This paper describes two cryogenic thermal switches (CTSWs) under development for instruments on the James Webb Space Telescope (JWST). The first thermal switch was designed to extend the life of the solid H2 dewar for the 6 K Mid Infrared Instrument (MIRI) while the second thermal switch is needed for contamination and over-temperature control of three 35 K instruments on the Integrated Science Instrument Module (ISIM). In both cases, differential thermal expansion (DTE) between two materials having differing CTE values is the process that underpins the thermal switching. The patented DTE-CTSW design utilizes two metallic end-pieces, one cup-shaped and the other disc-shaped (both MIRI end-pieces are Al while ISIM uses an Al/Invar cup and an Al disc), joined by an axially centered Ultem rod, which creates a narrow, flat gap between the cup (rim) and disc. A heater is bonded to the rod center. Upon cooling one or both end-pieces, the rod contracts relative to the end-pieces and the gap closes, turning the CTSW ON. When the rod heater is turned on, the rod expands relative to the end-pieces and the gap opens, turning the CTSW OFF. During testing from 6-35 K, ON conductances of 0.3-12 W/K and OFF resistances greater than 2500 K/W were measured. Of particular importance at 6 K was the Al oxide layer, which was found to significantly decrease DTE-CTSW ON conductance when the mating surfaces were bare Al. When the mating surfaces were gold-plated, the adverse impact of the oxide layer was mitigated. This paper will describe both efforts from design through model correlation.
This paper describes the design, manufacturing, modeling, and testing of a methane cryogenic diode heat pipe (CDHP) thermal switching system for the CRISM instrument onboard the NASA/JPL Mars Reconnaissance Orbiter. The purpose of the CDHP system is to enable three 1-year cryocoolers to provide 2 years of cooling to the 100 K CRISM sensor while minimizing the parasitic heat input from the two OFF (redundant) cryocoolers. Without the CDHP system, the parasitic heat input from the two OFF cryocoolers would prevent the CRISM sensor from being cooled to an acceptably low operating temperature. To provide sufficient structural support for launch with low parasitic heat input, the three methane CDHPs were supported by small diameter Kevlar tension cables attached to a shoebox-shaped cold shroud that enveloped the assembly. The cold shroud -- thermally coupled by a flexible link to the (cryoradiator cooled) cold side of the instrument housing -- was suspended from the warm side of the instrument housing by a second set of Kevlar cables, creating a dual-nested Kevlar cable thermal isolation/structural support system similar to that flown on the CRYOTSU flight experiment on STS-95. To accurately test the thermal switching system, a novel laboratory set-up was utilized involving three parallel heat metering cryocooler simulators (Q-meters). Numerous test runs were carried out to evaluate the impact of various system operating parameters. The parasitic heat leak predictions corresponded very closely to the measured data. The paper describes the effort from concept development through test data analysis.