That certainly doesn't sound like much fuel. Anyone know how long they expect it to last since re-fueling isn't an option?
It's primed and full of fuel, the James Webb Space Telescope is ready to be packed up prior to launch
Engineers have finished pumping the James Webb Space Telescope with fuel, and are now preparing to carefully place the folded instrument inside the top of a rocket, expected to blast off later this month. “Propellant tanks were filled separately with 79.5 [liters] of dinitrogen tetroxide oxidiser and 159 [liters of] hydrazine …
COMMENTS
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Tuesday 7th December 2021 03:40 GMT eldakka
Design lifetime is at least 5.5 years.
Hopeful of 10 years.
Assuming there are no actual failures that reduce its life (non-redundant systems breaking for example), it'll depend on how much fuel it will need for stationkeeping. The L2 is not 100% stable, it requires regular adjustments to maintain orbit around it. So there's a lot of educated guesswork in how much fuel they'll need. So they've put enough fuel for a worst case station-keeping fuel usage for 5.5 years, which will of course last longer if their worst-case estimates were too pessimistic, so they are hoping for ~10 years, maybe more, but only 'guarantee' 5.5.
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Tuesday 7th December 2021 08:22 GMT UCAP
It may not sound like a lot of fuel, however the main mechanism for turning the satellite will be several sets of momentum wheels; the thrusters are generally only used for very coarse pointing (normally a rare event) and when desaturating/spinning-up the momentum wheels. Also the thrusters don't actually use a lot of fuel; typically only a few grams each time they are used.
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Tuesday 7th December 2021 13:21 GMT Yet Another Anonymous coward
The fuel is mostly to nudge it back I to place, L2 isn't a stable minima so any slight movement will cause it to drift away.
Hubble uses the Earths magnetic field to lean on to point itself, ie to unload the reaction wheels. I suspect this will re-point a lot less than Hubble, it doesn't have to avoid the Earth/Sun/Moon as much, it just points mostly in the general direction of 'away'
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Tuesday 7th December 2021 02:30 GMT Anonymous Coward
Bon voyage
Now that the Ariane upper stage has been mounted, telescope has been mounted to the upper stage adapter (the kerfuffle when the clamp slipped), and the telescope has been fueled, the major sphincter tightening points before launch are securing the fairing around the telescope, mounting the telescope assembly on the Ariane upper stage, moving the Ariane to the launch pad, fueling it, static testing it, refueling it, and blastoff.
We're not out of the equatorial rain forest yet but I have faith in the boffins.
What a long strange trip it's been.
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Tuesday 7th December 2021 08:25 GMT UCAP
Re: Why so long to fill?
Also neither the fuel or oxidizer plan nice with the human skin/flesh - one splash and you have an extended stay in hospital plus the the possibility of the cancer of your choice down the road. Given how dangerous these are, standard procedure is to take your time and do it right.
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Tuesday 7th December 2021 11:43 GMT Filippo
Re: Why so long to fill?
The basic answer seems to be that the chemicals are insanely dangerous, and that for this sort of job you never, ever, do anything in a hurry. I expect that they'll put a little bit in, then check everything, then put a little bit more in, then check everything again, and so on.
I'm still a bit curious about what the details of the procedure are, thought.
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Tuesday 7th December 2021 11:30 GMT Pete 2
New! Improved! oxidiser!
> Oxidiser improves the burn efficiency of the hydrazine fuel
I have a sneaking suspicion that the NTO does a little more than "improve" the hydrazine. Without it, the telescope would just be squirting N2H4 out of its thrusters.
P.S. for a very readable book on the subject, try Ignition!: An Informal History of Liquid Rocket Propellants available from a south american river. Ideal for the christmas period when there's bugger-all on TV.
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Tuesday 7th December 2021 13:54 GMT ACZ
Re: New! Improved! oxidiser!
And includes a few words at the beginning from Isaac Asimov, including these two fabulous paragraphs:
"Now it is clear that anyone working with rocket fuels is outstandingly mad. I don't mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity.
There are, after all, some chemicals that explode shatteringly, some that flame ravenously, some that corrode hellishly, some that poison sneakily, and some that stink stenchily. As far as I know, though, only liquid rocket fuels have all these delightful properties combined into one delectable whole."
;)
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Tuesday 7th December 2021 15:26 GMT Anonymous Coward
Re: New! Improved! oxidiser!
I have a sneaking suspicion that the NTO does a little more than "improve" the hydrazine. Without it, the telescope would just be squirting N2H4 out of its thrusters.
Not at all! Hydrazine can be used as monopropellant using a catalyst. Such engines are very often used. Both Curiosity and Perseverance used hydrazine monopropellant engines to land on Mars in fact.. Process is 3N2H4 -> 4NH3 + N2 (very exothermic) followed by 4NH3 -> 2N2 + 6H2 (endothermic). Nice feature of these engines is that if you get them right exhaust is cold and pretty clean (if you look at Perseverance landing video you will see exhausts are clear). Also N2H4 easy to store of course.
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Tuesday 7th December 2021 17:56 GMT Yet Another Anonymous coward
Re: OS ?
Problem is if you want a rover with lots of image processing / autonomous decision / "AI", then you either need university labs full of AI researchers who know VxWorks, or you hire 1000s of engineers to port opensource libs and algorithms they don't understand to space-grade HW.
More common is to use regular Linux running all the very high level stuff as a job (don't know the technical details) under something like VxWorks that is in charge of uptime/reliability - rather than trying to put a realtime Linux on the bare metal that can run everything.
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Tuesday 7th December 2021 15:48 GMT Terje
Why hydrazine?
There's one thing I fail to understand, and that is why use ordinary hydrazine and not for example udmh (unsymetric dimethyl hydrazine) or a mix instead? Hydrazine freezes at -2 degrees so it must be kept relatively warm while you want to keep the main part of the telescope cold. Wouldn't that be easier to do with udmh (freezes at -57 degrees)?
Mines the one with Ignition in the pocket
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Tuesday 7th December 2021 16:41 GMT Anonymous Coward
Re: Why hydrazine?
Is good question. Turns out the thrusters live on the hot side of the spacecraft, are part of the 'spacecraft bus' (which is misleading term). Temperature on hot side is about 300K so well over freezing point of hydrazine. So hydrazine is fine.
[And this time I will reply to right comment...]
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Tuesday 7th December 2021 18:05 GMT Yet Another Anonymous coward
Re: Why hydrazine?
Hydrazine is well known, this mission has enough challenges that you don't want to find out that some other propellant eats through a seal after 5 years.
There is a lot of work on alternatives with more boring ingredients. Apart from the Nasa greenwashing it's a pain for university cubesats or SpaceX type quick turn around launches if the fill up has to be done by a guy in a bomb disposal suit and then everyone else has to hide in a bunker a mile away.
Hubble couldn't use propellants at all because of the need for manned servicing missions - this isn't stuff you want to get on your space suit and then bring indoors.
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Tuesday 7th December 2021 21:07 GMT Conundrum1885
Interesting aside
I have a bet that JWST will find a technosignature less than five months after launch.
I hav an additional bet that it will be on a system within 65 light years.
Not saying that there's some inside information here, but my sources suggest that the Wow!
signal was actually genuine, and there is a good reason for it not to have been repeated.
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Wednesday 8th December 2021 10:45 GMT Anonymous Coward
Re: Interesting aside
I don't know. But JWST certainly is interested in looking at IR emissions from exoplanets, for instance to try and infer compositions of atmosphere ... like oxygen perhaps. That's not technosignature of course but is likely signature of life since on Earth anyway oxygen concs would decline pretty quickly w/o life pumping out more as oxygen does whole oxidising thing.
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