Simulation of Rarefied Gas Heat Transfer for Acceleration of Thermal Transitions during James Webb Space Telescope (JWST) Thermal Vacuum Testing

The James Webb Space Telescope, scheduled for launch in 2018, must be cooled to temperatures below 50 K as part of a rigorous set of ground tests to verify operational performance. Due to the significant mass associated with the telescope components and ground support equipment needed for the tests, the time required to cool these components to operational temperatures will exceed acceptable durations in the program timeline unless the cool down and warm up operations can be accelerated. One method of accelerating these transient periods is to introduce gaseous helium at a low pressure level into the chamber. The gas acts as a heat transfer medium that increases the heat transfer between the chamber shroud walls and test assemblies. Without predictions of the effect of gaseous heat transfer and careful management of gas pressure, however, differences in temperatures between components could potentially exceed allowable thermally induced stresses in the hardware. Thus, several techniques of simulating rarefied gas heat transfer have been developed and correlated with component level tests conducted earlier that utilized helium for thermal transient acceleration purposes. The intent of this paper is to describe the modeling techniques, show correlation results and evaluate the extension of the techniques for simulating the JWST system level testing.