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Webb flies Ariane 5: watch the launch live on 25 December

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Re: Webb flies Ariane 5: watch the launch live on 25 December

#61

Post by jacquesm »

throw0101a wrote: post #60
dimtion wrote: post #42

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 eve…

> 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…

Regarding that helicopter example:

https://verticalmag.com/news/bell-electrically-distributed-a...

Re: Webb flies Ariane 5: watch the launch live on 25 December

#63

Post by jcims »

dimtion wrote: post #42

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 eve…

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

Re: Webb flies Ariane 5: watch the launch live on 25 December

#64

Post by WJW »

jacquesm wrote: post #55
dimtion wrote: post #42

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 eve…

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…

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".

Re: Webb flies Ariane 5: watch the launch live on 25 December

#65

Post by BurningFrog »

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?

Re: Webb flies Ariane 5: watch the launch live on 25 December

#66

Post by jacquesm »

WJW wrote: post #64
jacquesm wrote: post #55

Earlier quoted context omitted.

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…

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".

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.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#67

Post by Symmetry »

BurningFrog wrote: post #65

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?

It doesn't have enough fuel onboard to go from LEO to the Lagrange point itself.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#68

Post by argiopetech »

jpgvm wrote: post #47
ajross wrote: post #30

Earlier quoted context omitted.

Meh, but there's nothing to hit in space. The energy required to reach a routine geosynchronous orbit is already ~85% of escape velocity. The added boost is minimally more dangerous and the trip is just empty hours. "Time" does kill spacecraft, but not often. The Webb self-assembly is absolutely where the scary bits lurk.

Yeah valid. Though the whole "hurtling upwards on top of a bomb" part is still definitely scary in it's own right.

Particularly given the launch record of the Ariane 5 platform. I did some quick research, and no modern launch platform has as high a mission failure rate (4.5%.) The Delta family comes close at 4.4%, but that's a 50yr launch history. The Delta 4 iteration has had no failures.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#69

Post by ultramegachurch »

BurningFrog wrote: post #65

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?

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.

Re: Webb flies Ariane 5: watch the launch live on 25 December

#70

Post by Symmetry »

ajross wrote: post #8
jpgvm wrote: post #4

Earlier quoted context omitted.

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.

FWIW: given the extraordinarily complicated mechanical design, merely reaching orbit is the "easy part". The real imagined disasters won't happen until the mirrors assemble.

The Ariane 5 is batting 106 for 111 which is pretty good odds and perfectly respectable for a heavy lift vehicle... but not so much so that I don't wish it was launching on an Atlas 5 instead.
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