|MadSci Network: Engineering|
Dear Cheri, This wasn't an easy question to find information on. I had a hunch about the reasons for the black and white tiles, and I found a NASA website that verified my hunch. However, I don't know any ceramics engineers that may better explain the tiles, so I will try to do my best. Most people know that black colors will absorb more radiative heat than white will. Everyone knows you wear white clothing on a hot, sunny day to keep cooler. This is because black will absorb more of the light and heat from the sun than white will (White will reflect more of the light and heat). What most people don't know is that the exact opposite is also true, that black will emit more heat that white will! Let me give an example: Take two baseballs, one is its normal white and another is dyed all black. If they are both heated to the same temperature and you held your hand close to one and then the other, the black one would feel warmer. A temperature gauge would verify the same thing. Although they are the same temperature, there is more radiative heat from the black baseball. (Now this is only true for holding your hand close, not touching. That is a hole different ball game! If hot enough, both can burn you with equal efficiency). This is because the black color of the baseball is able to emit more heat than the white, just like it can absorb more heat than the white. So, what do hot black and white baseballs have to do with shuttle tiles? Out in space there is no protective atmosphere to protect you from the sun, and while it is very cold in space, if you are hit by all of the rays from the sun, they can warm you up in a hurry! Therefore, tiles that aren't critical for re-entry are white to help reflect the radiative energy from the sun, making the temperature control for the inside of the shuttle a little easier. So that explains the white tiles, they are there to help keep the shuttle cool. (That must sound strange since it is -200° C and colder in space!) But it is true. Think how the sun can warm you up on a cold winter day! So then what about the black tiles! Well, their purpose is much different than the others, and very important. They prevent the shuttle from burning up during re-entry into the atmosphere. One of the ways they are able to do this is by dissipating heat rapidly. By getting rid of the heat as it builds up, you collect less of a total heat build up. These tiles get rid of heat so fast, that you can pick up a tile with your bare hand seconds after it being removed from the oven, even with the center still glowing red hot! Part of this heat dissipation comes from the radiative emittance from the black colored tile. So there you have it, part of the reason for the different color tiles. (There may be more that I don't know about!). All of this information was gleaned from the following NASA website. The content is a little jargon laden so I hope that my explanation helps. However, there is a ton of information here about the Thermal Protection System for the shuttle: http://science.ksc.nasa.gov/shuttle/technology/sts- newsref/sts_sys.html#sts -tps Best of luck. BK
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