How does a thermal control system of a spacecraft basicaly work?

Question

I'd like to know how a thermal control system of a spacecraft basically works. As the only possibility to get rid of the thermal heat is through radiation, I'd like to get a basic qualitative understanding (without going to much into the maths) of how this specific process of "getting rid" of the heat through radiation works.

Answer 1

In the general case of "spacecraft" meaning any satellite, manned or otherwise, or upper stage the hierarchy for thermal control looks like this:

Passive systems

• materials chosen for their radiative finish (paint, mirrors etc)
• materials chosen for solid conductivity or insulating properties (aluminium face-skins on honeycomb panels, interface filler, CFRP spacers as insulators)
• aluminised mylar or kapton blanket material combining the above properties (ten layers with dacron spacers to reduce radiation)

Active systems

• electrical heaters (some argue these are so simple they count as passive)

• simple heat pipes (again some argue these are passive)

• loop heat pipes

• mechanically pumped fluid loops

• More specialised items (louvres, stirling coolers)

Simple heat pipe and loop heat pipes are two phase fluid systems where the heat causes evaporation and pressurisation of a liquid to gas which in turn causes fluid flow from hot to cold parts. The geometry is arranged so that the fluid condenses, i.e. loses heat, to the external radiator. A mechanically driven pump by contrast does not need to be two phase and gives more control to the operator as to what heat transfer capacity it has at a given time.

EDIT The driving power for a heat pump comes from the heat that is to be transported from a point of power dissipation in the payload to a comparatively cool radiator surface that can see deep space. The whole point is that it is hotter at one end of the heat pipe than the other. The increased gas pressure at the hot end of the pipe spreads out to the cold end causing mass transfer in the gas phase. The role of surface tension, or capillary action, is to even out the meniscus shapes by transferring liquid in the grooves from the cold end back to the hot end.

Simple heat pipes don't work well against acceleration. This means whilst they work in orbit they need to be tested on Earth whilst horizontal. Some loop heat pipes, which are a more complex design, can work against a 1-g acceleration.

Synthesis

Usually the design of an equipment item or a whole satellite will be to identify the power dissipating elements and then choose a primary heat loss path which is specifically made to have well known characteristics.

An example of passive design would be to finish items in interior cavities in paint so as to make a high emmisivity surface and thus create a quasi isothermal cavity through efficient exchange of radiation. The walls of the cavity could be an aluminium box or a honeycomb sheet with a dense interior mesh. Particularly high dissipating elements would be mounted directly on the outer walls, inside or outside. The outside of the honeycomb could be given white paint, or vacuum deposited aluminium or second surface mirrors.

If the power dissipated is too great for this the design might then progress to a fluid based system. Heat pipes are used extensively in high power geostationary satellites to spread heat within an outer wall and thus make best use of the radiator area. Loop heat pipes are used where the demand is highest. Mechanically driven pumps are used in some unmanned situations but, I believe, are more a feature of manned systems.

Answer 2

Similarly to a common refrigerator.

There's a working liquid of a low evaporation temperature (below room temperature) - e.g. ammonia. Flowing through sealed pipes in radiators of equipment and air circulation machinery, it evaporates, absorbing heat from the environment, evaporating in the process. Then it's pumped into radiator panels - large structures outside the spacecraft, mounted on pivot mounts, turning in such a way that narrow edge is directed at the Sun absorbing least amount of solar heat and large surfaces face the deep space, radiating heat out. The vapor gives out heat and condenses back into liquid, to be returned to the radiators inside the craft.

This may work on minimal pressure differential and only heat of condensation and evaporation (safer, but less efficient), or may be augmented by additional pressurization: the working fluid has even lower evaporation temperature at ambient pressure, but can be liquified by moderate pressurization. The vapor pumped into the radiators through a stronger pump, condenses due to high pressure; that additionally heats it; the extra heat is radiated out. Then, inside the ship, the heat-absorbing radiators contain nozzles where the pressure is released - the decompression process combined with vaporization draws much more heat from the environment than vaporization alone would.