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Comment by NikolaeVarius | Hacker News Reader

I'm wondering why is that, which part of the effort was dedicated to research and how much time and money would it cost to build a new JWST?

The reasoning that I heard (and I hope I'm relaying this right) was that if it doesn't work then it will most likely be because of something that needs to be redesigned, so there's no point in producing an exact replica.

I think this assumes a successful launch and some kind of issue with the deployment.

The other concern is, of course, what happens if it blows up on the launchpad.

Presumably they've actually quantified these probabilities and think that the chance of that is relatively very low compared to the chance of something going wrong in the design. These things are not cheap, so it makes sense not to have to make three if the launch succeeds and design fails.

Oh yes, I totally agree— this is a very proven rocket, and the deployment sequence is very, very much not proven. So it absolutely makes sense to have gone this way.

That's assuming that anything that goes wrong in the deployment sequence necessitates a clean-sheet redesign of the whole thing. No reason to think that.

But with so many things that could go wrong in such a tightly integrated system, it's impossible to predict which components or assemblies may be affected by a partial redesign.

If you say so.

Ariane 5 appears to have 96% mean reliability. Source: 2021 Space Launch Report – Launch Vehicle by Success Rate (spacelaunchreport.com) - https://www.spacelaunchreport.com/log2021.html#rate

https://news.ycombinator.com/item?id=29554274

The footnote about Lewis Point Estimates is super interesting.

"Lewis Point Estimate Determined as Follows.

  Maximum Liklihood Estimate (MLE)= x/n 
      where x=success, n=tries
  For MLE<=0.5, use Wilson Method = (x+2)/(n+4)
  For MLE Between 0.5 and 0.9, use MLE = x/n
  For MLE>=0.9, use Laplace Method = (x+1)/(n+2)
 
  Lewis, J. & Lauro, J., "Improving the Accuracy of Small-Sample 
   Estimates of Completion Rates", Journal of Usability Studies, 
   Issue 3, Vol. 1, May 2006, pp. 136-150."

Concur. You have to see this thread on it in the NASA Spaceflight forum - https://forum.nasaspaceflight.com/index.php?topic=39928.0

I think OP means "what if the rocket fails".

The answer is that building another telescope specimen would be quite expensive, even without any changes to the design. The supply chains aren't set up for volume and integration and testing is very involved. I can't put a number on "the second telescope would be x% cheaper", but it would be very disappointing.

Also Ariane 5 is one of the most reliable rocket capable of launching JWST on the correct orbit.

It's also going out pretty far (the L2 Lagrange point on the other side of the moon), so they also won't be able to perform any manned post-launch repair missions like they did with Hubble.

Actually I have a question about this. What happens if it fails at L2? Is that position like occupied or more difficult to deal with from then on?

The L2 point is unstable do energy needs to be expended to maintain position there.

Solar wind will push it out of L2 and into an orbit around the sun. This is already going to happen once JWT runs out of fuel to maintain its position.

> so they also won't be able to perform any manned post-launch repair missions like they did with Hubble.

In terms of hardware that's readily or soon to be available, it is technically possible. Starship, if it gets off the ground, is the obvious choice. Orion could do it, but it would mean dumping another two billion into JWST.

If it really came down to it, Falcon Heavy launched with a Crew Dragon sitting on top of it could probably reach it, although it might need an additional kick stage (I haven't done the maths), and it would need to be crew rated first (and SpaceX don't currently have an interest in doing that).

If you consider the human to be part of that hardware, it's over 4x the distance that a human has ever travelled. There would still be a lot of technical problems to be figured out.

Having a ship and a launcher capable of reaching the telescope is a bit different of being able to service it. I understand that just planning and preparing a mission on this scale, with current technology and processes, would take more time than the service life of the telescope itself.

Also, AFAIK, Crew Dragon does not have airlocks or other resources for EVAs. Not sure it the capsule can be evacuated for that too. But I would like more information in this issue ... could not find too much from reliable sources for now.

> Having a ship and a launcher capable of reaching the telescope is a bit different of being able to service it.

True. But generally when people bring up the statement that we can't service it, they usually back that up by citing the distance. There would still be a lot of hurdles to overcome, but none are insurmountable with relatively small investment.

All of that said, if Starship works, then the economics change drastically anyway. You could probably launch an entire fleet of less reliable, but much cheaper telescopes, for less than the cost of servicing.

Webb is going to the Sun-Earth L2 (about 1.5 million km from Earth), not the Earth-Moon L2 (which is about 61,000 km from the Moon and 445,000 km from the Earth.

https://webb.nasa.gov/content/about/orbit.html

I'd imagine they just cannot fabricate the mirrors as many as they want. The mirrors, arguably the most important parts of any telescope or optical instrument, need delicate manufacture due to the required precision. At the end of the day, this is not automobile industry.

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