The packaging design of a spaceborne computer is usually the result of a careful tradeoff among the requirements for cooling, reliability, volume, weight, mechanical integrity, cost, ease of maintenance, accessibility of test points, and interconnection schemes. The latter factor is extremely important since the number and type of interconnections have a strong effect on reliability and since the interconnection provisions take up a major part of the volume in the computer. For example, approximately 75% of the volume of the Apollo guidance computer is required for interconnection wiring (ref. 14).
Spaceborne computers have often been physically divided into modules corresponding to the organizational subsystems of the computer (i.e., logic, memory, power supplies, and input/output), for at least two reasons:
Different packaging techniques can be used for each section.
An individual subsystem (e.g., memory) can easily be replaced or revised without redesigning the whole package.
The number of packages for each subsystem is the result of a tradeoff since smaller modules reduce the cost of each throw-away unit and may allow for some module standardization but increase the interconnection problem.
In most early computers, the logic section was usually constructed of cordwood modules of discrete components mounted on single-layer, plug-in printed circuit boards. These techniques are now obsolete, having been replaced by the use of some form of integrated circuit. Typically, a number of these microcircuit flatpacks are mounted in a module, in one or more layers, and interconnected by a multilayer printed circuit board. These modules in turn plug into or are mounted on a header or main frame which is the basic structural element of the computer. The wiring in the main frame may be wire wrap, multilayer printed circuit, or wire and solder (ref. 68). In some small developmental computers using LSI circuitry, the entire logic subsystem may consist of only a few flatpacks, which are attached directly to the main header, thus omitting the intermediate module.
Few general statements can be made about packaging the memory subsection of the computer since there is such a variety of competing memory techniques - magnetic core, plated wire, semiconductors, drums, tapes, and many others in the development stage. Each of these techniques has its own special problems and characteristics which must be considered at the time of packaging and in the light of mission requirements.
Discrete components still require packaging in the power supply subsection primarily because of the high power levels and large physical sizes involved. Discrete component circuits are also used in the I/O section because some functions are not conveniently adapted to microcircuits. There is a tendency in some recent designs, however, to remove these special I/O functions from the computer in order to standardize the I/O interface and encourage off-the-shelf use. If this is done, the packaging for the part of the I/O section within the computer is governed by the same considerations as for the logic section.
The spaceborne computer is typically housed in a substantial case made of lightweight metal such as aluminum or magnesium, which is both mechanically and electromagnetically sealed. Many early designs and some recent computers that are intended for use on the smaller unmanned spacecraft have odd shapes to fit in the limited space available in the vehicle. Other computers, designed for larger vehicles, have generally rectangular shapes, which are usually more convenient to package and easier to seal properly.
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