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The internal cleanliness of heat pipe casings used in spacecraft thermal control systems must meet strict non-volatile residue (NVR) and particulate requirements. Presently, Trichloroethane (TCA) is used in the precleaning of heat pipes and 1,1,2 trichloro 1,2,2 Trifluoroethane (CFC-113) is used for precision cleaning. TCA removes oils and other organic contaminants while CFC-113 removes particles and fibers.
A new process was developed and tested for heat pipe casing cleaning using replacements for TCA and CFC-113 which meets NVR and particulate cleanliness requirements. An alkaline cleaner was used instead of TCA for precleaning and deionized water (DI) with a nitrogen gas aeration was used instead of CFC-113 for precision cleaning.
The new process was tested on five heat pipe casings and four of these met the cleanliness requirements of MIL-STD-1246B, level 250D. The five casings were subjected to life tests. After five months, all of the casings have acceptable levels of non-condensable gas (NCG). Solvent extraction (NVR), particulate counts and suface analysis alone do not provide adequate confidence that the heat pipes will perform as intended. A heat pipe's performance is degraded when NCG forms within the pipe. This phenomenon can only be assessed by life testing which involves charging the heat pipe with ammonia and periodically determining the NCG level.
ozone-depleting chemicals (ODC), heat pipes, deoxidizer, life testing, non-condensible gas (NCG), particulate cleanliness, non-volatile residue
Lockheed Missiles and Space Company, Inc., Materials and Processes Engineering, Sunnyvale, CA