A lot of people and engineers are saying that the JWST is a marvel of engineering, with truly inovative technical solutions and a giant step up compared to Hubble Telescope. And it does seems like so!
However, I'm always baffled how everyone seems proud that the telescope has something like 200 SPOF during deployment, and if even one of them fails the whole mission could fail.
I know that each step has probably been throughoutly tested, and that the acceptable probability of failure of each one of those steps has been deemed acceptable. But I'm still surprised that people are proudly conflating excellent engineering with a design that has a large number of spofs.
In my domain this would be considered as a terrible design (aka "hope is not a strategy"), even given the constraints of mass and volume that such project incur: 200 hundred low probability events, chained, can get in the realm of possible.
I can't imagine JSWT team doing "bad engineering", so I'm sure I'm missing a piece. Is it only PR that underline this aspect? Is JWST as brittle as the news want to make us think? Or are there technical reasons or acceptable failure modes that gives confidence that those steps are not as critical as the news let us people know?
I don't think that a raw metric of the number of SPOF is the right way to measure the risk of this spacecraft. It's a fun term for PR purposes (and emphasizing the risk here) but the actual risk posture is more complex.
I imagine that in the course of developing this, they worked out a possible strategy without all of those SPOF - but doing so doesn't eliminate the risk, and the impact to mission is likely massive.
Let's say a Falcon 9 launch is $90M. Falcon heavy let's say $200M.
So you take your 3x $3B. Put $200M/instrument into launch, have $2.8B per telescope leftover.
There just seems to be something wrong that it costs THIS much to build a telescope.
That said, the Thirty Meter Telescope is also a sort of "forever" job, the delays have stretched on and on.
I wonder if you did something like bid out and paid just on performance instead of this forever cost reimbursement thing. Right now if you can get onto one of these mega projects, and can stretch it out with delays, it basically can cover your career (ie, 20 year projects).
There is no room for redundancy in many aspects of the design, unlike, say a Boeing 777 or Airbus A350.
How can you have a redundant heat shield, or primary mirror (two parts of which swing)? I'm sure some computer systems have redundancy and perhaps comms.
But like with a helicopter: how can you have redundancy in the tail rotor?
So with the JWST: there's no way around many SPOFs.
https://verticalmag.com/news/bell-electrically-distributed-a...
Instead of ramping up a project, and building 1 of something, you would plan to do more than one, and you could iterate over time as you learn. SpaceX is doing a good job of this.
If 1 Webb telescope is valuable then wouldn't 3 or 5 also be valuable?
We have a number of proven space designs at this point: Soyuz, Spirit/Opportunity rovers.
They must've calculated that the overall chance of success, and they have a target, and they met their target. Unfortunately, tests and theoretical modelling have a tendency to not exactly replicate a space environment (or any true production environment), nobody's perfect at anticipating everything, and management has ways of manipulating engineering estimates.
The Space Review [1] quotes NASA as saying there are 344 SPOF. They talk mainly about the sun shield, so that's probably the biggest risk, but consider all of them as about equal...
If each SPOF has a 0.1% chance of failure, net success rate is only 71%. Presumably most of the estimated failure probabilities are less than that, and the sun shield—which probably comprises many of the SPOFs—averages (far?) more than 0.1% per SPOF, because everyone seems to be particularly worried about that working.
I wonder what that figure is. Has it been published anywhere? Dear NASA and ESA, what do your engineers say about overall chance of failure?
The JWST marketing seems to work under a similar premise, they proudly proclaim that they couldn't mitigate hundreds of single points of failure, and you're supposed the be impressed by how difficult their task is. Hopefully the engineering did a reasonable job and the marketing is just playing up the wrong thing.
I'm not sure I've seen anyone who is _proud_ of it, lots of people are just setting expectations. Probably due to the similarities with Hubble (although JWST can self-align it's mirrors!).
It also might be posturing to show how well the thing is built. Space is hard, like really hard, and these agencies keep knocking it out of the park.
It's also has to fit on the rocket hence the once off folding mechanism. And after deploying it has to be perfectly aligned (remember the Hubble with its slightly off mirror)
I think having redundancy for everything would just not make for a launchable spacecraft.
The thing to realize is that these are some of the hardest things humanity has tried their hand on and if it all works that's a great thing for all of us, if it fails we will learn something and we'll go back to the drawing board (but we won't have a JWST and that's a significant loss, besides the obvious future calls of 'look at what happened to JWST' which will no doubt have negative impact on finding funding for future space missions).
Also, I think you're mistaken about people being 'proud about the 200 SPOFs', if they could have made it one less they certainly would have because everybody involved wants this to succeed. Think of these as the ones that they simply could not get rid of no matter how hard they tried.
Apollo was wildly dangerous. Apollo 1 killed the whole crew. The contemporary calculated failure odds for a Saturn V launch were 1/8. Compare that to the current Dragon 2 projected LOC risk of 1/276 [1].
[1] https://en.wikipedia.org/wiki/Crew_Dragon_Demo-2#:~:text=NAS....
Reading the other comments, and maybe to contextualize to my question better, I'm more surprised by how the project is presented as marvelous to the public, rather than thinking that any technical part were overlooked.
While I'm sure that engineering teams at NASA and ESA have countless contingency plans, procedures and failure models. Medias around the project seem to focus on how fragile the deployment procedure is. Great engineering is an act of finding the best balance between opposing constrains, by building technically sound systems but also more importantly designing robust human or automated procedures.
In this story, in my opinion, the media presents a skewed explanation of why the project is incredible by highlighting that it would be incredible that such a brittle deployment procedure would even work.
Space is hard.
Genuinely curious: how would you have achieved the mission goals with fewer SPOF?
Why can't it unfold etc in Earth orbit, where a repair mission can be sent if needed, and then travel to its Lagrange point?
To get from low-earth orbit to the sun-earth Lagrange point 2 (where the JWT is headed) takes around 7 km/s of delta-v[0]. That's a lot of speed.
You could try to do this gently enough that the unfurled JWT won't be damaged by the acceleration. This isn't totally impossible, but you'd need a good Hall thruster (ion engine) with a huge amount of reaction mass, since the JWT is so big itself. It would need to run for longer than any other such thruster has. It would need massive solar panels to power it.[1]
Or you could have the original rocket just be bigger, and throw it all the way to the right orbit while everything is packed tight.
[0] https://en.wikipedia.org/wiki/Delta-v_budget [1] I'm guessing at this, but that's my intuition. I encourage anyone to correct me because space is too cool to be upset that I was wrong.
Does that really matter without air resistance?
Depending on how high you actually bring it. Like 500km away from earth is still an orbit (I think that's Hubble's orbit) but how much force do you need or will happen?
A broken JWT could wait a few years in orbit.
Another comment mentioned that it's not designed to accelerate while it's fully deployed, and that's true enough. You'd wreck it.
The other essential thing is that there's no way to give it and its instruments anything like their designed operating parameters (pretty hot on one side of the sunshade, something like 40 kelvin on the other side) in Earth orbit.
Let alone the current lack of in orbit service capabilities like we had when the space shuttle was still around.
I saw some interviews of engineers of the jwst and few of them had similar ideas, or at least to assemble them in leo then slingshot them to their final positions/orbits
4:20 am PST
7:20 am EST
"In the 2005 re-plan, the life-cycle cost of the project was estimated at US$4.5 billion. This comprised approximately US$3.5 billion for design, development, launch and commissioning, and approximately US$1.0 billion for ten years of operations.[18] ESA is contributing about €300 million, including the launch.[84] The Canadian Space Agency pledged $39 million Canadian in 2007[85] and in 2012 delivered its contributions in equipment to point the telescope and detect atmospheric conditions on distant planets."
It can launch anytime during that window.
It's going to be awesome for us to watch but I feel for all the folks that worked on this.
"In exchange for full partnership, representation and access to the observatory for its astronomers, ESA is providing the NIRSpec instrument, the Optical Bench Assembly of the MIRI instrument, an Ariane 5 ECA launcher, and manpower to support operations. The CSA will provide the Fine Guidance Sensor and the Near-Infrared Imager Slitless Spectrograph plus manpower to support operations."
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Par...
the falcon heavy probably didn't even exist when they were drawing up the contracts
But yes, the contract for the webb launch was probably locked a very long time ago.
(Though it's also possible that Falcon 9's fairing wont be able to accommodate Webb)
Reliability (as in: established track record).
Although, if Ariane decides to explode tomorrow, this comment will look ... odd.
All that said it's worth nothing that SpaceX's flight success rate is 98.5 (135/137), while Ariane V's is 95.5 percent (106/111).
The really gobsmaking thing about that is that this is that SpaceX's rate is over 11 years, while Ariane's is over 25 years.
It's time to stop thinking of SpaceX as the plucky, untrustworthy startup.
In the future space telescopes like this really need to be built in LEO, and then boosted to Lagrange points. The number of failure modes beyond the typical rocket / stage / fairing, secondary burns that the folding mechanism and the lack of a ability to test a ton of new technology in zero-g orbit makes this far more likely to fail then anyone is comfortable with, given the overall cost to this.
I think that in 2021, Falcon 9 ‘s track record arguably suggests it is more reliable than Ariane 5, but it doesn’t matter because the Falcon fairing is too small for JWST.
Right, it was originally planned for 2007.
It is sorta funny that after 14 years of delays they picked the week of Christmas.