2.4 Environmental Design Factors
A major factor in the design of a spaceborne computer is the precautions that must be taken to insure correct operation in the environment to which the computer will be subjected, both in space and during testing and checkout on the ground. This section discusses several aspects of this environment.
2.4.1 Thermal AspectsMost spaceborne computers have been cooled by conduction either to a liquid-cooled cold plate (e.g., Gemini and Apollo) or to a heat sink which radiates directly to space. The requirement to interface with the cold plate or heat sink imposes some mechanical constraints on the design of the computer. A few designs have been cooled by liquid flowing through passages within the computer (e.g., Saturn LVDC), but this method is becoming less common due to the development of more efficient thermal interface materials (ref. 25). Only minor thermal problems have been experienced with spaceborne computers, and these have been solved by minor modifications. For example, the radiational cooling of the Centaur computer was improved by simply changing the external finish from gold plating to white polyurethane paint (ref. 45).
2.4.2 Electromagnetic Interference
Digital circuitry is particularly susceptible to electromagnetic interference (EMI), of which there are many sources in a typical space vehicle. Relays and solenoid-operated devices, such as electrically-controlled valves or electrically-fired explosive charges used for stage separation, are especially troublesome generators of impulse noise. These voltages can enter a computer either on the power lines or through the signal interfaces, and their magnitudes can be quite high. For example, noise spikes of 50 to 100 V were observed on the power distribution buses in the Apollo spacecraft, and impulses of up to approximately 150 V were produced by the turn-on and turn-off of the Gemini rate gyros (ref. 15). EMI problems have been experienced at one time or another, particularly during testing, in nearly every spaceborne computer system. References 15 and 69, for example, contain a fairly complete discussion of the EMI problems encountered with the Gemini guidance computer and the EM susceptibility testing conducted on it. In general, most EMI problems have been either unique to ground checkout installations or solved by minor modifications such as shorter ground straps, decoupling capacitors on input lines, or precautionary flight procedures.
2.4.3 Power Bus Voltage Variations
The power supplies in a spacecraft are not ideal voltage sources, which leads to potential problems besides those associated with strictly EMI effects. The voltage output of a battery varies with the temperature and state of charge of the battery. Fuel cells, like those used on the Apollo spacecraft, are also temperature sensitive in addition to having transient voltage variations due to many factors such as the dynamics of the fuel supply system. Spacecraft power systems are also subject to occasional externally induced variations, such as low-voltage transients or temporary outages. To minimize the effect of these occurrences on the computations, many spaceborne computers have a circuit in the power input section which detects a low voltage condition before the voltage drops to a level where the computer will not operate, and which either shuts down the processing in a safe and orderly manner or initiates a switchover to a backup power source. For example, in the Gemini spacecraft, an auxiliary computer power unit (ACPU) furnished backup power for the computer to operate during low voltage transients of up to 100 msec on the primary supply (ref. 15). If the power was interrupted for a longer period, the ACPU shut down the computer in a controlled manner to prevent loss of memory contents.
2.4.4 Other Environmental Factors
Most of the other environmental factors that affect the spaceborne computer have a similar effect on other types of medium to low power equipment in the spacecraft. Among those factors whose effects have been found to be important during the early stages of design are shock, vibration and acoustic noise, humidity, vacuum, radiation, magnetic fields, and sterilization. As an example, the Centaur computer was repackaged to seal and pressurize the I/O unit and general purpose section and thus eliminate a corrosion problem caused by operating in a humid environment (ref. 45). The acoustic vibration environment experienced by the Atlas computer produced intermittent diode failures on several flights (ref. 70). The recommended solution was to modify the computer isolation mounting to further attenuate circuit-board vibration, but this was not implemented due to impending deactivation of the Atlas fleet. The use of magnetic material may be restricted if a sensitive magnetometer experiment is included in the spacecraft payload.
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