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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

#101

Post by bernulli »

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.

It's really not - just by having more hardware available (at higher total expenses) doesn't make the pool of money available (public research funding) to book time on these things more. These things are one-off, you build a new one if you expect 10x improvement over the old one.

We don't need a fleet of X1 to break the sound barrier for the first time. We do need many Airbus/Boeings to fly people and stuff from A to B.

Note that that is the case with the unique research hardware you cite as well - we're not sending another Spirit/Opportunity, but have graduated to something else.

Soyuz is a different use case, as there is an economic demand to be filled - that's why a private company like SpaceX is in that sector with its Dragon. On the other hand, you don't see SpaceX cranking out Spirits or JWSTs or Washington Monuments.

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

#103

Post by BurningFrog »

twistedpair wrote: post #99
eterm wrote: post #77

Earlier quoted context omitted.

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.

The scenario we're talking about is having JWT unfold in Low Earth Orbit, so it can be repaired there, if need be.

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

#104

Post by Stratoscope »

twistedpair wrote: post #94
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…

> 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... .

I think GP was being sarcastic.

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

#105

Post by BurningFrog »

ultramegachurch wrote: post #69
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.

We had the capability in 1993, and could of course develop it again.

A broken JWT could wait a few years in orbit.

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

#106

Post by whiteboardr »

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?

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.

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

#107

Post by devoutsalsa »

twistedpair wrote: post #93
beamatronic wrote: post #80

Earlier quoted context omitted.

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).

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.

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

#108

Post by mastax »

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…

Better helicopter example is the Jesus Nut.

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

#109

Post by devoutsalsa »

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?

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?

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

#110

Post by aliher1911 »

Mesisio wrote: post #95
MobiusHorizons wrote: post #90

Earlier quoted context omitted.

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.

Rocket engines could only be throttled down to certain power and manoeuvring thrusters would run out of propellant way before it reaches any orbit. Maybe something like electric propulsion can do it but it will take very long time to make it practical imho.
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