Space flight presents both a varied and extreme thermal environment to the space vehicle designer. Spacecraft thermal control is an important topic in its own right, and will be treated in more detail in Chapter 9. However, it is appropriate in this section to survey some of the environmental conditions that must be addressed in the thermal design.
The space vacuum environment essentially allows only one means of energy transport to and from the spacecraft, that of radiative heat transfer. The overall energy balance is therefore completely defined by the solar and planetary heat input, internally generated heat, and the radiative energy transfer properties that are determined by the spacecraft configuration and materials. The source and sink temperatures (from the sun with a characteristic blackbody temperature of 5780 K and dark space at 3 K, respectively) for radiative transfer are extreme.
Under these conditions, extremes of both temperature and temperature gradient are common. Thermally isolated portions of an Earth-orbiting spacecraft can experience temperature variations from roughly 200 K during darkness to about 350 K in direct sunlight. One has only to consider such everyday experiences as the difficulty of starting a car in very cold weather, with battery and lubrication problems, or very hot weather (which may cause carburetor vapor lock) to appreciate that most machinery functions best at approximately the same temperatures as do humans.
If appropriate internal conduction paths are not provided, temperature differences between the sunlit and dark sides of a spacecraft can be almost as severe as the extremes cited earlier. This results in the possibility of damage or misalignment due to differential expansion in the material. Space vehicles are sometimes rolled slowly about an axis normal to the sun line to minimize this effect. When this is impractical, and other means to minimize thermal gradients are not suitable, special materials having a very low coefficient of thermal expansion (such as Invar® or graphite-epoxy) may need to be employed.
The fatiguing effect on materials of repeated thermal cycling between such extremes is also a problem and has resulted in many spacecraft component and subsystem failures. One relevant example was that of LANDSAT-D, where the solar cell harness connections were made overly tight and pulled loose after repeated thermal cycling, ultimately disabling the spacecraft.
Thermal system design in vacuum is further complicated by the need for special care in ensuring good contact between bolted or riveted joints. Good thermal conductivity under such conditions is difficult to obtain, hard to quantify, and inconsistent in its properties. Use of a special thermal contact grease or pad is required to obtain consistently good conductive heat transfer.
The lack of free convection has been mentioned in connection with the Og environment; it is, of course, equally impossible in vacuum. Heat transfer internal to a spacecraft is therefore by means of conduction and radiation, in contrast to ground applications in which major energy transport is typically due to both free and forced convection. This results in the need for careful equipment design to ensure appropriate conduction paths away from all internal hot spots and detailed analytical verification of the intended design. This may sometimes be avoided by hermetically sealing an individual package or, as is common for Russian spacecraft, by sealing the whole vehicle. The disadvantage here is obviously that a single leak can result in loss of the mission.
The atmospheric entry thermal environment is the most severe normally encountered by a spacecraft, and vehicles designed for this purpose employ a host of special features to achieve the required protection. This is discussed in more detail in Chapters 6 and 9.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.