|
|
|
|
Space shuttle thermal protection system
|
| |
|
| |
The Space Shuttle thermal protection system (TPS) is the barrier that protects the Space Shuttle Orbiter during the searing 1650 °C (3000 °F) heat of atmospheric reentry. A secondary goal is to protect from the heat and cold of space while on orbit. The TPS covers essentially the entire orbiter surface, and consists of seven different materials in varying locations based on amount of required heat protection:
- Reinforced carbon-carbon (RCC), used in the nose cap and wing leading edges. Used where reentry temperature exceeds 1260 °C (2300 °F).
- High-temperature reusable surface insulation (HRSI) tiles, used on the orbiter underside. 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.
- Advanced flexible reusable surface insulation (AFRSI), a quilted, flexible blanket-like surface insulation. Used where reentry temperature is below 649 °C (1200 °F).
- Low-temperature Reusable Surface Insulation (LRSI) tiles, formerly used on the upper fuselage, but now mostly replaced by AFRSI. Used in temperatures ranges roughly similar to AFRSI.
- 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 are below 371 °C (700 °F).
Each type of TPS has specific heat protection, impact resistance and weight characteristics, which determine the location, amount, and type used.
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 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 neededThe orbiter's aluminum structure cannot withstand temperatures over 175 °C (350 °F) without structural failure. Aerodynamic heating during reentry would push the temperature well above this level in areas, so an effective insulator was needed.
Reentry heatingReentry heating is different from 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 is frictional heat -- similar to rubbing your hands together. The Orbiter reenters the atmosphere as a blunt body by having a very high (40 degrees) angle of attack, with its broad lower surface facing the direction of flight. 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 (an approach called "heat sink" thermal protection), a technique that 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.
Detailed description
The 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 AFRSI blankets were primarily adopted because of reduced maintenance, not thermal or weight reasons.
Much of the shuttle is covered with 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 31,000 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.
Tile typesHigh-temperature reusable surface insulation (HRSI)HRSI tiles (black in color) provide protection against temperatures up to 1260 °C. 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 2.54 cm (one inch) to 12.7 cm (five inches), depending upon the heat load encountered during reentry. Except for closeout areas, these tiles are normally 15.2 by 15.2 cm (6 by 6 inch) squares. The HRSI tile is basically a composite of high purity (99.8 %) silica fibers (10 %) and empty space (90 %) that exhibits ceramic bonding. The high percentage of voids is the reason for the low density (144 kg/m³, 9 lb/ft³) of the material making it light enough for spaceflight and strong enough to withstand the required G forces. The HRSI tiles have the black glossy appearance because of waterproofing coating made of tetrasilicide and borosilicate glass.
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) to the high temperatures of re-entry (caused by friction between the gases of the upper atmosphere & the hull of the Space Shuttle, typically around 1600 °C).
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 649 °C (1200 °F). The LRSI tiles are manufactured in the same manner as the HRSI tiles, except that the tiles are 20.3 x 20.3 cm (8 by 8 inch) squares and have a white coating made of silica compounds with shiny aluminum oxide.
These tiles are reusable for up to 100 missions with refurbishment. 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, yet allow for thermal expansion and flexing of the underlying vehicle skin.
Non-tile TPSAdvanced Flexible Reusable Surface Insulation (AFRSI). 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 AFRSI 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 1510 °C (2750 °F) protects the wing leading edges and nose cap. Each of the Orbiters’ wings has 22 RCC panels. These panels are about 0.635 cm (0.25 inch) to 1.27 cm (0.5 inch) 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.
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.
Weight considerationsWhile RCC has the best heat protection characteristics, it is also much heavier than the silica tiles and AFRSI 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
- RCC: 1,986 kg/m³ (124 lb/ft³)
- HRSI tiles: 352 kg/m³ (22 lb/ft³)
- FRCI tiles: 192 kg/m³ (12 lb/ft³)
- LRSI tiles: 144 kg/m³ (9 lb/ft³)
- AFRSI blankets: 144 kg/m³ (9 lb/ft³)
Early TPS problems 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 develop 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. Another problem was ice or other debris impacting the tiles during ascent.
Early tile repair plansThese 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 would probably stay on. The first shuttle mission did suffer several tile losses, but they were fortunately in non-critical areas.
Columbia accident and aftermathOn February 1, 2003, the Space Shuttle Columbia was destroyed on reentry due to a failure of the TPS. A piece of foam debris punctured an RCC panel on the left wing leading edge and allowed hot plasma from the reentry to enter the wing and break the shuttle apart from within. For more information and a timeline, see Space Shuttle Columbia disaster.
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) in preparation for future launches into space. It is one of several modifications made by NASA during the two years in which the Space Shuttle was grounded.
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 15.2 m-(50-foot)-long 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° rotation, allowing all areas of the vehicle to be photographed from ISS. Two gap fillers were protruding out from the orbiter's underside more than the nominally allowed distance, and the agency decided it would be best to attempt to remove the fillers or cut them flush rather than risk the increased heat they would cause. Even though each one protruded less than 3 cm (1.18 inch), it is believed that leaving them in that state could cause heat increases of 25 % upon reentry.
Because the orbiter doesn't have any handholds on its underside, 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, but NASA officials felt that leaving the gap fillers alone could be more troublesome. In the event, Robinson was able to pull the gap fillers free by hand, and caused no damage to the TPS on Discovery.
See also
External links
|
| |
|
|
