Cryogenic diode heat pipe system for cryocooler redundancy

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.