristics was also so fragile, due to the very low density, that one could easily crush a TPS tile by hand.

Why TPS is needed
Discovery’s under wing surfaces are protected by thousands of High-Temperature Reusable Insulation tiles.The orbiter's aluminum structure cannot withstand temperatures over 175 °C (347 °F) without structural failure. Aerodynamic heating during reentry would push the temperature well above this level in areas, so an effective insulator is needed.

Reentry heating
A closer view of the tiles under the forward fuselage and the front end of the left wing. The corner of the nose-gear door can be seen at the lower left. The dark solid black tiles are new ones which have never been through a reentry yet. (At top, the white object is the open left cargo bay door.)Reentry heating differs from the normal atmospheric heating associated with jet aircraft, and this governs TPS design and characteristics. The skin of high-speed jet aircraft can become hot from atmospheric friction, but this frictional heating is similar to rubbing your hands together. The Orbiter reenters the atmosphere as a blunt body by having a very high (40-degree) angle of attack, with its broad lower surface facing the direction of flight. Over 80% of the heating the Orbiter experiences during reentry is caused by compression of the air ahead of the ultrasonic vehicle, in accordance with the basic thermodynamic relation between pressure and temperature. A hot shock wave is created in front of the vehicle, which deflects most of the heat and prevents the orbiter's surface from directly contacting the peak heat. Therefore reentry heating is largely convective heat transfer between the shock wave and the orbiter's skin through superheated plasma. The key to a reusable shield against this type of heating is very low-density material, similar to how a thermos bottle inhibits convective heat transfer.

Some high temperature metal alloys can withstand reentry heat; they simply get hot and re-radiate the absorbed heat. This technique, called "heat sink" thermal protection, was planned for the X-20 Dyna-Soar winged space vehicle.  However, the amount of high-temperature metal required to protect a large vehicle like the Space Shuttle Orbiter would have been very heavy and entailed a severe penalty to the vehicle's performance. Similarly, ablative TPS would be heavy, possibly disturb vehicle aerodynamics as it burned off during reentry, and require significant maintenance to reapply after each mission. (Unfortunately, TPS tile, which was originally specified never to take debris strikes during launch, in practice has also needed to be closely inspected and repaired after each landing, due to damage invariably incurred during ascent, even before new on-orbit inspection policies were established following the loss of Space Shuttle Columbia.)

Detailed description
Silica tile from AtlantisThe TPS is a system of different protection types, not just silica tiles. They are in two basic categories: tile TPS and non-tile TPS. The main selection criteria is using the lightest weight protection capable of handling the heat in a given area. However in some cases a heavier type is used if additional impact resistance is needed. The FIB blankets were primarily adopted for reduced maintenance, not for thermal or weight reasons.

Much of the shuttle is covered with LI-900 silica tiles, made from essentially very pure quartz sand.  The insulation prevents heat transfer to the underlying orbiter aluminum skin and structure. These tiles are such poor heat conductors that one can hold one while it is still red hot. There are about 24,300 unique tiles individually fitted on the vehicle, for which the Orbiter has been called "the flying brickyard".

The tiles are not mechanically fastened to the vehicle, but glued. Since the brittle tiles cannot flex with the underlying vehicle skin, they are glued to Nomex felt Strain Isolation Pads (SIPs) with RTV silicone adhesive, which are in turn glued to the orbiter skin. These isolate the tiles from the orbiter's structural deflections and expansions.

High-temperature reusable surface insulation (HRSI)
HRSI tiles (black in color) provide protection against temperatures up to 1,260 °C (2,300 °F). There are 20,548 HRSI tiles which cover the landing gear doors, external tank umbilical connection doors, and the rest of the orbiter's under surfaces. They are used in areas on the upper forward fuselage, parts of the orbital maneuvering system pods, vertical stabilizer leading edge, elevon trailing edges, and upper body flap surface as well. They vary in thickness from 1 to 5 inches (2.5 to 13 cm), depending upon the heat load encountered during reentry. Except for closeout areas, these tiles are normally 6 by 6 inches (15 by 15 cm) squares. The HRSI tile is composed of high purity silica fibers. Ninety percent of the volume of the tile is empty space giving it a very low density (9 lb/cu ft, 140 kg/m3) making it light enough for spaceflight.  The uncoated tiles are bright white in appearance and look more like a solid ceramic than the foam-like material that they are.

The black coating on the tiles is Reaction Cured Glass (RCG) of which tetrasilicide and borosilicate glass are some of several ingredients. RCG is applied to all but one side of the tile to protect the porous silica and to increase the heat sink properties. The coating actually is also absent from a small margin of the sides adjacent to the uncoated (bottom) side. To waterproof the tile dimethylethoxysilane is injected into the tiles by syringe. Densifying the tile with tetraethyl orthosilicate (TEOS) also helps to protect the silica and waterproof. An uncoated HRSI tile held in the hand feels like a very light foam, less dense than styrofoam, and the delicate, friable material must be handled with extreme care to prevent damage. The coating feels like a thin, hard shell and encapsulates the white insulating ceramic to resolve its friability, except on the uncoated side. Even a coated tile feels very light, lighter than a same-sized block of styrofoam. As expected for silica, they are odorless and inert.

HRSI is used in conjunction with stronger, waterproof materials in the Space Shuttle heatshielding to give a balance of strength and resistance to the high re-entry temperatures experienced in Earth's upper atmosphere.

HRSI is primarily designed to withstand transition from areas of extremely low temperature (the void of space, about −270 °C / −454 °F) to the high temperatures of re-entry (caused by interaction, mostly compression at the hypersonic shock, between the gases of the upper atmosphere & the hull of the Space Shuttle, typically around 1,600 °C / 2,910 °F).

Fibrous Refractory Composite Insulation Tiles (FRCI)
The black FRCI tiles provide improved strength, durability, resistance to coating cracking and weight reduction. Some HRSI tiles were replaced by this type.

Toughened unipiece fibrous insulation (TUFI)
A stronger, tougher tile which came into use in 1996. TUFI tiles come in high temperature black versions for use in the orbiter's underside, and lower temperature white versions for use on the upper body. While more impact resistant than other tiles, white versions conduct more heat which limits their use to the orbiter's upper body flap and main engine area. Black versions have sufficient heat insulation for the orbiter underside but have greater weight. These factors restrict their use to specific areas.

Low-temperature reusable surface insulation (LRSI)
White in color, these cover the upper wing near the leading edge. They are also used in selected areas of the forward, mid, and aft fuselage, vertical tail, and the OMS/RCS pods. These tiles protect areas where reentry temperatures are below 1,200 °F (649 °C). The LRSI tiles are manufactured in the same manner as the HRSI tiles, except that the tiles are 8 by 8 inches (20 by 20 cm) squares and have a white RCG coating made of silica compounds with shiny aluminum oxide. The white color is by design and helps to manage heat on orbit when the orbiter is exposed to direct sunlight.

These tiles are reusable for up to 100 missions with refurbishment (100 missions is also the design lifetime of each orbiter). They are carefully inspected in the Orbiter Processing Facility after each mission, and damaged or worn tiles are immediately replaced before the next mission. Fabric sheets known as gap fillers are also inserted between tiles where necessary. These allow for a snug fit between tiles, preventing excess plasma from penetrating between them, yet allow for thermal expansion and flexing of the underlying vehicle skin.

Prior to the introduction of FIB blankets, LRSI tiles occupied all of the areas now covered by the blankets, including the upper fuselage and the whole surface of the OMS pods.

Flexible Insulation Blankets (FIB)
Developed after the initial delivery of Columbia. The white low-density fibrous silica batting material has a quilt-like appearance. The vast majority of the LRSI tiles have been replaced by FIB blankets. They require much less maintenance than LRSI tiles yet have about the same thermal properties.

Reinforced Carbon-Carbon (RCC)
The light gray material which withstands reentry temperatures up to 1,510 °C (2,750 °F) protects the wing leading edges and nose cap. Each of the Orbiters’ wings has 22 RCC panels. These panels are about 1⁄4 to 1⁄2 inch (6.3 to 13 mm) thick. T-seals between each panel allow thermal expansion and lateral movement between these panels and the Orbiter's wing.

RCC is a laminated composite material made from graphite rayon cloth and impregnated with a phenolic resin. After curing at high temperature in an autoclave, 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 silicon-carbide coating protects the carbon-carbon from oxidation. The RCC is highly resistant to fatigue loading that is experienced during ascent and entry. It is stronger than the tile and is used around the socket of the forward attach point of the Orbiter to the External Tank to accommodate the shock loads of the explosive bolt detonation. The RCC is the only TPS material that also serves as structural support for part of the orbiter's aerodynamic shape: the wing leading edges and the nose cap. All other TPS components (tiles and blankets) are mounted onto structural materials that support them, mainly the aluminum frame and skin of the orbiter.

Nomex Felt Reusable Surface Insulation (FRSI)
The white, flexible fabric offers protection at up to 371 °C (700 °F). FRSI covers the Orbiter's wing upper surface, the upper payload bay doors, a portion of the OMS/RCS pods, and aft fuselage.

Gap fillers
Gap fillers are placed at doors and moving surfaces to minimize heating by preventing the formation of vortices. Doors and moving surfaces create open gaps in the heat protection system that must be protected from heat. Some of these gaps are safe, but there are some places on the heat shield where surface pressure gradients would cause a cross flow of boundary layer air in those gaps.

The filler materials are made of either white AB312 fibers or black AB312 cloth covers (which contain alumina fibers). These materials are used around the leading edge of the forward fuselage nose caps, windshields, side hatch, wing, trailing edge of elevons, vertical stabilizer, the rudder/speed brake, body flap, and heat shield of the shuttle's main engines.

On STS-114, some of this material was dislodged and determined to pose a potential safety risk. It was possible that the gap filler could cause turbulent airflow further down the fuselage, which would result in much higher heating, potentially damaging the orbiter. The cloth was removed during a spacewalk during the mission.

Weight considerations
While RCC has the best heat protection characteristics, it is also much heavier than the silica tiles and FIB blankets, so it is limited to relatively small areas. In general the goal is to use the lightest weight insulation consistent with the required thermal protection. Weight per unit volume of each TPS type Total area and weight of each TPS type (used on Orbiter 102) (pre-1996)  :
 * RCC: 1,986 kg/m³ (124 lb/ft³)
 * LI-2200 tiles: 352 kg/m³ (22 lb/ft³)
 * FRCI tiles: 192 kg/m³ (12 lb/ft³)
 * LI-900 (black or white) tiles: 144 kg/m³ (9 lb/ft³)
 * FIB blankets: 144 kg/m³ (9 lb/ft³)

Concern over "zipper effect"
The tile TPS was an area of concern during shuttle development, mainly concerning adhesion reliability. Some engineers thought a failure mode could exist whereby one tile could detach, and resulting aerodynamic pressure would create a "zipper effect" stripping off other tiles. Whether during ascent or reentry, the result would be disastrous.

Concern over debris strikes
Another problem was ice or other debris impacting the tiles during ascent. This has never been fully and thoroughly solved, as the debris has not been eliminated, and the tiles remain susceptible to damage from it. NASA's current strategy for mitigating this problem, which so far seems successful, is to aggressively inspect for, assess, and address any damage that may occur, while on orbit and before reentry, in addition to on the ground between flights.

Early tile repair plans
These concerns were sufficiently great that NASA did significant work developing an emergency-use tile repair kit which the first shuttle crew STS-1 could use before deorbiting. By December 1979 prototypes and early procedures were completed, most envisioning astronauts equipped with a special in-space repair kit and a jet pack called the Manned Maneuvering Unit, or MMU, developed by Martin Marietta.

Another element was a maneuverable work platform which would secure an MMU-propelled spacewalking astronaut to the fragile tiles beneath the orbiter. The concept used electrically-controlled adhesive cups which would lock the work platform into position on the featureless tile surface. About one year before the 1981 STS-1 launch, NASA decided the repair capability was not worth the additional risk and training, so discontinued development. There were unresolved problems with the repair tools and techniques; also further tests indicated the tiles were unlikely to come off. The first shuttle mission did suffer several tile losses, but they were fortunately in non-critical areas, and no "zipper effect" occurred.

Columbia accident and aftermath
Main article: Space Shuttle Columbia disasterOn February 1, 2003, the Space Shuttle Columbia was destroyed on reentry due to a failure of the TPS. The investigation team found and reported that the probable cause of the accident was that a piece of foam debris punctured an RCC panel on the left wing leading edge and allowed hot gases from the reentry to enter the wing and disintegrate the wing from within, leading to eventual loss of control and breakup of the shuttle.

The Space Shuttle's thermal protection system has received a number of controls and modifications since the disaster. They have been applied to Space Shuttle Discovery (as well as to the remaining shuttles: Atlantis and Endeavour) in preparation for future mission launches into space.

On 2005's STS-114 mission, in which Discovery made the first flight to follow the Columbia accident, NASA took a number of steps to verify that the TPS was undamaged. The 50-foot-long (15 m) Orbiter Boom Sensor System, a new extension to the Remote Manipulator System, was used to perform laser imaging of the TPS to inspect for damage. Prior to docking with the International Space Station, Discovery performed a Rendezvous Pitch Maneuver, simply a 360° backflip rotation, allowing all areas of the vehicle to be photographed from ISS. Two gap fillers were protruding from the orbiter's underside more than the nominally allowed distance, and the agency cautiously decided it would be best to attempt to remove the fillers or cut them flush rather than risk the increased heating they would cause. Even though each one protruded less than 3 cm (1.2 in), it is believed that leaving them in that state could cause heating increases of 25% upon reentry.

Because the orbiter doesn't have any handholds on its underside (as they would cause much more trouble with reentry heating than the protruding gap fillers of concern), astronaut Stephen K. Robinson worked from the ISS's robotic arm, Canadarm2. Because the TPS tiles are quite fragile, there had been concern that anyone working under the vehicle could cause more damage to the vehicle than was already there, but NASA officials felt that leaving the gap fillers alone was a greater risk. In the event, Robinson was able to pull the gap fillers free by hand, and caused no damage to the TPS on Discovery.

Tile giveaway
With the planned Space Shuttle retirement, NASA is giving TPS tiles to schools and universities for $23.40 USD each. About 7000 tiles are available on a first-come, first-served basis, but limited to one each per institution.