WST PMBSS Deployable Wings Thermal Management during Cryo- Cycle at XRCF



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44th International Conference on Environmental Systems


The JWST Optical Telescope Element (OTE) assembly is the largest optically stable infrared-optimized telescope currently being manufactured and assembled, and scheduled for launch in 2018. The JWST OTE is designed to be passively cooled and operate at near 45K. The critical core of OTE is the composite structure that supports 12 Beryllium Primary Mirror Segment Assemblies (PMSA) at the Center Section (CS) and 6 additional PMSA at 2 ‘Wing’ sections that are hinged from the Center Section. The Backplane Support Frame (BSF) is a composite structure that is designed to support the telescope infrared science instruments. The combined CS and BSF make up the Primary Mirror Backplane Support Structure (PMBSS) that provides a stable platform, nearly motionless when subjected to temperature excursions of a few tenths of a degree Kelvin, and perfectly aligned between the telescope optical elements and the sensitive instruments during operations of the JWST. As a part of the cryogenic acceptance of these composite structures, the PMBSS and ‘Wings’, separately, have recently gone through two cryo-cycle tests, from normal ambient temperature to approximately 25K at NASA Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF). This paper describes the PMBSS ‘Wings’ thermal model in the XRCF chamber environment. This thermal model was used to establish the pre-test chamber shroud cool-down rate that meets the absolute maximum allowable temperature gradients within the structures, developed by performing thermal stress analyses, in support of the project test schedule timeline. This paper also describes the use of gaseous helium during the cool-down in order to accelerate the cool-down rate by enhancing heat transfer between the test articles and the chamber shroud, especially at cryogenic temperatures where normal radiation heat transfer is dramatically reduced. The thermal models were used to provide inputs to thermal-stress analysis and this paper also describes an unique method of summarizing the local linear temperature gradients in terms of ‘delta’-temperatures per unit length (Kelvin-per-meter, K/m). Furthermore, this paper gives insights into the post-test model comparisons against the test results focusing briefly on the gaseous helium conductive heat transfer coefficients.


Tucson, Arizona
The 44th International Conference on Environmental Systems was held in Tuscon, Arizona, USA on 13 July 2014 through 17 July 2014.
Sang C. Park, Smithsonian Astrophysical Observatory, USA
Mark D. Freeman, Smithsonian Astrophysical Observatory, USA
Lester M. Cohen, Smithsonian Astrophysical Observatory, USA