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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Space Shuttle thermal protection system | 2/5 | https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system | reference | science, encyclopedia | 2026-05-05T13:22:37.256899+00:00 | kb-cron |
The TPS was 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 used the lightest weight protection capable of handling the heat in a given area. However, in some cases a heavier type was used if additional impact resistance was needed. The FIB blankets were primarily adopted for reduced maintenance, not for thermal or weight reasons. Much of the shuttle was covered with LI-900 silica tiles, made from essentially very pure quartz sand. The insulation prevented heat transfer to the underlying orbiter aluminium skin and structure. These tiles were such poor heat conductors that one could hold one by the edges while it was still red hot. There were about 24,300 unique tiles individually fitted on the vehicle, for which the orbiter has been called "the flying brickyard". Researchers at University of Minnesota and Pennsylvania State University are performing the atomistic simulations to obtain accurate description of interactions between atomic and molecular oxygen with silica surfaces to develop better high-temperature oxidation-protection systems for leading edges on hypersonic vehicles. The tiles were not mechanically fastened to the vehicle, but glued. Since the brittle tiles could not flex with the underlying vehicle skin, they were glued to Nomex felt Strain Isolation Pads (SIPs) with room temperature vulcanizing (RTV) silicone adhesive, which were in turn glued to the orbiter skin. These isolated the tiles from the orbiter's structural deflections and expansions. Gluing on the 24,300 tiles required nearly two man-years of work for every flight, partly due to the fact that the glue dried quickly and new batches needed to be produced after every couple of tiles. An ad-hoc remedy that involved technicians spitting in the glue to slow down the drying process was common practice until 1988, when a tile-hazard study revealed that spit weakened the adhesive's bonding strength.
=== Tile types ===
==== High-temperature reusable surface insulation (HRSI) ====
The black HRSI tiles provided protection against temperatures up to 1,260 °C (2,300 °F). There were 20,548 HRSI tiles which covered the landing gear doors, external tank umbilical connection doors, and the rest of the orbiter's under surfaces. They were also 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. They varied in thickness from 1 to 5 inches (2.5 to 12.7 cm), depending upon the heat load encountered during reentry. Except for closeout areas, these tiles were normally 6 by 6 inches (15 by 15 cm) square. The HRSI tile was composed of high purity silica fibers. Ninety percent of the volume of the tile was empty space, giving it a very low density (9 lb/cu ft or 140 kg/m3) making it light enough for spaceflight. The uncoated tiles were bright white in appearance and looked more like a solid ceramic than the foam-like material that they were. The black coating on the tiles was Reaction Cured Glass (RCG) of which tetraboron silicide and borosilicate glass were some of several ingredients. RCG was applied to all but one side of the tile to protect the porous silica and to increase the heat sink properties. The coating was absent from a small margin of the sides adjacent to the uncoated (bottom) side. To waterproof the tile, dimethylethoxysilane was injected into the tiles by syringe. Densifying the tile with tetraethyl orthosilicate (TEOS) also helped to protect the silica and added additional waterproofing.
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 was primarily designed to withstand transition from areas of extremely low temperature (the void of space, about −270 °C or −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 or 2,910 °F).
==== Fibrous Refractory Composite Insulation Tiles (FRCI) ==== The black FRCI tiles provided improved 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 came 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 conducted more heat which limited their use to the orbiter's upper body flap and main engine area. Black versions had sufficient heat insulation for the orbiter underside but had greater weight. These factors restricted their use to specific areas.