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JWST, like many other spacecraft, has reaction wheels to orient itself. The reason Kepler ran out of fuel when it did was that it was expending more maneuvering
by ak217 5y ago
JWST, like many other spacecraft, has reaction wheels to orient itself. The reason Kepler ran out of fuel when it did was that it was expending more maneuvering fuel for attitude control because two of its four reaction wheels failed. Hubble and the ISS also had similar failures.
We're getting better all the time at building more reliable components (including reaction wheels and cryocoolers) though. Until a few years ago, the life of something like JWST would be limited by the amount of liquid helium on board to cool the components. Modern cryocooler technology (aka a space grade refrigerator) is good enough to cool it indefinitely. Solid state cryocoolers, previously unachievable, are now apparently available for some applications (important not only for reliability but also to reduce vibrations).
Reaction wheels can be used for attitude control but they still have to be unloaded by thrusters after maneuvering for a while. You're right that you could use a rudder (probably two rudders would be required for 3d attitude control) and have to have a balanced solar wind profile (JWST does actually have a solar wind balancing flap, but I don't think it's adjustable like a rudder). But solar wind won't act fast enough if you want to quickly change attitude for observations. And you can't use reaction wheels for stationkeeping. It very much matters where the telescope is, since if it drifts too far away from Earth it will be much harder to send high bandwidth data, and if it's too close to Earth, Moon etc. it will have no way to orient without heating up or blinding itself with the IR sunlight reflected by them.
- terramex 5y ago> The reason Kepler ran out of fuel when it did was that it was expending more maneuvering fuel for attitude control because two of its four reaction wheels failed. Hubble and the ISS also had similar failures. And to calm down anyone afraid of JWST sharing the same fate - construction of reaction wheels have been changed some time ago to make them significantly more reliable. The source of issues on Hubble, Kepler, FUSE, Hayabusa, Dawn and TIMED was electrical arcing between metal parts of reaction wheels. Static charge was building up like when you rub a ballon against your head. That charge caused arcing that in turn caused metal pitting and increased friction leading to failures. That failure mode was understood only in late 2007, when Kepler was already fully build and ready for launch. JWSt uses new generation ceramic bearing in its reaction wheels, they have been used in spacecrafts since 2010 with great performance.
- dtgriscom 5y agoGreat info: citation? More details?
- terramex 5y agoHere is a paper detailing metal reaction wheels failure modes going into detail on how charge was built up and solar influence on this process: https://www.esmats.eu/esmatspapers/pastpapers/pdfs/2017/bialke.pdf https://www.esmats.eu/esmatspapers/pastpapers/pdfs/2017/bial... I cannot find primary source for JWST using ceramic bearings right now, but I know they are manufactured by Rockwell Collins Deutchland and have good track record: https://jwst.nasa.gov/content/forScientists/faqScientists.html#wheels https://jwst.nasa.gov/content/forScientists/faqScientists.ht... TELDIX (today owned by Rockwell Collins Deutchland) invented use of ceramic bearing in reaction wheels for spacecraft use in 1978: https://patents.google.com/patent/DE3027209A1/en https://patents.google.com/patent/DE3027209A1/en so it would make sense that JWST uses ceramic bearings. Other manufacturers (ITHACO) have switched to ceramic too.
- Laremere 5y agoScott Manley video: https://www.youtube.com/watch?v=KibT-PEMHUU https://www.youtube.com/watch?v=KibT-PEMHUU
- magicalhippo 5y ago> JWST, like many other spacecraft, has reaction wheels to orient itself. It should be noted that reaction wheels can saturate when the motor reaches its top speed. One then needs to spend fuel to provide a counter-force while the wheel to spins down. So even with reaction wheels running off solar panels or similar you need fuel, though much less.
- dreamcompiler 5y agoThat's what "unloading" the reaction wheels refers to in some comments here, in case others were wondering.
- dTal 5y agoPerhaps in a future design the reaction wheel could be unloaded by solar rudder, reducing the need for maneuvering fuel (although station keeping fuel would still be required without an outright solar sail). edit - apparently they do do something like this? https://news.ycombinator.com/item?id=29857031 https://news.ycombinator.com/item?id=29857031