Low Outgassing Accelerometers and Cables for Thermal Vacuum and Vibration Test Environments
Exposure to the high vacuum level of a space environment induces material outgassing in ordinary accelerometers and cables. Any substance subjected to a vacuum has the potential to release trapped gasses. Contaminants from outgassing can condense onto nearby surfaces, such as photo-optic devices, and obscure them, rendering them useless during their intended application.
During random vibration, swept sine or shock testing prior to flight, spacecraft payloads are often fitted with accelerometers in hard-to-reach mounting locations. As the space structure is built up around them, it can become impossible to remove the accelerometers. Sensors installed for ground vibration testing may, therefore, remain on the structure — even if they are no longer needed for testing purposes.
In any application involving a thermal vacuum environment, care must be taken to select the proper accelerometers and cables prior to vibration testing. Accelerometer designs with hermetic housings and connectors can have low outgassing qualities. For all nonmetallic materials outside of a hermetic package, such as cables with polymer strain relief that do not typically have low outgassing qualities, verification is required to ensure that the materials have less than or equal to 1% TML (total mass loss) and a CVCM (collected volatile condensable mass) less than or equal to 0.1%. This is verified either using NASA documentation or test results from an outside laboratory.
Given these design parameters, a series of accelerometer and cable designs for the thermal vacuum environment will be discussed in this paper. They have been specifically designed or tested for low outgassing properties in accordance with the report NASA RP-1124, “Outgassing Data for Selecting Spacecraft Materials.” [Reference: NASA Reference Publication 1124 (January 1997)]
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