back to article 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 …

  1. Mark 85 Silver badge

    That certainly doesn't sound like much fuel. Anyone know how long they expect it to last since re-fueling isn't an option?

    1. eldakka Silver badge

      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.

    2. UCAP Silver badge

      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.

      1. Yet Another Anonymous coward Silver badge

        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'

        1. Stoneshop Silver badge

          it just points mostly in the general direction of 'away'

          And occasionally farts at it.

    3. Ian Johnston Silver badge

      Apparently robotic refuelling isn't currently planned, but is a future possibility.

  2. HildyJ Silver badge

    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.

    1. spold

      Re: Bon voyage

      ...charge the flux capacitor. It will be on the planned schedule.

  3. Sleep deprived

    Why so long to fill?

    Why does it take 10 days to fill 238 liters of fuel and oxidizer? This is a genuine question.

    1. msobkow Silver badge

      Re: Why so long to fill?

      Well, among other things, unlike in a car, a bubble in a rocket's fuel tanks can have explosive consequences...

      1. Yet Another Anonymous coward Silver badge

        Re: Why so long to fill?

        The two chemicals are hypergolic = basically explode on contact with each other.

        The craft cost $10bn and took 20 years to build

        They do not wish to become featured in a future "Who, me?" column

        1. UCAP Silver badge

          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.

          1. Yet Another Anonymous coward Silver badge

            Re: Why so long to fill?

            Well obviously they are also toxic and corrosive - it wouldn't be any fun if they weren't poisonous AND explosive

          2. Ian Johnston Silver badge

            Re: Why so long to fill?

            It's not immediately obvious that "taking longer over it" makes a dangerous process safer.

    2. Filippo Silver badge

      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.

      1. ClockworkOwl
        Thumb Up

        Re: Why so long to fill?

        Picture for perusal...

        Seems like they're taking it seriously...

        1. Yet Another Anonymous coward Silver badge

          Re: Why so long to fill?

          Is good reason tovarisch

  4. eldakka Silver badge

    I am feeling two conflicting emotions

    I'm so excited I could pee my pants.

    I'm so terrified that something goes wrong I could puke.

    1. Doctor Syntax Silver badge

      Re: I am feeling two conflicting emotions

      Please stand right over there. No, further that that. Right back some more.

    2. Antron Argaiv Silver badge
      Thumb Up

      Re: I am feeling two conflicting emotions

      My fingers have been crossed so long they're beginning to cramp.

      Good luck!

      1. ravenviz Silver badge

        Re: I am feeling two conflicting emotions

        Mine have fused together!

      2. idiot taxpayer here again Bronze badge

        Re: I am feeling two conflicting emotions

        I think @eldakka needs to cross more than his fingers

  5. NanoMeter

    Hope they made a copy of JWST

    Because what can possibly go wrong?

    1. 42656e4d203239

      Re: Hope they made a copy of JWST

      Pretty sure they don't have an offsite backup...

  6. adam 40 Silver badge

    It’s large mirror

    will have a good butcher's' at some star's and galaxy's

    1. TheRealRoland

      Re: It’s large mirror

      "have a butcher's" - "have a butcher's hook" - "have a look" - got it :-)

      *sighs contentedly* it was a good day. learned something.

    2. Mark Exclamation

      Re: It’s large mirror

  7. imanidiot Silver badge

    Pucker factor rising.

    I suspect some people will be crapping diamonds when it's in space and unfolded

  8. Pete 2 Silver badge

    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.

    1. Antron Argaiv Silver badge
      Thumb Up

      Re: New! Improved! oxidiser!

      1. 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."


      2. John Robson Silver badge

        Re: New! Improved! oxidiser!

        And still in print, so you can actually buy it....

      3. Doctor Syntax Silver badge

        Re: New! Improved! oxidiser!

        A lovely description of a monopropellant: "a molecule with one reducing (fuel) end and one oxidizing end, separated by a pair of firmly crossed fingers,"

    2. Hublot

      Re: New! Improved! oxidiser

      Will this text explain to me how horrid, sticky droplets of inefficiently oxidised hydrazine won't end up all over their nice shiny mirror(s)?

      1. Yet Another Anonymous coward Silver badge

        Re: New! Improved! oxidiser

        It's spraying out into space with a velocity.

        In space there's no wind to blow it back.

        Although it's a problem not to end up in a cloud of exhaust

        1. John Robson Silver badge

          Re: New! Improved! oxidiser

          Quite a high velocity hopefully

      2. the small snake

        Re: New! Improved! oxidiser

        Suspect one purpose of the oxidiser is to scavenge unused hydrazine from the otherwise-monopropellant engine although not sure.

    3. the small snake

      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.

      1. ClockworkOwl

        Re: New! Improved! oxidiser!

        Yup, and from a link provided below, it seems they are using both types of thruster.

        The biprops are for orbital manouvers and station keeping, the monoprops are for pointing and momentum management...

    4. druck Silver badge

      Re: New! Improved! oxidiser!

      Which flavour of hydrazine? Good old MMH or funky UDMH?

  9. Arthur the cat Silver badge

    When it's finally in orbit and definitely working

    I hope they'll hire a large opera diva to sing an aria, so we know it's over.

  10. Miss Config

    OS ?

    A recent Mars rover was/is run by a highly technical version of Linux.

    I was about to ask whether JWST was similar till I realised that JWST was probably designed BEFORE Linux was invented.

    Anyway does it have an OS of its own ?

    1. the small snake

      Re: OS ?

      Ingenuity uses Linux (and it is still flying which is amazing!). Perseverance & Curiosity run VxWorks. Not sure but would expect JWST to use VxWorks too: it's what I would use, anyway.

      1. Yet Another Anonymous coward Silver badge

        Re: OS ?

        JWST uses VxWorks. Remember it was basically designed/built 10years ago and has a much simpler task than the various Mars rovers. Point to next target, trigger instrument, store data, transmit back to Earth.

        1. the small snake

          Re: OS ?

          Well I would use it even if tasks were complicated! Apart from political considerations VxWorks is fine OS, very well tested including in many spacecraft, lots of understanding of it in NASA etc I expect.

          1. Yet Another Anonymous coward Silver badge

            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.

  11. 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

    1. the small snake

      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...]

      1. Yet Another Anonymous coward Silver badge

        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.

  12. Joe Gurman

    An understatement

    "Fingers crossed the telescope will finally take to space on 22 December."

    How about all movable parts crossed, and all bodily sphincters puckered up?

    1. This post has been deleted by its author

  13. Dante Alighieri


    Counting the days...

    Cheers to all involved!

  14. HammerOn1024

    Place Your Bets!

    I wonder what the Vegas odds are on this thing blowing up on the pad?

    With it's luck, and white elephant budget, it would be the perfect capstone.

  15. Ian Johnston Silver badge

    My money's on the solar panels failing to open properly, because every time NASA launch anything important, the solar panels fail to open properly. Skylab? Tick. Hubble? Tick. ISS? Tick. JWST ... ?

  16. 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.

    1. Ian Johnston Silver badge

      Re: Interesting aside

      Wouldn't that be a more likely task for a radio telescope. Are there likely to be "technosignatures" in the infrared that aren't swamped by those geet big infrared emitters dotted around the cosmos?

      1. the small snake

        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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