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While most of JWST uses passive cooling to get instruments to low operating temperature the MIRI requires active measures to get to 7K. I've been reading about the complex system developed to cool MIRI and... holy cow! At least the other instruments will continue to function if MIRI cooling fails.

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  • $\begingroup$ Did you find any paper with reliability / leakage studies about the cryo cooler design? $\endgroup$
    – asdfex
    Commented Apr 9, 2022 at 16:18
  • $\begingroup$ @asdfex sorry, no. I did find an early trade study that compared a dewar flask consumable fluid cooling system with active closed system. sorry, didn't keep link. $\endgroup$
    – BradV
    Commented Apr 9, 2022 at 16:28

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JWST's weakest link is the propellant needed to maintain its pseudo-orbit about the Sun-Earth L2 point and to desaturate the vehicle's momentum wheels. That propellant will eventually run out. Shortly before that happens the JWST intentionally will be made to exit the vicinity of the L2 point. The JWST was not designed to have this propellant resupplied.

The helium used for active cooling of the JWST MIRI is a closed system. There will be leaks (helium leaks at a slow rate through any container / plumbing system). However, assuming a lack of failures and a nominal leak rate, the helium used for cooling the MIRI will last longer than will the propellant.

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    $\begingroup$ the propellant to maintain station keeping was a designed in factor and so I wouldn't consider it a "weak link". To me a weak link is something that is not supposed to break or fail or run out but DOES unexpectedly before it should. To me, the complexity of the cryocooler means a high number of failure modes with potential for any number of its many single-point fail points possibly degrading cooling beyond effective use. I come from a background of avionics systems Mean Time Between Failures (MTBF), Mean Time Between Removals (MTBR), etc $\endgroup$
    – BradV
    Commented Apr 9, 2022 at 16:11
  • $\begingroup$ I cannot believe it.... I sort of disagree with David Hammen on something. OMG! ;-) $\endgroup$
    – BradV
    Commented Apr 9, 2022 at 16:18
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    $\begingroup$ @Starfish Prime we've been designing/building these station keeping systems for many years and have had lots of success. This unique cryocooler seems to be a one-of-a-kind spacebased system. $\endgroup$
    – BradV
    Commented Apr 9, 2022 at 16:24
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    $\begingroup$ @BradV Strictly speaking, the MIRI cooling system accounts for a few of the 344!!! single points of failure on the JWST. Most of these single points of failure have already succeeded. Some were brand spanking new systems. (I'm quite amazed that it has all worked.) The MIRI cooling system, while complex, is not brand spanking new. This is tried and true technology. $\endgroup$ Commented Apr 9, 2022 at 17:30
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    $\begingroup$ @David Hammen I too am amazed at how each and every thing has performed. Absolutely inspiring engineering! This cryocooler uses 2 moving piston pumps. Each pump uses dual opposed pistons to help minimize vibrations for a total of 4 pistons. Also uses a number of pulse tube compressors. All TRL 8+ (Technology Readiness Level). Hope can last 10+ years! $\endgroup$
    – BradV
    Commented Apr 9, 2022 at 22:28

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