Shuttle Thermal Tiles: Description and Suggestions


by PhysicsPost
Tags: description, shuttle, suggestions, thermal, tiles
PhysicsPost
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Nov4-11, 04:13 PM
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Friction with the air is often neglected, but when traveling at Mach 18 it becomes a huge factor. To cope with this, NASA engineers have ingeniously developed sand-based thermal tiles that are designed to withstand temperatures that can reach up to 2600 degrees Fahrenheit.

There are 3 types of heat tiles that are found on the space shuttle: high-temperature reusable surface insulation tiles (HRSI), fibrous refractory composite insulation tiles (FRCI), and low-temperature reusable surface insulation tiles (LRSI).
The technology behind these tiles is quite interesting and very important to their function, especially in light of the Columbia crash. The tiles can be exposed to temperatures of 2300 degrees Fahrenheit (the melting point of lead is 621 degrees, and lead boils at 3180 degrees) and be cool enough to touch within a manner of seconds. This is due to the fact. Their amazing ability to dissipate heat results from that fact that while very strong, they are made of tiny fibers, making their composition almost empty space. However there is an inherent problem with this construction; all types of tiles are brittle and subject to damage from stress. Impact with foam during liftoff may have caused this kind of stress. These factors make them ideal for reentry, where temperatures reach approximately 2600 degrees Fahrenheit. It has been suggested that damage to a tile could have caused the disaster; however this an unlikely event as damage to a small number of tiles (1-2) would not cause the observed temperature spikes.

The first type of tile, the HSRI tile, is composed of 99.8% silica amorphous fiber. The tiles are typically 1 to 5 inches thick, and have two different models of density. The higher density, 22lbs/cubic foot, is employed in areas that are exposed to the highest temperatures upon reentry, on and around the nose of the shuttle and the tips of the wings. The lower density models, at 9lbs/cubic foot, are used for the remainder of the bottom of the shuttle.

The seconds type of tile, FRCI, are being used to replace the HSRI tiles. These tiles are essentially the same, only more resistant to damage thanks to added alumina-borosilicate fibers. These fibers act like an internal support structure for the tiles. They are also coated with a special type of glass, called reaction-cured glass, which reduces susceptibility to damage as a result of handling. Additionally, FRCI tiles are lighter than HSRI tiles (by 10lbs/cubic foot) and have a higher operating temperature. Given this benefits, the obvious recommendation is to step up implementation of these tiles instead of HRSI tiles. Due to their increased strength, they might have reduced damaging stress that occurued if they had been more completely implemented.

LSRI tiles are designed to operate in lowers temperatures. They are white rather than black, and are not placed at the areas the are exposed to the most heat. The are essentially the same, however the key difference arises in the thickness. LSRI tiles are only 0.2 to 1.4 inches thick. There is a surprising inconsistence that requires immediate attention. Shuttle Columbia was the only shuttle that had certain regions containing these tiles replaced by only gap fillers, instead of FRCI tiles or HRSI tiles and gap fillers. Plasma flow in these areas was indicated, and I question why all other shuttles received new tiles, but shuttle Columbia did not. It is quite possible this played a secondary role in the disaster.

Research also indicates a thorough inspection schedule of 10 percent of tiles every three flights to check if bonding is weakening. This is not to be confused with normal inspections to determine which tiles need replacement. This schedule is not acceptable; the most detailed examinations should be conducted prior to each flight. Furthermore, it is known that some areas are up to 80% more susceptible to debris impacts, and these areas should be thoroughly inspected before launch, as well as while in orbit. Given the cost factor between implemented an inspection method that could be conducted in orbit and the cost of losing a shuttle ($5 billion+), it certainly financially desirable as well as in the best interest of safety.

The tile technology is the most crucial protection system during reentry. While it is not likely damage to heat tiles was explicitly responsible for the crash, it is very likely that they had a secondary role. For this reason, the recommendations will ensure increased safety in the future: more careful examination of just where FRCI tiles need to be, a method of examining tiles for damage prior to reentry, and a complete inspection of all tiles prior to launch.

“Risk Management for the Tiles on the Space Shuttle” M. Elisabeth Paté and Paul S. Fischbeck http://www.informs.org/Press/SpaceShuttle.pdf

“Shuttle Thermal Protection System” U.S. Centennial of Flight Commission http://www.centennialofflight.gov/es...TPS/Tech41.htm
NSTS 1988 News Reference Manual http://science.ksc.nasa.gov/shuttle/...tsref-toc.html

-Jon Johnson
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boneh3ad
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Nov6-11, 01:16 PM
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Quote Quote by PhysicsPost View Post
It has been suggested that damage to a tile could have caused the disaster; however this an unlikely event as damage to a small number of tiles (1-2) would not cause the observed temperature spikes.
Quote Quote by PhysicsPost View Post
While it is not likely damage to heat tiles was explicitly responsible for the crash, it is very likely that they had a secondary role.
Source? Unless you have a credible source, I am just going to assume you made this up. Why do I think that? Because it doesn't take much to trip the boundary layer from laminar to turbulent, and a turbulent boundary layer transfers heat to a surface an order of magnitude more efficiently than a laminar boundary layer. In other words, if you have a chunk of tile missing and it chips the boundary layer, it would get the hotter and more quickly than in a laminar boundary layer. Combine that with a compromised heat shield and I don't think it is very far-fetched to imagine the damage being the primary cause.

For example, see what a 0.25-inch protrusion did to the boundary layer when tested intentionally on STS-119.

What, pray tell, do you suggest the primary cause of the crash was?


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