Space Shuttle thermal protection system:
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 Columbia.)
High-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 composed of high purity silica fibers. Ninety percent of the volume of the tile is empty space giving it a very low density (144 kg/m³, 9 lb/ft³) 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) 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 1600 °C)
On 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) in preparation for future 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 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° 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 (1.18 inch), 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.
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