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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

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?

ufmace 5y ago
Absent all of the other practical considerations, even in LEO, a repair mission would probably be so expensive that it would be cheaper to build and launch a new one instead.
Symmetry 5y ago
It doesn't have enough fuel onboard to go from LEO to the Lagrange point itself.
- CyanBird 5y ago
  It wouldn't need to, you could park it there a secondary liquid fuel booster attached, and then after the origami shenanigans if it unfolds correctly the second booster could send it away to its final position I saw some interviews of engineers of the jwst and few of them had similar ideas, or at least to assemble them in leo then slingshot them to their final positions/orbits
  - bendhoefs 5y ago
    What booster would you use? Another booster up in the fairing? That would need to be quite heavy. It would be a totally custom thing for this specific mission. You would need a suitable storable propellant. Leave the booster from the Ariane attached? The lh2 and lox would boil off after a few days. What do you do if the deployment fails? SpaceX's dragon can't do space-walks on its own, it doesn't have an airlock. There would be no way to fix it short of developing a whole new space craft for that task.
    - samwillis 5y ago
      For space walk accessibility maybe they should have docked it to the ISS for initial setup before then boosting it to its final point?
      - zrail 5y ago
        ISS is in a very low and eccentric orbit. I think boosting from ISS to a geo transfer orbit, let alone escape, is more expensive than just going straight to escape velocity.
ultramegachurch 5y ago
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.
- BurningFrog 5y ago
  We had the capability in 1993, and could of course develop it again. A broken JWT could wait a few years in orbit.
  - ultramegachurch 5y ago
    That may be true, but designing a mission based off that hypothetical is a bad idea. The reality is we currently don’t have the capability for humans to service satellites, and developing that capability would probably take years and cost >$100 million. And NASA can’t just decide to take on that endeavor, it would require congress and months of political bickering. JWST was designed for what is currently feasible and practical.
    - BurningFrog 5y ago
      $100M is 1% of what JWST cost. Looks like a reasonable repair cost (and only if) it turns out to be broken.
      - ultramegachurch 5y ago
        It’s still not that simple, unfortunately. Ironically, there are too many single pint failures. Maybe JWST broke in a way that can’t be repaired. Maybe congress doesn’t approve the repair mission. Maybe the repair mission would actually cost $1 billion. Maybe the repair mission fails. Now imagine you’re the mission designer. You could trade increased complexity for some small chance of a repair mission maybe being possible. Or you you could decrease complexity and just accept that repair won’t be possible. The answer becomes pretty clear.
eterm 5y ago
How feasible is a repair mission without the shuttle?
- twistedpair 5y ago
  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.
  - usrusr 5y ago
    I believe what was GP was getting at is that even in a "convenience orbit", a repair mission would be very unlikely.
  - BurningFrog 5y ago
    The scenario we're talking about is having JWT unfold in Low Earth Orbit, so it can be repaired there, if need be.
  - buryat 5y ago
    in Armageddon the Shuttles flew over the moon
    - dragonwriter 5y ago
      The shuttle-like craft were explicitly special, more capable craft (visually hinted at by the extra set of boosters and other design differences.) Not that Armageddon was particularly focussed on reality.
whiteboardr 5y ago
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.
devoutsalsa 5y ago
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?
- mabbo 5y ago
  Can we crowdfund 3 more this way, I wonder?
justin66 5y ago
> 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? Another comment mentioned that it's not designed to accelerate while it's fully deployed, and that's true enough. You'd wreck it. The other essential thing is that there's no way to give it and its instruments anything like their designed operating parameters (pretty hot on one side of the sunshade, something like 40 kelvin on the other side) in Earth orbit.
mabbo 5y ago
It's a problem of energy and orbits. To get from low-earth orbit to the sun-earth Lagrange point 2 (where the JWT is headed) takes around 7 km/s of delta-v[0]. That's a lot of speed. You could try to do this gently enough that the unfurled JWT won't be damaged by the acceleration. This isn't totally impossible, but you'd need a good Hall thruster (ion engine) with a huge amount of reaction mass, since the JWT is so big itself. It would need to run for longer than any other such thruster has. It would need massive solar panels to power it.[1] Or you could have the original rocket just be bigger, and throw it all the way to the right orbit while everything is packed tight. [0] https://en.wikipedia.org/wiki/Delta-v_budget https://en.wikipedia.org/wiki/Delta-v_budget [1] I'm guessing at this, but that's my intuition. I encourage anyone to correct me because space is too cool to be upset that I was wrong.
- deleted 5y ago
  [deleted]

ufmace 5y ago

Absent all of the other practical considerations, even in LEO, a repair mission would probably be so expensive that it would be cheaper to build and launch a new one instead.

Symmetry 5y ago

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

CyanBird 5y ago
It wouldn't need to, you could park it there a secondary liquid fuel booster attached, and then after the origami shenanigans if it unfolds correctly the second booster could send it away to its final position I saw some interviews of engineers of the jwst and few of them had similar ideas, or at least to assemble them in leo then slingshot them to their final positions/orbits
- bendhoefs 5y ago
  What booster would you use? Another booster up in the fairing? That would need to be quite heavy. It would be a totally custom thing for this specific mission. You would need a suitable storable propellant. Leave the booster from the Ariane attached? The lh2 and lox would boil off after a few days. What do you do if the deployment fails? SpaceX's dragon can't do space-walks on its own, it doesn't have an airlock. There would be no way to fix it short of developing a whole new space craft for that task.
  - samwillis 5y ago
    For space walk accessibility maybe they should have docked it to the ISS for initial setup before then boosting it to its final point?
    - zrail 5y ago
      ISS is in a very low and eccentric orbit. I think boosting from ISS to a geo transfer orbit, let alone escape, is more expensive than just going straight to escape velocity.

ultramegachurch 5y ago

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.

BurningFrog 5y ago
We had the capability in 1993, and could of course develop it again. A broken JWT could wait a few years in orbit.
- ultramegachurch 5y ago
  That may be true, but designing a mission based off that hypothetical is a bad idea. The reality is we currently don’t have the capability for humans to service satellites, and developing that capability would probably take years and cost >$100 million. And NASA can’t just decide to take on that endeavor, it would require congress and months of political bickering. JWST was designed for what is currently feasible and practical.
  - BurningFrog 5y ago
    $100M is 1% of what JWST cost. Looks like a reasonable repair cost (and only if) it turns out to be broken.
    - ultramegachurch 5y ago
      It’s still not that simple, unfortunately. Ironically, there are too many single pint failures. Maybe JWST broke in a way that can’t be repaired. Maybe congress doesn’t approve the repair mission. Maybe the repair mission would actually cost $1 billion. Maybe the repair mission fails. Now imagine you’re the mission designer. You could trade increased complexity for some small chance of a repair mission maybe being possible. Or you you could decrease complexity and just accept that repair won’t be possible. The answer becomes pretty clear.

eterm 5y ago

How feasible is a repair mission without the shuttle?

twistedpair 5y ago
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.
- usrusr 5y ago
  I believe what was GP was getting at is that even in a "convenience orbit", a repair mission would be very unlikely.
- BurningFrog 5y ago
  The scenario we're talking about is having JWT unfold in Low Earth Orbit, so it can be repaired there, if need be.
- buryat 5y ago
  in Armageddon the Shuttles flew over the moon
  - dragonwriter 5y ago
    The shuttle-like craft were explicitly special, more capable craft (visually hinted at by the extra set of boosters and other design differences.) Not that Armageddon was particularly focussed on reality.

whiteboardr 5y ago

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.

devoutsalsa 5y ago

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?

mabbo 5y ago
Can we crowdfund 3 more this way, I wonder?

justin66 5y ago

> 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? Another comment mentioned that it's not designed to accelerate while it's fully deployed, and that's true enough. You'd wreck it. The other essential thing is that there's no way to give it and its instruments anything like their designed operating parameters (pretty hot on one side of the sunshade, something like 40 kelvin on the other side) in Earth orbit.

mabbo 5y ago

It's a problem of energy and orbits. To get from low-earth orbit to the sun-earth Lagrange point 2 (where the JWT is headed) takes around 7 km/s of delta-v[0]. That's a lot of speed. You could try to do this gently enough that the unfurled JWT won't be damaged by the acceleration. This isn't totally impossible, but you'd need a good Hall thruster (ion engine) with a huge amount of reaction mass, since the JWT is so big itself. It would need to run for longer than any other such thruster has. It would need massive solar panels to power it.[1] Or you could have the original rocket just be bigger, and throw it all the way to the right orbit while everything is packed tight. [0] https://en.wikipedia.org/wiki/Delta-v_budget https://en.wikipedia.org/wiki/Delta-v_budget [1] I'm guessing at this, but that's my intuition. I encourage anyone to correct me because space is too cool to be upset that I was wrong.

deleted 5y ago
[deleted]