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Space Shuttle Tiles

  1. Feb 5, 2012 #1

    cepheid

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    I came across an example figure in my first-year physics textbook depicting a tile of the same material used for the heat shield on the space shuttle. The tile is hot enough to be glowing red, and yet a person is holding it by the edges. The caption explains that this is due to the "extremely small thermal conductivity and small heat capacity of the material."

    Small thermal conductivity makes sense. You want something that can insulate the orbiter (which is apparently made of aluminium, at least in part) from the heat from the shock front of compressed gas during re-entry. But I'm confused about the small heat capacity. My first instinct is that you would want a material with a large heat capacity so that you could dump a lot of energy into it without it heating up too much. I'm not sure where that reasoning goes wrong. The only other thought I've had is that maybe you want the tiles to heat up quickly to the temperature of the gas, so that there won't be any subsequent heat transfer to the tiles, which are already in thermal equilibrium with the gas.

    Can anyone explain the true reasoning to me?
     
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  3. Feb 5, 2012 #2

    davenn

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    IF you are going to dump lots of heat into it, that heat will get radiated somewhere ...
    guess where a lot will go ?

    yeah out the other side of the tile and into the orbiter framework ... that can only end badly !

    cheers
    Dave
     
  4. Feb 5, 2012 #3

    cepheid

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    I don't get it. If it has a large heat capacity, that means that dumping a lot of heat into it does not raise its temperature too much. The amount by which it radiates depends on T^4. So what am I missing?
     
  5. Feb 5, 2012 #4

    D H

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    What you're missing is that it is the atmosphere, not the Shuttle, that is dumping the heat. If the tiles had a high heat capacity, all of that thermal energy from the atmosphere streaming past would be transferred through the tiles to the underlying structure. Thanks to the low heat capacity, there was very little transfer of energy from the atmosphere to the Shuttle's structure.
     
  6. Feb 5, 2012 #5
    As Davenn said you do not want them conducting inwards to the shuttle. With a low heat capacity them heat up fast as the shuttle enters the atmosphere. After the entry they cool down quickly.
     
  7. Feb 5, 2012 #6

    cepheid

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    No, I do get that that is the situation. I just am not very good at applying physics to it, I guess.

    Why not? I.e. why would the energy not be transferred anyway?

    He was worried about them radiating energy into the shuttle, not conducting it. In any case we know they won't conduct, because they have been designed to be poor thermal conductors.
     
    Last edited: Feb 5, 2012
  8. Feb 5, 2012 #7

    cepheid

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    Okay, I've thought about it some more, and I think I've found the flaw in my reasoning: for the high heat capacity case, I was assuming that the tiles would magically remain at a temperature lower than the air. But I think that's wrong. I think that regardless of the heat capacity, the tiles will eventually warm up to the same temperature as the surrounding gas, at which point heating will stop. If that's true, then in the high heat capacity case, it will take a tremendous amount of energy going into the tiles before they heat up to be the same temperature as the surroundings. All of that heat will then start to be conducted (albeit poorly) from the tiles to the body of the shuttle, which would be bad.

    In the low heat capacity case, the tiles will heat up to the same temperature as the surroundings much more quickly, and much less heat will be transferred in doing so. This means that much less heat will be conducted to the body of the shuttle through the tiles.

    Am I correct?

    EDIT: simplified version: you want the tiles to have a low heat capacity so that their capacity to take in heat from the atmosphere is lower.
     
    Last edited: Feb 5, 2012
  9. Feb 5, 2012 #8

    D H

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    Correct. In a material with a high heat capacity, heat conduction dominates over radiative transfer.

    Those Shuttle tiles also took advantage of emissivity. The tiles on the bottom of the Shuttle had a black coating (high emissivity) but the bulk of the tile was white (low emissivity).
     
  10. Feb 5, 2012 #9

    DaveC426913

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    Right. An air layer is an excellent insulator because air has a low heat capacity.
     
  11. Feb 5, 2012 #10

    cepheid

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    Hmm. So what is the significance of that? Are you saying that even though the tile with high C doesn't warm up as fast (and hence won't be radiating as much), it will conduct all of the heat that's transferred to it?

    Now is the high emissivity so that they can cool radiatively as much as possible? (Which presumably helps lessens the amount of heat they conduct to the interior).

    What about the white tiles? Are they deliberately painted white because a poor emitter is also a poor absorber (since it reflects most of the incident light)? In other words, is the whiteness more advantageous in space where all of the heating is radiative, and you don't want a black shuttle that will absorb a lot of radiant heat?

    I'm sorry to badger you, I'm just trying to make sure I really understand the concepts here.

    Still air is also a poor thermal conductor, is it not?
     
  12. Feb 6, 2012 #11
    It makes sense that the tiles have low C so that during the fireball phase of re-entry they quickly begin shedding, or re-radiating, heat almost as quickly as absorbed. The word is "almost" because some of that heat is not re-radiated but diffuses deeper into the tile in at a rate hampered its low conductance.

    So it is a race. The fireball phase must end before too much heat penetrates into the tiles. As soon as the fireball phase ends, the tile quickly radiates its heat back into space -- thanks to its low C. Any remnant diffused deep into the tile will reach the shuttle skin, but not by a dangerous amount.

    If the tile has high C, much more heat will be stored in the tile during the fireball phase, the tile will take much longer to cool, and during that time, much more heat will reach the shuttle's skin, with perhaps lethal results.
     
  13. Feb 6, 2012 #12

    Vanadium 50

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    Maybe an analogy would help. Think of the Shuttle as a thermos bottle.
     
  14. Feb 6, 2012 #13

    sophiecentaur

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    I think the thermal capacity of the tiles is of secondary interest. When you get down to it, they are there to delay the temperature rise of the shuttle skin until it's down in cool air. I guess that they will cool down quicker, once they are in the cool atmosphere, because they have low heat capacity but the major aspect of their construction is that the surface gets very hot so it radiates loads of friction-generated energy rather than passing it on to the shuttle body.

    I asked a question a while ago (now it seems daft) about why they don't 'fly' the shuttle gradually, down through the atmosphere in order to limit the maximum surface temperature reached. Of course, there is a certain amount of energy to dissipate, one way or another and the whole shuttle would actually get much hotter if the braking process were spread over minutes - or even hours, just like supersonic aircraft only worse.
    It's clearly better to 'get it over with quickly' because heat is radiated much faster from a white hot surface in a quick burst.
     
  15. Feb 6, 2012 #14

    DaveC426913

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    Agreed. I am not sure how one might see a distinction between poor thermal conductor and poor thermal capacity. I mean, I know the difference but I don't know where you might find one without the other.
     
  16. Feb 6, 2012 #15

    sophiecentaur

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    Water?
     
  17. Feb 6, 2012 #16

    Q_Goest

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    The thermal gradient through the tile at steady state wouldn't be any different (assuming the thermal conductivity is identical) between a tile with a high heat capacity and a tile with a low one. Assuming the heat is rejected via radiation heat transfer from the hot surface first which I understand is how these tiles are supposed to work, and only secondarily to the air frame, the inner surface of the tile under steady state conditions is dictated by the thermal conductivity, not the heat capacity.

    The only difference will be during the transient when the tile heats up or cools down. For a tile with higher heat capacity, the thermal gradient will set up more slowly and disipate more slowly.

    I believe the point of having a low heat capacity is that once you heat up the tille to red hot under a bunsen burner for example, then it will cool down much more quickly so someone can pick it up if it has a low heat capacity. So the low heat capacity is a feature that allows for an impressive demonstration in a classroom, but I don't think a low heat capacity has anything to do with the function of the tile on the shuttle.*

    As a side note, I've also heard the tiles have a thermal conductivity which is anisotropic, though I don't know how true that is. I think the coating has a relatively high thermal conductivity so it can conduct heat along the outer surface rapidly. In other words, the thermal conductivity perpendicular to the face is low but the thermal conductivity parallel to the face is high. This allows heat to flow out to the edges and be rejected in relatively cool areas. Unfortunately, I couldn't find anything on the net to confirm or deny this though.

    * EDIT: The same thing applies to the heat capacity of aluminum foil for example. If you ever cooked a slice of pizza in a hot oven on top of a piece of aluminum foil, you probably have found you can open the oven and pick up the aluminum foil by the edges and remove your pizza. Even though the foil was obviously way too hot to touch in the oven, the foil doesn't have enough heat capacity to feel hot when you touch it because it cools down so quickly and any residual heat can be absorbed by your hands without getting your skin hot.
     
    Last edited: Feb 6, 2012
  18. Feb 6, 2012 #17

    sophiecentaur

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    I guess a laminated construction could well help to even out the surface temperatures but still insulate the skin from the outside. Smart thinking, actually, imo.
     
  19. Feb 6, 2012 #18

    D H

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    Both the volumetric heat capacity and thermal conductivity are of interest, and they go somewhat hand in hand. A low volumetric heat capacity reduces the magnitude of the heat pulse while a low thermal conductivity delays the heat pulse (and also reduces it as a consequence of the delay).

    If you're talking about powered flight, yeah, that's pretty much daft. If you're talking about a different reentry angle or a different angle of attack, well that's (possibly) not so daft. The Shuttle had a skip entry capability (shallow reentry angle). It was never used, however.



    I alluded to that in post #8. The coating of the tiles on the bottom of the Shuttle had a higher thermal conductivity and a higher emissivity than the main bulk of the tile. The higher emissivity was the key factor here, IIRC.
     
  20. Feb 6, 2012 #19

    sophiecentaur

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    Yebbut I think the issue is to do with the total energy that needs to be dissipated. That would be so much that the whole shuttle temperature would rise to an unacceptable value if you were just to rely on 'forced convection' in the atmosphere on the way down. Radiating at 'several kK' is much more effective - although spectacularly dangerous looking.
     
  21. Feb 6, 2012 #20

    Q_Goest

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    Just to clarify a few things:
    • "Volumetric heat capacity" is the heat capacity multiplied by density?
    • The heat pulse referred to is the total heat input during reentry?
    • The main consideration regards the transient condition and not the steady state one?
    Perhaps you can elaborate and also explain why (assuming I've understood correctly) a low heat capacity might be of benefit for the transient condition.
     
    Last edited: Feb 6, 2012
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