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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

#91

Post by GuB-42 »

Symmetry wrote: post #70
ajross wrote: post #8

Earlier quoted context omitted.

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.

Ariane 5 had some early problems, the first failure even became a case study in critical software development. But it had only had one partial failure after the 17th launch: the payload was launched to the wrong orbit because the wrong coordinates were put in the computer. A huge QA problem but not the fault of the launcher which did exactly as told.

Now, both Ariane 5 and Atlas 5 are extremely reliable, mature rockets.

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

#92

Post by twistedpair »

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…

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.

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

#93

Post by twistedpair »

beamatronic wrote: post #80
throw0101a wrote: post #60

Earlier quoted context omitted.

> 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…

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.

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).

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

#94

Post by twistedpair »

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…

> Unlike say, the Apollo program, which had a guaranteed successful outcome?

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....

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

#95

Post by Mesisio »

MobiusHorizons wrote: post #90
Mesisio wrote: post #86

Earlier quoted context omitted.

Interesting though. 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?

When rockets are firing lots of acceleration is applied that the delicate structures are only designed to handle when stowed. Think long arms on hinges. They can take acceleration in one axis, but not at 90 degrees to it.

I guess the acceleration needed to leave orbit might need so much more fuel if you need to do it slowly than doing it fast?

Otherwise it would just take longer.

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

#96

Post by soperj »

rrss wrote: post #44
jacquesm wrote: post #37

Earlier quoted context omitted.

If you start planning your rocket launches around religious holidays then that is going to be a bit of a problem, there are just too many of them: https://nationaltoday.com/religious-holidays/

How about planning around the single most celebrated holiday in the US, religious or not: https://en.m.wikipedia.org/wiki/Public_holidays_in_the_Unite...

If you worked on something since 1996, do you think you would give two flying figs what day it launches on?

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

#97

Post by HideousKojima »

Mesisio wrote: post #86
ultramegachurch wrote: post #69

Earlier quoted context omitted.

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.

Interesting though. 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?

The parts of a ship that thrusters are directly attached to experience acceleration first, the other parts that are further out from the thrusters won't accelerate immediately and if the acceleration is too sudden or extreme could be damaged or break off entirely.

And to answer your question in the other response, many thrusters have a minimum thrust, and even that minimum may be too much for the parts when deployed.

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

#98

Post by twistedpair »

Mesisio wrote: post #86
ultramegachurch wrote: post #69

Earlier quoted context omitted.

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.

Interesting though. 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?

Rockets can easily accelerate with enough force to kill a human (cargo flights and unmanned flights use different launch profiles for this reason).

The less Gs you need to design a component for, the lighter/simpler it can be, so why unfurl early and add that extra mass and complexity to the design?

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

#99

Post by twistedpair »

eterm wrote: post #77
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?

How feasible is a repair mission without the shuttle?

L2 is very far from earth, well beyond the Moon. The Shuttle could never leave Low Earth Orbit, so it's a non-factor in repair missions once JWST is on station.
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