Introduction

This section consists of a few points about the role of the above for temperature regulation in space colonies or spaceships.

Any habitat needs light. This may be provided as sunlight or artificial light, but whichever, it will introduce energy, much of which will end up as heat, thus tending to increase the temperature of the habitat. The activities of the inhabitants and their artifacts may add to the heat generated, particularly if they rely on further imported energy (e.g. as food, electricity etc.). In spaceships, the engines would be yet another heat source.

Nothing too remarkable about that, just simple physics. The temperature of the entire habitat would increase until it reached an equilibrium value at which it was losing as much energy by radiation to space as was being put in from whatever source. From the point of view of this section, the thing to be careful of in habitat design is that said equilibrium value should not be so high as to prevent the habitat's functioning. This could be achieved by increasing the rate of radiative heat loss to a point at which the equilibrium temperature was low enough to allow optimum habitat function. Radiators, in this context, are devices for increasing the rate at which heat can be dumped to space.

For efficiency, a radiator needs to be as thin as possible, to maximise surface (from which radiation takes place) to volume (and thus mass) ratio. A radiator that radiates from both faces will be twice as efficient as one that can only radiate from one face, all else being equal. However, if radiating from both surfaces, the radiator must be flat and not curve towards either face, since the concave face would be, in part, radiating onto itself. A one sided radiator can be curved provided that the convex face is the one that is radiating.

Relative to the habitat that is being cooled, a radiator needs to be positioned so that it radiates as little as possible back towards the colony. For a two faced radiator, the can be achieved by placing the radiator as far as is practical from the habitat and perpendicular (edge on) to the habitat. Any deviation from the perpendicular would mean that one face was radiating back towards the habitat more than was necessary. Another way to look at this is that the habitat should 'see' as little as possible of the radiating surface(s) of the radiator. For a one sided radiator the position can be as close as desired to the habitat, provided the habitat is 'behind' the radiator. Such a radiator could be positioned on the outer surface of the habitat.

The next important aspect of radiator position is its orientation with respect to the sun. To avoid the radiator acting, also, as a solar heat collector it is important that it is edge on to the sun. If the habitat involved has a vertical axis of rotation, with respect to the plane of the ecliptic, then keeping the radiator edge on to both the sun and the habitat seems to inevitably involve a rotating connection, with any complications that entails*. If the habitat axis of rotation is perpendicular to the sun, then the radiator could rotate with the colony and stay edge on to both the sun and the habitat. Rotation of the radiator would impose structural stresses and require increased structural strength (and possibly mass). Thus, even though not geometrically necessary, a radiator on a habitat with an axis of rotation perpendicular to the sun may be best if provided with a rotating connection, so that it may itself not have to rotate.

Erosion problems

Figure 1.

Partitioning the Problem

In some cases, the problem of dumping waste heat may be reducible by reducing the heat input to the system to be cooled. For example, it might be better if you could 'intercept' the heat (of sunlight, for example) in a non-rotating part of the structure (and thus dump it through a non-rotating radiator) leaving the rest of the light to enter the rotating habitat. This may allow the rotating habitat to get away with one-sided surface heat dumping, where otherwise that wouldn't have been the case. The intercepting could be done by, for example, passing the sunlight (or artificial light, if this was a deep space, interstellar or otherwise artificially lit habitat) through an infra-red absorbing, but otherwise transparent, fluid. Said fluid would then carry the heat away to a non-rotating radiator. An example of such a fluid would be water. This type of thing is done by photographers when trying to adequately illuminate small, but fast moving organisms for filming without cooking said organisms. I think they call it cold light.