One of the big numbers that been thrown around is that JWST has 344 single points of failure in its mission. Now that deployment has been completed, is there somewhere that lists how many of those points we have passed?
https://twitter.com/SpcPlcyOnline/status/1479900221131964421
As always, it's useful to post this link:
Those adjustments, as well as unloading of the reaction wheels used to position JWST, and the fuel burns reqiured for them, are the principle determinants of the JWAT's lifespan, estimated at 10 years.
Nasa have lowballed such estimates for numerous missions in the past (Hubble and numerous Mars lander and rover missions come to mind), and it's possible JWST will exceed the planned lifetime, potentially by a wide margin. Initial indications are that the Ariane V insertion should have spared much potential fuel use in orbital corrections and insertion.
I expect the space geek set will have speculation, hopefully reasonably informed, on this in the not-too-distant future.
https://www.space.com/3833-nasa-adds-docking-capability-spac...
https://jwst.nasa.gov/content/about/faqs/faq.html#serviceabl...
But I don't think that means it only has 10 years of fuel, and I don't think they have ever said explicitly said that. I suspect it's more like 15 years in the best case.
The fact that they aren't already planning a refuelling mission implies that it's not a critical limiting factor.
There are a set of telescope plans which are presently in consideration, including WIRST, the wide-angle infrared telescope; HabEx, the Habitable Exoplanet Imaging Mission; Lynx, a next-generation X-ray telescope; and the Origins Space Telescope, an infra-red telescope even larger than Webb.
https://www.universetoday.com/139461/what-comes-after-james-...
For telescopes, among factors I'm aware of are:
- The total number of devices. More 'scopes means more points of the sky which can be imaged at any one time. This permits detecting either rare or transient events.
- Wavelength specificity. Infra-red, radio, visible light, UV, and X-Ray sensitivity all permit detection of different phenomena. Devices suited to one wavelength may not be suitable for others. Specific research goals may favour specific observational methods.
- Other sensing modes. Gravity, gamma ray, and particle sensors (e.g., neutrino sensors, cosmic-ray detectors) may afford other options. There are proposals for space-based gravity-wave detectors, for example.
- Compound devices. The HabEx system in particular has two components, the telescope itself, and a sunshield used to block the light of an observed star, which would operate at a separation of 100s of thousands of km.
- Collector size. Larger mirrors permit gathering of more light. This permits shorter collection periods for brighter events, and imaging of previously undetectable phenomena. The Hubble Deep Field views are an example of the latter, and pushed the boundaries of known and and observable phenomena tremendously.
- Storage, processing, and communications capabilities. I don't know how much this contributes to observation capabilities, though I suspect it has an impact.
- Developments in phsyics, materials, and sensing, generally. Looking through lists of physics and chemistry Nobel awards since the 1970s, a surprisingly large number have concerned capabilities rather than fundamental characteristics or properties of matter or the universe. Many of these afford new capabilities in sensing and detection.
- Probes. Rather than a single instrument which views distant objects, probes get close to a specific object, or set of objects, and make close or direct observations of these. Various landers, impactors, flyby, orbiter, and similar missions, to date all to objects within the Solar System, would be examples of these. These compete with other missions (manned, long-distance sensing).
- Earth observation. Probably the largest class and most productive set of space-based observation platforms has been looking at our own planet.
It's also worth thinking through what has made JWST possible, including launch platforms, experience with complex deployments, manufacturing, sensing, and control capabilities. These will have impacts on future missions, and further development might also extend their capabilities.
Finally: most technological improvement tends to follow a sigmoidal curve: an early period of slow development, a period of rapid attainment, then a slower period of approaching theoretical maximum efficacy. New developments or combinations of technologies may restart that curve, but often 15 years doesn't deliver transformational development. Rather older technologies are refined, reliability improved, costs reduced, or flexibility increased.
https://blogs.nasa.gov/webb/2021/12/29/nasa-says-webbs-exces...
I've seen lots of people talk about how some Lagrange points are stable and others are unstable, but I've never been able to find a source for how much this matters.
How long would it take for Webb to move so far off L2 that we can't communicate with it anymore, or for it to be at risk of being completely thrown out of orbit to the point that it would impact earth or the sun?
[1] https://jwst.nasa.gov/content/webbLaunch/deploymentExplorer....
Some other parts don't and cannot have redundancy due to design.
I just wish there was more elaboration of things they are able to accommodate for as things inevitably pop up and not this hyper focus on a number of supposed single point failures.
If one or two layers of the sunshade failed to correctly deploy, JWST is still usable, but the temperature of the mirrors would be higher than expected, and thus the spectrum of IR light it can image would be reduced.
If the momentum flap fails to deploy, then more fuel is needed to keep the JWST at the right attitude/position, shortening the lifespan of the mission.
But some others were make or break: deployment of the solar panels, deployment of the secondary mirror and a few others. If those had failed then the mission would be over.
How can a system have more that one single point of failure?
So long as the chain is not loaded near its tensile limit, then the grain boundaries that support the load within each link do so in parallel and are therefore redundant.
The selection of the allegory of the chain was intentional -- each link must be properly formed, or the entire chain will fail. If it breaks, it is surely correct to say, "the chain broke", but in truth, it was actually link-86.
For JWST, the remarkable/audacious thing is that many links in the chain from launch to observation are potential single-point failures. Furthermore, many of them haven't ever been tested independently in space... ever. It is a hell of a triumph that JWST has gotten this far already.
If even a small fraction of the instrumentation works at this point, we are going to learn a ton about the universe, simply due to JWST's position, collecting-area, and mirror-diameter.
Also, I don't want to underplay JWST's success or its challenges. But, when saying 300+ SPOF conditions, one has to specify at what abstraction level. Otherwise, it can be misleading.
Single point of failure doesn’t mean “weakest link”. It means if this one piece (“single point”) fails, the whole system will fail.
Contrast a second chain, or double linked chain, so if any link fails the load is not lost.
If you know the strength of every link in the chain with perfect accuracy, and you know that the only potential cause of failure is too much weight being placed on the chain, then the only link that can fail is the weakest[1] one because no other failure can happen before that one.
But really you need to design for the idea that various things might happen. Someone else gave the example of a person choosing a link to cut with bolt cutters. The person's choice is what's not predictable in that example.
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[1] And if you assume it's not possible to have two links that are exactly as strong as each other.
Which isn't relevant since (obviously) no one was talking about how many links are the weakest link.
"it is at that link where the chain will break"
Wars have been lost by thinking this way. e.g., D-Day was a successful assault on a strong link.
That's not to say that cables or ropes cannot fail, but when they do so, failure of many individual components is required.
A recent example is the failure of support cables for the Arecibo radio observatory in Puerto Rico in 2020. This was the result of a progressive failure played out over months, though at an accelerating rate, until the final catastrophic failure and loss of the instrument as a whole. (This wasn't wholly unforseen and was precipitated by a long period of rather intentional neglect of maintenance.)
Even after the failure of two complete cables, the remaining cables supported the load of Arecibo's instrument platform. On 1 December 2020, strands of one of the remaining cables began breaking, at an accelerating rate, leading to the total failure of that cable, then a second and third, and with it loss of the instrument platform. The final moments were captured on video.
Sequence begins at about 55s here: https://yewtu.be/watch?v=b3AASKr_iHc
Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.
If any one of 344 pieces were to fail during deployment, then all of the deployment has failed and the entire $10B was a loss. Consider the engine in your car- how many single pieces of it could fail before the entire engine can't work? The difference with Webb is that most of those points were single actions that had to work once.
And we're now at a stage where most of them did not fail.
>I think linguistically the term can be hard to parse.
Yeah, "solitary" might make more sense ... or "non-redundant". Or "point of single failure".
> what if the front fell off?
All structural components are redundant and have redundant attachments. The wing spars are doubled, for example.
The AL flight cracked its nut, and the balls fell out. I don't know the details of that design, but on the 757 the ice scrapers on both sides of the nut will hold the nut in place if the balls fall out.
If I recall correctly, if the AL pilot had simply left the trim system alone after it failed, and landed the airplane, it would have been fine. Instead, he kept fiddling with it, driving it up and down, until it tore the nut to pieces. The loud screech it made when running in a half broken configuration should have chilled anyone's blood to bone at 30,000 feet.
As that incident, and the later MAX crashes clearly demonstrated, once something goes wrong with the stab trim system, you move it only enough to get the airplane into a flyable state. Then, you turn it off and leave it the frack alone. Land it, and let the mechanics figure it out with it safely in the hangar.
One incident where the stab trim failed and the poor pilots were just passengers to their doom was that 747 that took off from the base, and some tank in the cargo hold broke loose, slid back, smashed through the bulkhead and broke off the whole stab trim system.
That really wasn't the stab trim system's fault, you can't really design for a tank falling on it. It was the fault of no redundancy in the straps holding the tank in place.
The video of the crash, shown endlessly on the news, makes my gut turn to water.
If either one fails, you die.
https://en.wikipedia.org/wiki/Single_point_of_failure
"A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working."