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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.

BradV
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  • Did you find any paper with reliability / leakage studies about the cryo cooler design? – asdfex Apr 09 '22 at 16:18
  • @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. – BradV Apr 09 '22 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.

David Hammen
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    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 – BradV Apr 09 '22 at 16:11
  • I cannot believe it.... I sort of disagree with David Hammen on something. OMG! ;-) – BradV Apr 09 '22 at 16:18
  • @BradV why should a cryocooler be more complex and more at risk of failure than the multiple systems required for halo orbit station keeping? – Starfish Prime Apr 09 '22 at 16:19
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    @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. – BradV Apr 09 '22 at 16:24
  • @BradV weren't the devices flown on the Planck and Spitzer missions pretty similar? I'm also seeing experiments that were certainly run in space in the late 90s that appear to be pretty direct precursors. I'm also not seeing huge numbers of spacecraft in halo orbits. Stationkeeping in such places is somewhat more effort than in more conventional orbits, no? – Starfish Prime Apr 09 '22 at 16:48
  • @BradV, space based cryocoolers seem relatively mature, "We have cryocoolers in orbit now that have been operating with no change in performance for over 20 years" – GrapefruitIsAwesome Apr 09 '22 at 16:49
  • @GrapefruitIsAwesome do note that >70K cryocoolers are quite different beasts from <10K cryocoolers. There haven't been many of the latter, and I don't think any have run for more than a few years. – Starfish Prime Apr 09 '22 at 16:51
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    @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. – David Hammen Apr 09 '22 at 17:30
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    @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! – BradV Apr 09 '22 at 22:28
  • @BradV TRL 8+ means tried and true technology. It does not mean infallible. Even TRL 9 technology does fail, for example TRL 9 reaction wheels based on steel ball bearings failed to due space weather events. So yes, let's hope in can last 10+ years. – David Hammen Apr 12 '22 at 14:25