8 ms·

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

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?

darknavi 5y ago
> 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.
nharada 5y ago
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.
jacquesm 5y ago
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.
- WJW 5y ago
  Perhaps it's a case of "we're proud that we got it all the way down to only 200 SPOFs in the final design, the earlier designs were way worse".
  - jacquesm 5y ago
    There is a fair chance of that, actually. If you start enumerating the ones that are obvious even to lay people (one rocket, one satellite, one set of mirrors and so on) then this is probably an extremely impressive low number.
    - shmageggy 5y ago
      I've been binging JWST content and definitely recall John Mayer saying in at least one interview that 344 single points of failure was as low as they could get it. And I can believe it. If you watch some of the more detailed interviews, he always stresses how every design choice was heavily labored over and reviewed.
      - jacquesm 5y ago
        It's an absolutely amazing effort, from the design through all of the problems they had to deal with along the way to seeing it sit there on top of that booster. A few months from now we could be in a completely new era of astronomy.
sgt101 5y ago
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.
nkrisc 5y ago
> 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?
throw0101a 5y ago
> 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.
- jacquesm 5y ago
  Regarding that helicopter example: https://verticalmag.com/news/bell-electrically-distributed-anti-torque-edat/ https://verticalmag.com/news/bell-electrically-distributed-a...
- beamatronic 5y ago
  The answer to your redundancy question is: Assembly line. 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.
  - twistedpair 5y ago
    They calculated that. Building a second JWST would have added 10% to the budget, but the budget committee nixed that. YOLO (you only launch once).
    - devoutsalsa 5y ago
      The JWST only has a planned mission duration of 10 years. If it’s as epic as everyone claims it could be, there must be a follow up mission planned. So even if it fails, we’ll still see a successor at some point.
jcims 5y ago
I think this is your earnest attempt at it but its a good place to apply the story from yesterday 'be curious, not judgemental'.
- kataklasm 5y ago
  That's literally what he is doing, isn't it? He thought something was off, and instead of spewing his opinion all over the place he requested information from folks more versed in the domain than him. He is literally being curious, not judgemental.
  - jacquesm 5y ago
    > 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 see this as judgmental, am I wrong in that?
    - MobiusHorizons 5y ago
      It expresses an opinion, but I don’t think that has to be judgmental. The OP has to explain their position in order to ask for additional information.
- dimtion 5y ago
  I'd like to apologize if this sounded judgmental. English is not my main language, and perhaps my phrasing sounded like I assumed my domain specific opinion is better than the actual people working on the the project. 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.
wslack 5y ago
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.
- onphonenow 5y ago
  What I don't understand is for $11 billion - can't we do 3 $3B space telescopes that work reasonably well? Spread risk? More science? 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).
  - azalemeth 5y ago
    I'd bet that the vast majority of that cost isn't going to be materials -- it'll be staff time to design and optimise the telescope and make something that can work, including where necessary how to make new materials or processes to make that telescope. I'm not saying the actual hardware is cheap by any stretch of the imagination, but making three different telescopes isn't a linear function of that budget. They're literally pushing the envelope of what's possible here. If something terrible happens to the rocket, lots of forms will be written and people will be sad but fundamentally I think they'll build something a bit better on a few years and nail it. A bit like New Horizons (awesome Mars rover) vs Beagle 2 (awesome Mars rover that died on arrival to Mars).
twistedpair 5y ago
While a farm tractor might have a factor of safety of 10, spacecraft are usually closer to 1.3, due to mass and efficiency constraints. Space is hard.
GekkePrutser 5y ago
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.
yk 5y ago
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.
- jdiez17 5y ago
  I don't think you are supposed to be impressed by the number of things that could go wrong. At least, that's not how I see it. They talk about the deployment phase with all these complex things that have to go exactly right as "the 30 days of terror." And that's a pretty accurate description of what many people with a stake in the JWST will be feeling. Not awe, but terror. This is a big part of space missions at the bleeding edge of science and technology, and I think it's great that many people are hearing about it!
harshreality 5y ago
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? [1] https://www.thespacereview.com/article/4303/1 https://www.thespacereview.com/article/4303/1

darknavi 5y ago

> 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.

nharada 5y ago

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.

jacquesm 5y ago

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.

WJW 5y ago
Perhaps it's a case of "we're proud that we got it all the way down to only 200 SPOFs in the final design, the earlier designs were way worse".
- jacquesm 5y ago
  There is a fair chance of that, actually. If you start enumerating the ones that are obvious even to lay people (one rocket, one satellite, one set of mirrors and so on) then this is probably an extremely impressive low number.
  - shmageggy 5y ago
    I've been binging JWST content and definitely recall John Mayer saying in at least one interview that 344 single points of failure was as low as they could get it. And I can believe it. If you watch some of the more detailed interviews, he always stresses how every design choice was heavily labored over and reviewed.
    - jacquesm 5y ago
      It's an absolutely amazing effort, from the design through all of the problems they had to deal with along the way to seeing it sit there on top of that booster. A few months from now we could be in a completely new era of astronomy.

sgt101 5y ago

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.

nkrisc 5y ago

throw0101a 5y ago

> 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.

jacquesm 5y ago
Regarding that helicopter example: https://verticalmag.com/news/bell-electrically-distributed-anti-torque-edat/ https://verticalmag.com/news/bell-electrically-distributed-a...
beamatronic 5y ago
The answer to your redundancy question is: Assembly line. 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.
- twistedpair 5y ago
  They calculated that. Building a second JWST would have added 10% to the budget, but the budget committee nixed that. YOLO (you only launch once).
  - devoutsalsa 5y ago
    The JWST only has a planned mission duration of 10 years. If it’s as epic as everyone claims it could be, there must be a follow up mission planned. So even if it fails, we’ll still see a successor at some point.

jcims 5y ago

I think this is your earnest attempt at it but its a good place to apply the story from yesterday 'be curious, not judgemental'.

kataklasm 5y ago
That's literally what he is doing, isn't it? He thought something was off, and instead of spewing his opinion all over the place he requested information from folks more versed in the domain than him. He is literally being curious, not judgemental.
- jacquesm 5y ago
  > 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 see this as judgmental, am I wrong in that?
  - MobiusHorizons 5y ago
    It expresses an opinion, but I don’t think that has to be judgmental. The OP has to explain their position in order to ask for additional information.
dimtion 5y ago
I'd like to apologize if this sounded judgmental. English is not my main language, and perhaps my phrasing sounded like I assumed my domain specific opinion is better than the actual people working on the the project. 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.

wslack 5y ago

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.

onphonenow 5y ago
What I don't understand is for $11 billion - can't we do 3 $3B space telescopes that work reasonably well? Spread risk? More science? 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).
- azalemeth 5y ago
  I'd bet that the vast majority of that cost isn't going to be materials -- it'll be staff time to design and optimise the telescope and make something that can work, including where necessary how to make new materials or processes to make that telescope. I'm not saying the actual hardware is cheap by any stretch of the imagination, but making three different telescopes isn't a linear function of that budget. They're literally pushing the envelope of what's possible here. If something terrible happens to the rocket, lots of forms will be written and people will be sad but fundamentally I think they'll build something a bit better on a few years and nail it. A bit like New Horizons (awesome Mars rover) vs Beagle 2 (awesome Mars rover that died on arrival to Mars).

twistedpair 5y ago

While a farm tractor might have a factor of safety of 10, spacecraft are usually closer to 1.3, due to mass and efficiency constraints. Space is hard.

GekkePrutser 5y ago

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.

yk 5y ago

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.

jdiez17 5y ago
I don't think you are supposed to be impressed by the number of things that could go wrong. At least, that's not how I see it. They talk about the deployment phase with all these complex things that have to go exactly right as "the 30 days of terror." And that's a pretty accurate description of what many people with a stake in the JWST will be feeling. Not awe, but terror. This is a big part of space missions at the bleeding edge of science and technology, and I think it's great that many people are hearing about it!

harshreality 5y ago

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? [1] https://www.thespacereview.com/article/4303/1 https://www.thespacereview.com/article/4303/1