The thermal protection tiles on the Space Shuttle that protect it during the inferno of reentry are black. Seems counter-intuitive because black absorbs heat, but someone told me that black tiles actually release heat during reentry. How does that work, and why does black absorb heat here on Earth’s surface, but not in high atmosphere?
If the Shuttle had used white thermal-protection tiles during reentry, what would happen? Would they absorb heat? Would the Shuttle melt?
Everything that absorbs heat releases it also, and vice-versa. The laws of thermodynamics guarantee that it can’t be any other way. On the flip side, a polished silver or stainless steel teapot will keep the tea hot for a long time, because polished stainless steel or silver reflects almost all light that hits it, and thus emits almost none, either.
Color is only relevant for absorbing or releasing heat radiantly, via light. Reflective objects will absorb or release very little light, while black objects will absorb and release a lot. But most of the heat transferred to the Space Shuttle during re-entry is transferred via conduction, from the air being directly in contact with the tiles. Color doesn’t matter here, though it might matter some to the re-radiation of heat out into the larger environment. So the tiles being black might produce some small benefit.
In the end, though, I don’t think they’re chosen for their color. I think it’s just that the higher heat-tolerance materials just happen to be black, and the less-tolerant but cheaper materials used for most of the vehicle just happen to be white, and there’s no point in artificially changing the color of either.
I believe that’s just the color of the material itself. The heat generated upon re-entry into the atmosphere is caused by friction, and I don’t think the color has anything to do with that. On the other hand, black does absorb radiative heat, which is actually a good thing in the terrible coldness of space.
You’re probably thinking about the fact that wearing a white t-shirt while standing in the sun will keep you cooler than wearing a black t-shirt while standing in the sun, because white reflects the light and black absorbs it. But if there’s a hot metal bench and you sit on it, it won’t matter what color your pants are because you’re not getting the heat in the form of radiation, you’re getting it from conduction.
When the shuttle is re-entering, the most significant source of heat is from conduction. The air in underneath the shuttle can’t get out of the way fast enough, so it compresses rapidly and hence the temperature goes up too. So the bottom of the shuttle is touching very hot air, and the heat from that air is transferred by conduction to the tiles on the bottom of the shuttle. The color makes no difference in that situation.
… and, yes, otherwise the Space Shuttle would melt when re-entering.
The black heat shield tiles are not made of any form of carbon. The tiles are pure white, made of high purity silica fibers. They are then coated with black borosilicate glass which gives them their black appearance.
I though it was ablation or another term sublimation is similar. The conversion of the solid tiles surface to a gas/plasma, the phase change energy input and the leaving of the heated tile material that was converted to the gas/plasma form was the main part of cooling. IIRC the tiles were designed to be changed out every 3 flights, but it was always retiled every flight.
Ablation is a consequence of the heat transfer, not a mechanism by which it happens.
Ablative heat shields were what was used on everything previous to the Shuttle (Mercury, Gemini, Apollo). Those shields are designed to melt into plasma as a way of dissipating the heat. The Shuttle’s tiles, like the rest of it, were meant to be almost completely reusable, IOW they get hot but insulate the underside of the craft extremely well and simply radiate the heat away directly. Turned out to be a terrible idea. Getting the tiles to work right was the biggest cause of delay in getting the Shuttle flight ready and their fragility was the cause of the loss of Columbia.