I do have a question about JWST that I wasn't able to find a fine answer in other forums.
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 suggest the book "Failure is not an option" by Gene Kranz, an Apollo flight director (played by Ed Harris in Apollo 13). He describes how the primary work of flight controllers in all missions is risk management. You are constantly balancing mission needs, fuel needs, mass needs, temperature needs, and etc etc.
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.
> 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?
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.
I think all the SPOF talk is expectation management in case it fails. It's part space telescope mission, part engineering challenge. Even if the space telescope part fails, the engineering effort that's gone into it means something.
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?
I hope it's just badly thought through marketing. There is currently a AWS spot during nfl games, that shows a spectacular catch and then proclaims that the catch probability is only 3.6% or something. You are meant to be somehow impressed by the unlikelihood I believe, but that their model thought the catch is unlikely and the guy caught it implies a rejection of their model with p=3.6% < 5%.
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.
It has to fit in the fareing, survive launch stress and vibration, fly further than the moon and it has to weigh 6000 kgs +including fuel. If it works it will be one of the greatest engineering feats of history.
I'm guessing it's a case where all the various extreme requirements simply do not allow for redundancy in the places the engineers would prefer to have it. The options are likely (1) okay performance with a lower chance of failure or (2) extreme performance with a higher chance of failure.
> 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'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.
Every kilo launched costs a proverbial and probably nearly literal tonne of fuel so things are not as simple as that.
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.
Unlike say, the Apollo program, which had a guaranteed successful outcome?
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.
> 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.
Genuinely curious: how would you have achieved the mission goals with fewer SPOF?
We're told that since JWT will travel very far away before it unfolds and activates all its systems, there is no practical way to service it if something would go wrong.
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.
I suspect the deployed structure cannot handle the acceleration required for escape velocity. That also may require much more propellant. Then on top of that, we don’t have the capability for humans to service satellites other than the ISS. So this is all a moot point.
We probably wouldn’t have a way to service it in earth orbit either. We needed a space shuttle to operate on Hubble. And using some other commenter’s estimate a 2nd JWST would cost 10% of the 10 billion USD price of the first one. A billion dollars is ballpark what it costed to launch a space shuttle. So even if we had the shuttle, would we fix the first one or just build & launch a second one?
Absent all of the other practical considerations, even in LEO, a repair mission would probably be so expensive that it would be cheaper to build and launch a new one instead.
> 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?
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.
Orbiting around earth would require it to constantly course correct / rotate in order to avoid the device from getting to hot from the sun’s radiation - there isn’t enough fuel to “play it safe” around earth for this long since it will be needed at L2.
Let alone the current lack of in orbit service capabilities like we had when the space shuttle was still around.
"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."
Decades of meticulous planning and they couldn't give their teams a day off on Christmas? Of course it could be that consensus was this beats any other kind of activity that day in which case fair enough. :-)
Well, according to the original planning it should have launched before Christmas. It was only the weather delaying it by one day. I guess once they were ready for the launch, they want to avoid to delay it any longer than needed. Not sure, how long the rocket can be "stored" in launch-ready state before it has to be serviced again.
I think spending Christmas wringing your hands unsure if your $10B decades-in-development baby is going to make it to orbit sounds incredibly stressful...
It's going to be awesome for us to watch but I feel for all the folks that worked on this.
The launch partner/platform wasn't chosen recently and it's part of what ESA is providing as a partner in the program.
"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."
Interesting they found a way to add a hidden risk variable to the launch, by sending it up on a day people are having to choose to not be with their families. Have a bad feeling about this :/
dimtion|4 years ago
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?
wslack|4 years ago
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.
throw0101a|4 years ago
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.
harshreality|4 years ago
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?
[1] https://www.thespacereview.com/article/4303/1
yk|4 years ago
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.
sgt101|4 years ago
nharada|4 years ago
darknavi|4 years ago
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.
GekkePrutser|4 years ago
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.
jacquesm|4 years ago
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.
jcims|4 years ago
twistedpair|4 years ago
Space is hard.
nkrisc|4 years ago
Genuinely curious: how would you have achieved the mission goals with fewer SPOF?
BurningFrog|4 years ago
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?
mabbo|4 years ago
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.
ultramegachurch|4 years ago
devoutsalsa|4 years ago
ufmace|4 years ago
justin66|4 years ago
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.
whiteboardr|4 years ago
Let alone the current lack of in orbit service capabilities like we had when the space shuttle was still around.
Symmetry|4 years ago
eterm|4 years ago
Ankaios|4 years ago
meepmorp|4 years ago
Mesisio|4 years ago
ape4|4 years ago
captn3m0|4 years ago
kitd|4 years ago
guerrilla|4 years ago
notjustanymike|4 years ago
notjustanymike|4 years ago
jessriedel|4 years ago
_joel|4 years ago
Mesisio|4 years ago
keewee7|4 years ago
jacquesm|4 years ago
"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."
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope
ourmandave|4 years ago
It can launch anytime during that window.
neals|4 years ago
C19is20|4 years ago
anotheryou|4 years ago
drunkonvinyl|4 years ago
Sosh101|4 years ago
mulcahey|4 years ago
foobarian|4 years ago
_ph_|4 years ago
furyofantares|4 years ago
jpgvm|4 years ago
It's going to be awesome for us to watch but I feel for all the folks that worked on this.
malermeister|4 years ago
jacquesm|4 years ago
https://nationaltoday.com/religious-holidays/
sp332|4 years ago
unknown|4 years ago
[deleted]
amichail|4 years ago
adventured|4 years ago
"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...
_fizz_buzz_|4 years ago
varjag|4 years ago
ddalex|4 years ago
sprucely|4 years ago
fnord77|4 years ago
the falcon heavy probably didn't even exist when they were drawing up the contracts
NikolaeVarius|4 years ago
ur-whale|4 years ago
Reliability (as in: established track record).
Although, if Ariane decides to explode tomorrow, this comment will look ... odd.
unknown|4 years ago
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gfodor|4 years ago
daenney|4 years ago
unknown|4 years ago
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