Internal heat transfer

The temperature of the spacecraft surfaces may be driven by their radiative properties, and/or convection and conduction. To some extent the equipment inside the probe can be decoupled from these surfaces via insulators, subject to the structural needs to carry mechanical loads to the interior. Heat produced inside the spacecraft is due largely to dissipation in electrical systems. For cold environments, electric heaters may be added at key locations. Radioisotope

Heater Units (RHUs) are also used for providing heat, as was the case for Huygens; heat is supplied continuously, requiring no electrical power.

Russian designs tended to use pressurized equipment compartments (even in space) with fans to ensure adequate convective heat transfer.

Heat pipes are devices with variable heat conductance. In one direction thermal conduction through the walls and through the modest vapour in the pipe is quite small. However, if the 'wet' end of the pipe is the warmer one, the higher vapour pressure of the working fluid (typically ammonia or Freon) causes vapour, and thus latent heat, to flow to the cold end, resulting in much higher conductance.

In planetary surface applications, such as to maintain low temperatures around the supports of the Trans-Alaska oil pipeline (which are embedded in permafrost that could become unstable if allowed to thaw) the heat transfer liquid is transferred to the 'hot end' by gravity. In microgravity, this will obviously not work, and a 'wick' is used to draw the liquid along the pipe by surface tension.

Deforming structures can also be used, either to modify effective thermal conductance by contacting surfaces, or by controlling surface exposure via louvres. Historically these were bimetallic strips - two metals with different coefficients of expansion. Today shape-memory alloys like Nitinol are sometimes used. Another approach (used on the Mars Exploration Rovers, e.g. Novak et al., 2005) is a paraffin-wax actuator, used as a thermal switch to dump heat from the insulated interior when warm, and isolate it from the environment during the cold night.

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