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Aerospace Heat dissipation In Space!

  1. Apr 29, 2010 #1
    Hi there, new to the forum. Just finished my Electrical engg. undergrad at the University of Calgary. Anyway I had a question regarding thermal transfer in a vacuum.

    Over my internship I did a substantial amount of work with cooling electronics using heat sinks and/ liquid, so I have a fairly basic understanding of the nature of thermal transfer in atmosphere. On with the question:

    So far as I understand in space, since there is no (very, very little) atmosphere, cooling based on convection wouldn't work, obviously conduction wouldn't be of use once apart from moving the heat around the satellite, leaving radiation as the only mechanism that could be used to cool the device.

    Would an arbitrary "heat sink" placed in space naturally cool to the 4K or so of space? What kind of thermal resistance could be expected in this situation? What would change depending on whether the device was in the sun or shade? I can only assume that the cooling must be in the shade.

    And, if anyone knows: how do they actually cool devices in space?

    Whew, that was a little more long winded that I expected.

    Thanks.

    B

    Edit: also, why isn't radiant cooling used terrestrially?
     
  2. jcsd
  3. Apr 29, 2010 #2

    russ_watters

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    Staff: Mentor

    Correct.
    Would an arbitrary heat sink on earth cool to room temperature? Not if there is heat applied: it reaches some equilibrium above room temperature.
    Thermal radiation is via the Stefan-Boltzman equation: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html
    Absolutely - or facing earth!
    Besides radiation, in some cases evaporation/sublimation is used if more cooling is needed in a smaller package.
    It isn't used as much because you're radiating against a higher ambient temperature. Try playing with the Stefan-Boltzman equation.
     
  4. Apr 29, 2010 #3

    D H

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    Correct.

    Only if the object is far removed from any heat source (i.e., star). For example, astronomers recently assessed the surface temperature of Pluto to be 43 K. They expected to see a temperature of about 53 K based on the solar irradiance at Pluto's orbit and based Pluto's reflectivity. The observed temperature was about 10 K lower than expected. Apparently Pluto has a thin atmosphere. (See this article for more.)

    For satellites in Earth orbit, the chief thermal problem is not that space is "cold". The biggest thermal challenges for Earth-orbitting satellites are
    • Getting rid of excess heat. Spacecraft are exposed to sunlight that is about 25% stronger in space than on the ground. (Space is free of an attenuating atmosphere or reflecting clouds.) The spacecraft's internal components add to this solar heating.
    • The potentially huge temperature difference between sunlit and shaded parts of the satellite. Were no measures taken, the temperature difference between sunlight and shadowed portions of a vehicle could exceed 500 F.
    • For vehicles in low-Earth orbit, passing in and out of Earth shadow can create significant thermal stress. Think of what happens when you bend a coat hanger many times.

    Lots of different measures. Some operational measures include rotating the spacecraft so that parts are not perpetually sunlit / shadowed. For example, one of the operational modes for the Space Shuttle is "barbeque mode". The Shuttle is aligned so that its long axis is normal to the Sun vector. The vehicle slowly rotates around its long axis so that parts do not excessively heat up / cool off. Another example are spin-stabilized satellites. The primary reason these satellites are rotating is to help stabilize the orientation. Moderating the temperature is a side benefit of this spinning.

    Many spacecraft employ passive thermal control techniques. Chief among these is the Multi-Layer Insulation used on many spacecraft. Think of it as a big blanket over the spacecraft. The insulation keeps the spacecraft from overheating due to solar radiation and from overcooling due to shadowing.

    The downside of those passive thermal techniques is that they can do too good a job. All of the electrical power used by a spacecraft eventually ends up in the form of heat. Some spacecraft need to use active thermal control to counteract this. Such vehicles have thermal radiators that are typically mounted orthogonal to the solar power panels. The radiators will be edge-on to the Sun when the solar panels are oriented to face the Sun. The radiators are not covered with a thermal blanket. Some radiators work passively, others use a fluid that circulates between the spacecraft proper and the radiators.
     
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