[HN Gopher] NASA Says Webb's Excess Fuel Likely to Extend Its Li...
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NASA Says Webb's Excess Fuel Likely to Extend Its Lifetime
Expectations
Author : ahamez
Score : 117 points
Date : 2021-12-29 15:44 UTC (7 hours ago)
(HTM) web link (blogs.nasa.gov)
(TXT) w3m dump (blogs.nasa.gov)
| wolverine876 wrote:
| NASA may be the greatest engineering organization in human
| history; I can't think of any competitors. They are also masters
| at consulting: Underpromise, overdeliver.
|
| 'The rover is planned to last one year.' 'Amazingly, it is still
| working after three years!'
| quesera wrote:
| Just to enumerate some recent missions: * NASA
| Curiosity: "2 years" -> 9.5 years, and counting. * NASA
| Spirit: "90 days" -> Just over 6 years, ending in 2010. *
| NASA Opportunity: "90 days" -> Over 14 years, ending in 2018.
|
| NASA has had some failures too, of course. These stats are
| cherry-picked but still impressive.
| wolverine876 wrote:
| My point is that it's not impressive, it's just an old sales
| technique. Curiosity could have been designed for 20 years,
| but all NASA needs to do is say '2 years' and the project
| sounds like a resounding success if it fails in 10.
|
| (It is extremely impressive to have rovers and helicopters
| traversing Mars.)
| quesera wrote:
| That's a cynical perspective. though.
|
| In each of the above cases, the mission end date was
| extended because the hardware was capable of doing more
| science (or for more time) than it was designed for (and
| budgeted for).
|
| Like JWST, some of the procedures required for a successful
| mission had never been done before, and could not be tested
| fully on Earth. Many of those procedures have implications
| on mission longevity. Very smart and dedicated people make
| their most educated guesses, build in what they know, try
| to handle the unknowns, and hope and pray -- and more often
| than not, are proven correct. That's pretty close to my
| personal definition of "impressive engineering".
|
| There's a political angle too, of course. We don't know
| what NASA's "estimate fudge factor" is. I'd be amazed to
| learn that it is 5x, 24x, or 56x.
| wolverine876 wrote:
| > the mission end date was extended because the hardware
| was capable of doing more science (or for more time) than
| it was designed for (and budgeted for)
|
| That is circular, it assumes that the announced end date
| isn't just underpromising what they know will be a later
| date. Neither of us is there when these dates are chosen,
| so we don't know what they are thinking. Arguably, NASA
| should be better able to estimate project lifetimes by
| now.
|
| NASA is doing amazing things; I'm not so cynical.
| Sharlin wrote:
| Big props to Ariane 5 and the whole Arianespace team for
| executing such a precise insertion.
| kitd wrote:
| Agreed. This whole operation has been a superb advert for
| Ariane 5.
| mrfusion wrote:
| I never understood how we launch to earth sun langrange points?
|
| By starting from earth we're already at the correct orbital
| velocity around the sun so do we just cruise out there and cancel
| out the delta v we used to get there?
| kibwen wrote:
| Take a look at the dashboard and note the current speed:
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html
|
| Right now it's traveling at about 0.55 miles per second. When I
| checked the other day that was over 0.7, and a bit after launch
| it was over 1.
|
| Putting JWST into L2 is essentially the world's biggest curling
| toss ( https://en.m.wikipedia.org/wiki/Curling ).
|
| As the OP alludes to, JWST's operational life is hard-limited
| by its fuel capacity. L2 (like L1 and L3, but unlike L4 and L5)
| is an unstable orbit, and without regular corrections it will
| drift out of position. This means that you want to save ALL
| your fuel for making those corrections, and thus spend as
| little of it as possible on getting there in the first place.
| So over the next month it's going to slowly crawl into place,
| cruising on no power other than what the Ariane initially gave
| it, getting continually slower and slower via the effect of the
| earth and sun's gravity until it stops perfectly in position.
| khuey wrote:
| > cruising on no power other than what the Ariane initially
| gave it
|
| Webb has already done multiple burns to get to L2 after being
| released from the Ariane upper stage. Preserving time on
| orbit is important but the bigger issue is that Webb can only
| fire its thrusters in one direction (because it can't point
| the optics at the sun) so any overshoot is catastrophic.
| Ariane thus was set to intentionally undershoot L2 and Webb
| is making up the difference with its fuel.
| vidanay wrote:
| If curling stones had attitude adjustment thrusters and PID
| control loops and astronomical observations and precision on-
| board time keeping.
| mattmoose21 wrote:
| wouldnt those be the brooms?
| blacksmith_tb wrote:
| Automation is coming[1] (though presumably only as a fun
| challenge).
|
| 1: https://www.smithsonianmag.com/smart-news/curly-curling-
| robo...
| [deleted]
| mabbo wrote:
| Remember that no matter how far from the earth you get, you're
| always being pulled by it, decreasing velocity. The Webb left
| earth moving at 7km/s, but now is going less than 1km/s.
|
| It's going to let Earth's gravity slow it down so that it
| reaches (effectively) zero at exactly the right spot.
|
| It's literally rocket science to do the equations to make it
| work, but by God we've gotten pretty good at it.
| BbzzbB wrote:
| Zero? Won't it keep spinning on a plane perpendicular to our
| orbit and "pointing" the Earth and Sun?
| jacquesm wrote:
| Yes, but that's 90 degrees to the current direction of
| travel. The L2 orbit insertion burn will achieve that.
| BbzzbB wrote:
| Oh, right. Makes sense thank you.
| nabla9 wrote:
| Hershel operated 4 years and one month, but this was because it
| needed liquid helium coolant and it eventually ran out. Hershel
| was planned to operate 3.5 years.
|
| btw. Hershel wast the first optical telescope placed in L2. It
| was also it was also the largest optical space telescope before
| JWST.
| julienchastang wrote:
| "though many factors could ultimately affect Webb's duration of
| operation." JWST also needs helium coolant with no possibility
| to recharge therefore limiting its lifespan to ~10 years, I
| believe. Interesting about Herschel and L2.
| bryanlarsen wrote:
| IIRC, the helium coolant is only used on one of JWST's 4
| instruments. So if they run out of helium the other three can
| still be used.
| terramex wrote:
| It does use helium only as refrigeration fluid in closed
| cycle, it does not get depleted. Building a cryocooler
| capable of achieving so low temperatures without boiling
| helium off was a major hurdle in JWST development.
| vlovich123 wrote:
| Isn't helium notoriously hard to contain without leakage
| even in a closed system?
| dnautics wrote:
| that's hydrogen. IIRC from my chemistry days, Helium is
| hard to contain in plastic closed systems, but "generally
| OK" in metal tubing.
| shepardrtc wrote:
| It actually uses a closed system:
|
| https://webb.nasa.gov/content/about/innovations/cryocooler.h.
| ..
| jerjerjer wrote:
| Doesn't liquid helium always evaporate?
| [deleted]
| Something1234 wrote:
| Wait if Hershel was the largest optical telescope in orbit, how
| come it didn't catch the public imagination like Hubble did?
| adventured wrote:
| In the late 1990s you'd walk into Walden Books or the
| equivalent store and this would be staring at you:
|
| https://i.imgur.com/Go5VDi1.jpg
|
| And it seemed truly astounding. Like we were getting to
| journey across the universe. The visuals that Hubble
| presented, the variety and scale of that style of imagery
| (great for PR), and the way it cultivated the imagination of
| the mass public, is still unrivaled.
| nabla9 wrote:
| Because it must be American to catch attention in American
| media.
|
| Hershel took awesome images. For example:
|
| Galactic outflow with the Suzaku X-ray astronomy satellite ht
| tps://www.esa.int/var/esa/storage/images/esa_multimedia/im...
|
| Herschel's view of new stars and molecular clouds https://www
| .esa.int/var/esa/storage/images/esa_multimedia/im...
|
| The Little Fox and the Giant Stars https://www.esa.int/var/es
| a/storage/images/esa_multimedia/im...
|
| Star formation on filaments in RCW106 https://www.esa.int/var
| /esa/storage/images/esa_multimedia/im...
|
| Feathery filaments in Mon R2 https://www.esa.int/var/esa/stor
| age/images/esa_multimedia/im...
| Gravityloss wrote:
| Herschel was more modest in size, 3.5 meters vs JWST:s 6.5
| meters. Also ESA PR is not as great as NASA. NASA is the gold
| standard in this regard.
| jcims wrote:
| Also because it was imaging far infrared with wavelengths
| 100-1000 times that of visible light, the resolution of the
| optics was much lower and yielded kind of bla images (with
| a few notable exceptions):
|
| https://www.herschel.caltech.edu/images
| ATsch wrote:
| I think a big reason Hubble managed to do that was that it
| captures in the visible spectrum due to being a repurposed
| spy satellite. That spectrum just isn't that interesting
| these days afaict, especially since ground based
| observatories have gotten a lot better at seeing through the
| earth's atmosphere. The false color images of most of today's
| missions are just a lot harder for the public to understand.
| It was also launched in a time where NASA desperately needed
| good publicity.
| izacus wrote:
| "... due to being a repurposed spy satellite."
|
| What? Where did this come from?
| segfaultbuserr wrote:
| KH-11 ("Keyhole-class") spy satellites. It's generally
| believed that Hubble's optical technology came directly
| from that.
|
| > KH-11s are believed to resemble the Hubble Space
| Telescope in size and shape, as the satellites were
| shipped in similar containers. Their length is believed
| to be 19.5 meters, with a diameter of up to 3 meters (120
| in). A NASA history of the Hubble, in discussing the
| reasons for switching from a 3-meter main mirror to a
| 2.4-meter (94 in) design, states: "In addition, changing
| to a 2.4-meter mirror would lessen fabrication costs by
| using manufacturing technologies developed for military
| spy satellites".
|
| https://en.wikipedia.org/wiki/KH-11_KENNEN
| krallja wrote:
| https://space.stackexchange.com/questions/3448/was-
| hubble-re...
| beezle wrote:
| I don't believe Hershel was optical at all - pretty certain it
| was far infra-red
| jacquesm wrote:
| Infra-red is optical. Just not visible light but it uses
| lenses and mirrors like every other visible light telescope.
| The main difference is that those lenses are designed to be
| as transparent to IR as possible instead of accidentally
| filtering it out.
|
| The big trick for Webb is that it will allow for extremely
| red shifted IR to be received, which translates into 'light
| from very long ago and very far away'. To be able to do that
| the sensors are cooled to extremely low temperatures to
| ensure that the instrument noise level is brought down as low
| as possible, allowing for extreme sensitivity.
| nabla9 wrote:
| Optical refers to principles that work in visible,
| ultraviolet, and infrared light.
|
| JWST is also primarily infrared telescope. It as some
| capability in red and the yellow part of the visible
| spectrum.
| wolverine876 wrote:
| > Optical refers to principles that work in visible,
| ultraviolet, and infrared light.
|
| Is there something qualitatively different about infrared
| and ultraviolet that they share with visible, that they are
| all grouped into 'optical'?
| Maxburn wrote:
| See where Webb is on the way to L2 orbit;
| https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html Fun to
| see where it is on deployment.
|
| It's also interesting to me that the mirror appears to be fully
| exposed to space unlike Hubbell. If anyone knows something about
| that design choice I've like to hear it.
| 19870213 wrote:
| Main reason I heard was that enclosing it would be preferable,
| but then it wouldn't fit in any currently in use fairing.
| wmf wrote:
| If the optical path was enclosed the JWST would probably be too
| big to fit on any existing rocket.
| Laremere wrote:
| Factors I can think of:
|
| 1. Bigger mirror = better. Enclosing a mirror requires mass and
| volume, limiting mirror size.
|
| 2. The L2 orbit is unstable. Far less risk of being hit by
| debris than low Earth orbit.
|
| 3. Webb's sun shield blocks the primary light pollution angle.
| Hubble would need more protection from all angles due to the
| Sun, Earth, and Moon.
| Maxburn wrote:
| Did not realize the light pollution angle, that's a major
| one.
|
| I also had no idea L2 was unstable and found a little
| discussion on it.
| https://space.stackexchange.com/questions/284/why-should-
| the...
| miohtama wrote:
| Here is a good video about the stability of L1 - L5 points
|
| https://youtu.be/7PHvDj4TDfM
| anonAndOn wrote:
| 4. Any variations in temperature in the mirror array affect
| geometry/optics
| Maxburn wrote:
| I was just reading that after L2 insertion it's going to
| take five months to get everything cooled off and aligned
| properly. Amazing.
| jacquesm wrote:
| Cooldown has already started with the deploying of the
| sunshield from it's launch position, even before it is
| unfolded fully and stacked properly the delta is already
| a whopping 60 degrees Celsius across a bit over two
| meters.
| latchkey wrote:
| Maybe we will get lucky and find out something like this
| announcement.
| terramex wrote:
| Hubble needed to hide mirror because it is passing from
| sunlight into shadow every 47 minutes and mirror has to be
| protected from thermal stress. Also Earth emits some light even
| on night side that would interfere with observations.
|
| JWST's mirror is always in shadow and stray light from sides
| will not be a problem at L2.
| Maxburn wrote:
| That makes a lot of sense, thanks.
| y4mi wrote:
| If you're interested in a more indepth view of how it's
| supposed to work, Real Engineering did a video about it
| recently.
|
| No idea if what they're saying is true however, I've no
| knowledge about the domain.
|
| https://youtu.be/aICaAEXDJQQ
| vikingerik wrote:
| Fascinating that it's already covered 40% of the distance, but
| it's only on day 4 out of 30. That makes sense once you realize
| that its radial velocity needs to slow to zero when it reaches
| L2. I'd love to see the shape of how the velocity curve slows.
| Maxburn wrote:
| They link to this which shows the gravity gradients in play.
| I could have sworn they charted the velocity involved but I
| can't find it now.
|
| https://upload.wikimedia.org/wikipedia/commons/3/36/Lagrangi.
| ..
| jacquesm wrote:
| I've been glued to that page since the launch. The temperature
| delta just came online so you can now see the 'top' and
| 'bottom' temperature.
|
| In the end it will be several hundred degrees difference, which
| given the distances involved is extremely impressive.
| Maxburn wrote:
| The explore all deployments section is worth a read too.
| Wealth of info on the whole site and it's designed really
| well. Clearly catering to the education / general public good
| will with this site. https://jwst.nasa.gov/content/webbLaunch
| /deploymentExplorer....
| yeetaccount4 wrote:
| NASA likes to pull the old mechanic's trick of underpromising and
| over delivering.
| beckingz wrote:
| It's career ending to not do that.
| Salgat wrote:
| It's so consistently done by NASA that I'm no longer impressed
| when they reach past EOL. It just shows that they were covering
| their asses.
| bryanlarsen wrote:
| That's just a natural consequence of designing with safety
| margins.
|
| In other words, it should be the norm and not the exception.
| But of course the vast majority of us work in fields where
| overpromising is the norm.
| vikingerik wrote:
| And think about how many layers of safety margins. If there
| are ten subsystems each with a 20% safety margin to say that
| their designers did their jobs right, then you might have an
| overall system with as much as 200% safety margin. (yeah not
| everything is linear like that, but illustrating the point.)
| asplake wrote:
| Tempting fate perhaps even to ask, but I wonder if an unmanned
| refuelling mission might be possible?
| tekno45 wrote:
| They are planning one!
| Tepix wrote:
| Source?
| Arubis wrote:
| Physics doesn't make this impossible, but it's _enormously_
| expensive to get payload to L2.
| toomuchtodo wrote:
| I am not familiar with the station keeping needed for
| maintaining an L2 orbit; would it be possible to use an Earth
| orbiting laser to apply photonic force against JWST's solar
| shield (turning it into a laser sail) to balance it "falling
| back" towards Earth without onboard propellant? My
| understanding is that it has a gravitational weighting
| towards Earth to prevent slipping away.
| vikingerik wrote:
| Is it enormously expensive because of physics or because of
| bureaucracy and politics?
|
| Like, serious question: how much cost are the raw materials
| for the fuel and spacecraft/rocket, and how much are the
| costs like human labor and ground transportation to the
| launch site?
| jstrodd wrote:
| There is a very good and detailed answer to your question here:
| https://space.stackexchange.com/questions/38389/is-it-possib...
| bryanlarsen wrote:
| That's a very good answer. A few more points, because the
| answer is a 2.5 years old.
|
| - the docking adapter discussed in the answer did not make
| it. However, the answer offers a couple of alternatives, so
| +1.
|
| - they painted some crosses on the JWST to make alignment
| easier.
|
| - JWST's L2 location is really far from Earth. It's about 4X
| the distance from the moon. So a manned mission is currently
| infeasible. But maybe in the future.
|
| - while fueling may be possible, any other sort of
| maintenance probably isn't. To save weight, the JWST is
| mostly glued together, unlike Hubble which used spacesuit
| accessible bolts.
| terramex wrote:
| I've read that optical tracking/docking markers were added
| around mating ring that connected JWST to Ariane upper
| stage. It does not mean that docking to it will be easy but
| it was at least considered.
| zackbloom wrote:
| It's also worth noting that much of JWST is now twenty year
| old technology. Assuming it isn't a total failure which
| needs to be salvaged for PR reasons, it will almost
| certainly make more sense to build a new observatory in ten
| years rather than rescuing this one. If Starship exists at
| that point it will also be a much less complex undertaking
| to get it into space, and it could likely use an even
| bigger mirror.
| vlovich123 wrote:
| The complexity of the undertaking was not building the
| rocket. The complexity is the telescope itself. It would
| be interesting what the costs come down to if rather than
| starting from scratch they iterated on existing designs.
| Anyone know what the incremental cost of building the
| same telescope again would look like?
| pirate787 wrote:
| My understanding is that a lot of the complexity is
| making the telescope able to fit in limited space and
| then unfurl. That constraint is lifted significantly with
| Starship.
| wolfd wrote:
| I think that the other commenter was referring to the
| relative complexity in JWST's unfolding/deployment. It
| might be possible to deploy something from Starship that
| doesn't need to be packed as tightly (and save on
| engineering costs there) and still get around the same
| performance.
|
| A complicated sunshield deployment seems somewhat
| inevitable, personally, but I'd be happy to be proven
| wrong there.
| yupper32 wrote:
| > So a manned mission is currently infeasible. But maybe in
| the future.
|
| Given the increased interest in manned missions and recent
| technology advancements/investments, I wouldn't be
| surprised if we could get there in 15-20 years, which
| sounds likely to be when it'd need to be refueled.
|
| If it's worth it or too dangerous to send an astronaut to
| deal with the dangerous refueling operation are whole other
| questions. But actually getting there? I think we can do it
| in 15-20 years.
| pirate787 wrote:
| Maybe but it might be obsolete by then, especially
| considering how much lift capacity Starship will bring
| online. Will be a lot cheaper to just replace Webb.
| jacquesm wrote:
| That propellant tank is massive.
| blackoil wrote:
| Can anyone with knowledge provide a rough estimate of how much
| would it cost to manufacture a second telescope considering all
| the research cost is assigned to first one.
|
| It is a hypothetical question to understand if we had lost it in
| launch would it have been a complete loss or can we have 2nd one
| within couple of billions and 3 years.
| dr_orpheus wrote:
| From my experience the cost breakdown of a typical [0]
| satellite is on the order of 60% recurring engineering, 40%
| non-recurring engineering. I don't have a good sense of the
| breakdown for JWST but the assembly and verification campaign
| is huge compared to most satellites and they would have to do
| that again so I would expect the recurring engineering to be
| higher. Coupled with the fact that it wouldn't be directly
| build-to-print due to parts obsolescence and technology
| upgrades they would want to make the cost would probably be
| higher still. So its not going to be a huge cost savings but
| you could maybe do it for something like 70-80% of the original
| cost.
|
| As for the timeline, that's going to be a hard one to change.
| Consider that all of the mirror segments and optics were
| delivered by the end of 2013. So the integration and test
| campaign of the spacecraft took 8 years. There would definitely
| be some optimization and lessons learned from that and you
| could likely shave off a couple years. However, if we are also
| looking at building it "now" the supply chain issues also hit
| the aerospace industry. There are typical things that you would
| expect like chip shortages, but also random things like its
| difficult to buy the epoxy we normally use to glue solar cells
| on to panels.
|
| [0] "Typical" varies wildly here. So for typical I am assuming
| something like a large NASA earth science mission with building
| one spacecraft of a relatively unique design.
| bryanlarsen wrote:
| A lot of those people hang out in the comments on Eric Berger's
| posts on Ars. Here's one example:
| https://arstechnica.com/science/2021/11/james-webb-space-tel...
|
| He estimates that a second would cost about 90% of the first.
| pas wrote:
| So what's next?
|
| Modular telescopes? Couldn't the whole manufacturing of the
| module (semi-)automated? (So no need for so much testing,
| verification, etc.) Bring unit costs down, and launch a few
| of the modules each year. Eventually they'll have as big a
| mirror as they want.
| miohtama wrote:
| A telescope that is assembled in space.
| idealmedtech wrote:
| A lot of the core research wouldn't have to be repeated, but
| the build and verification costs would not go away, and to do
| proper verification takes many many years.
| zeristor wrote:
| A key thing is that several years ago they realised that the
| reaction wheels were failing earlier than planned, some nifty
| deep analysis found the issue and I hope that this means they'll
| last long enough.
|
| The Reaction wheels are used to rotate the space craft without
| the use of fuel, and Scott Manly, as ever did a great video on
| this:
|
| https://youtu.be/KibT-PEMHUU
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