[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?

[mabbo]

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 [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.

[ultramegachurch]

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.

[Mesisio]

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?

[twistedpair]

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?

[MobiusHorizons]

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.

[Mesisio]

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.

[aliher1911]

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.

[HideousKojima]

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.

[martyvis]

By definition, as long as thrusters are firing, you are accelerating.

[chriswarbo]

Acceleration is defined as the change in velocity, and velocity is defined as the change in position (w.r.t. some inertial reference frame). There will always be some bending when thrust is applied at one part of a body; nothing is perfectly rigid.

As an extreme example, imagine a stick one lightyear in length: if we ignite a rocket on one end, firing perpendicular to the stick's length, then the other end cannot start moving for at least a year.

[BurningFrog]

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

A broken JWT could wait a few years in orbit.

[ultramegachurch]

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.

[kaashif]

> developing that capability would probably take years and cost >$100 million. And NASA can’t just decide to take on that endeavor

So what you're saying is, this could easily be funded by some billionaire, e.g. Jeff Bezos who already sells billions of dollars in Amazon stock per year to fund Blue Origin?

Not saying this should be done privately, but if funding is the problem, that problem can be solved.

Space travel is less expensive than most people think, it just isn't very high up on our list of priorities.

[ultramegachurch]

“Maybe Jeff Bezos could fund this” is not a good parameter to design a mission around.

[BurningFrog]

$100M is 1% of what JWST cost.

Looks like a reasonable repair cost (and only if) it turns out to be broken.

[ultramegachurch]

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.

[superjan]

Referring to hubble? The JWT is designed to observe from the Lagrange point in permanent shadow of the earth. It won’t work from earth orbit. First parking it in orbit, and then restarting the engine after unfolding comes with a whole new set of risks and tradeoffs.

[alserio]

What about an unmanned mission to fix it? And maybe to refuel it.

[ultramegachurch]

Incredibly unlikely. First, it would have to fail in a way that’s possible to fix. We don’t have robots that can replace screws, solder joints, and polish mirrors in space. Then we’d have to design a brand new spacecraft and mission. That would take years, lots of money, and political will. NASA would likely cut its losses, document the lessons learned, and try again.

[guerrilla]

I was watching a documentary about James Webb and they claimed that NASA has plans of using unmanned robots to potentially refuel it but no mention of fixing.

[nexuist]

But who repairs the repairers?

[devoutsalsa]

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]

Can we crowdfund 3 more this way, I wonder?

[ufmace]

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.

[justin66]

> 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.

[whiteboardr]

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.

[Symmetry]

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

[CyanBird]

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]

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]

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]

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.

[eterm]

How feasible is a repair mission without the shuttle?

[twistedpair]

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]

I believe what was GP was getting at is that even in a "convenience orbit", a repair mission would be very unlikely.

[BurningFrog]

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

[buryat]

in Armageddon the Shuttles flew over the moon

[dragonwriter]

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.