NASA - James Webb Space Telescope - Discussion Thread 2

[Jim]

I'm a frequent reader of the Economist, and while I think its very good, it does still make mistakes. I was reading their article of JWST (https://www.economist.com/science-and-technology/2021/11/23/a-new-eye-on-the-heavens (but paywall)) and it had the following text:

The cloud of delay, meanwhile, has had silver linings. ...
There is also a chance that the wait will permit the telescope’s lifetime to be extended. It will probably use up the thruster fuel needed to keep it on station within a decade. In the past, that would have been that. In light of advances in space technology, however, NASA has installed an arrangement which would let a service vessel dock and offer a top-up.

They only were going add a docking adapter.  There is no ability to refuel.

[whitelancer64]

Good question.  Taking Ariane 5 out of the equation, it might be cheaper to build a performance-equivalent telescope with a much simpler (thus less costly) 6m monolithic mirror.  Perhaps the wider payload fairings on FH or SLS* would permit this?

Not true. 
a. A monolithic mirror would be too heavy.  And it would be more complex to fly.
b.  Even large follow ons to JWST are planned to be segmented.

"Monolithic" is obviously the wrong term.  Perhaps a segmented-mirror telescope that does not have to be unfurled?  Like Keck, albeit smaller.  You eliminate all those complex unfurling steps involving each segment.  I guess my fear is based on Galileo's high gain antenna, Lucy's solar array, etc.

PS:  I read somewhere that one of the designs of a JWST follow-on would have much larger aperture (segmented of course) and be robotically serviceable.

Thanks for the clarifications.

The mirror is not the issue. The two "wings" on the JWST primary mirror fold out on two hinges. The secondary mirror folds out on five.

The complexity is virtually entirely in the sunshade, which has nearly 600 moving parts.

[Blackstar]

The complexity is virtually entirely in the sunshade, which has nearly 600 moving parts.

At the risk of being pedantic, I think it is useful to discuss what kind of complexity. I think you're primarily referring to mechanical complexity.

When I visited JWST at NG back around November 2019, we discussed with the program manager what was the most difficult part of the telescope to design. I forget most of the details, but he said that it was the connection points to the back of the mirrors. The thermal (cryo) requirements for that were extreme, so that was the most complex part of the telescope development.

[kessdawg]

I'm a frequent reader of the Economist, and while I think its very good, it does still make mistakes. I was reading their article of JWST (https://www.economist.com/science-and-technology/2021/11/23/a-new-eye-on-the-heavens (but paywall)) and it had the following text:

The cloud of delay, meanwhile, has had silver linings. ...
There is also a chance that the wait will permit the telescope’s lifetime to be extended. It will probably use up the thruster fuel needed to keep it on station within a decade. In the past, that would have been that. In light of advances in space technology, however, NASA has installed an arrangement which would let a service vessel dock and offer a top-up.

They only were going add a docking adapter.  There is no ability to refuel.

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

[whitelancer64]

The complexity is virtually entirely in the sunshade, which has nearly 600 moving parts.

I think you're primarily referring to mechanical complexity.

As is AS_501, who I was replying to. Context is key.

[whitelancer64]

The complexity is virtually entirely in the sunshade, which has nearly 600 moving parts.

At the risk of being pedantic, I think it is useful to discuss what kind of complexity. I think you're primarily referring to mechanical complexity.

When I visited JWST at NG back around November 2019, we discussed with the program manager what was the most difficult part of the telescope to design. I forget most of the details, but he said that it was the connection points to the back of the mirrors. The thermal (cryo) requirements for that were extreme, so that was the most complex part of the telescope development.

Note also: You said you asked what was most difficult to design, not what the most complex part of JWST is. Some things can be relatively simple, but fiddly to design.

[jbenton]

I'm a frequent reader of the Economist, and while I think its very good, it does still make mistakes. I was reading their article of JWST (https://www.economist.com/science-and-technology/2021/11/23/a-new-eye-on-the-heavens (but paywall)) and it had the following text:

The cloud of delay, meanwhile, has had silver linings. ...
There is also a chance that the wait will permit the telescope’s lifetime to be extended. It will probably use up the thruster fuel needed to keep it on station within a decade. In the past, that would have been that. In light of advances in space technology, however, NASA has installed an arrangement which would let a service vessel dock and offer a top-up.

They only were going add a docking adapter.  There is no ability to refuel.

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

Someone asked this awhile back on the Hubble thread. The extreme pointing requirements of large space telescopes might preclude this approach:

When Hubble finally dies - and I hope that wont be for awhile - that's it; it's over. Money spent to refurbish Hubble would be better spent on  either new technology, more capable space telescopes or new technology, ground-based telescopes.

What about an MEV-style robotic service module to take over attitude control and allow for orbit raising?  The optics/instruments on Hubble are likely to outlast the reaction wheels and control systems by a fair margin. 

No.  Hubble's pointing accuracy needs to be sub milli-arcsecond which would be hard without the scope.

[Paul451]

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

Someone asked this awhile back on the Hubble thread. The extreme pointing requirements of large space telescopes might preclude this approach

He's suggesting the MEV to take over the thruster role. JWST doesn't achieve high level pointing with thrusters, just station-keeping and reaction-wheel de-saturation burns.

[kessdawg]

Right. I understand that Jim has mentioned issues with docking damaging the telescope, but from a layman's perspective potentially a degraded telescope would still be worth it? I'm not a scientist (rocket or otherwise) and I'm sure the professionals are considering this. Just asking for my own understanding.

[jbenton]

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

Someone asked this awhile back on the Hubble thread. The extreme pointing requirements of large space telescopes might preclude this approach

He's suggesting the MEV to take over the thruster role. JWST doesn't achieve high level pointing with thrusters, just station-keeping and reaction-wheel de-saturation burns.

That's right, and neither does Hubble:
https://www.nasa.gov/content/goddard/hubble-space-telescope-pointing-control-system

I realized as I was typing that last night that pointing control didn't seem to me to fully answer the question, but I figured that maybe I was missing something obvious

Perhaps Lee Jay was suggesting that the added mass in a new place would be too much of a challenge for the onboard pointing system? As a complete layman myself, I'd assume that this could just require software upgrade (albeit, a complicated one) but I'm probably missing something.

There's also the issue of running out of helium coolant. WISE has had a fairly successful warm mission doing less demanding tasks (as did Spitzer). I have no idea if that's possible for JWST's instruments.
EDIT: I forgot that JWST's cooling systems don't work the way that the other telescopes I mentioned work. "Running out of coolant" might not be a concern at all.

[kdhilliard]

... There's also the issue of running out of helium coolant. ...

JWST's MIRI cryocooler doesn't actively consume helium, does it?
Is the concern leakage?

[Lee Jay]

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

Someone asked this awhile back on the Hubble thread. The extreme pointing requirements of large space telescopes might preclude this approach

He's suggesting the MEV to take over the thruster role. JWST doesn't achieve high level pointing with thrusters, just station-keeping and reaction-wheel de-saturation burns.

That's right, and neither does Hubble:
https://www.nasa.gov/content/goddard/hubble-space-telescope-pointing-control-system

I realized as I was typing that last night that pointing control didn't seem to me to fully answer the question, but I figured that maybe I was missing something obvious

Perhaps Lee Jay was suggesting that the added mass in a new place would be too much of a challenge for the onboard pointing system? As a complete layman myself, I'd assume that this could just require software upgrade (albeit, a complicated one) but I'm probably missing something.

There's also the issue of running out of helium coolant. WISE has had a fairly successful warm mission doing less demanding tasks (as did Spitzer). I have no idea if that's possible for JWST's instruments.

Look at the question I was answering:

When Hubble finally dies - and I hope that wont be for awhile - that's it; it's over. Money spent to refurbish Hubble would be better spent on  either new technology, more capable space telescopes or new technology, ground-based telescopes.

What about an MEV-style robotic service module to take over attitude control and allow for orbit raising?  The optics/instruments on Hubble are likely to outlast the reaction wheels and control systems by a fair margin. 

Notice the "take over attitude control" part.

[deadman1204]

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

The thrusters used to approach, move away,  and maneuver would shred the sunshield. At that point the telescope would be worthless.

[Redclaws]

Would anything prevent something like the Northrop Grumman Mission Extension Vehicle being used? That adds a thruster pack I think.

The thrusters used to approach, move away,  and maneuver would shred the sunshield. At that point the telescope would be worthless.

Really?  The sunshield can't handle the thruster firings required to approach?  Even if they're as small as possible?  It would be shredded?  Like so much confetti, no doubt.

Sorry, no.  Thruster firings can be incredibly small and gentle and largely pointed away from the shield.

[Blackstar]

This again? Just go read the rest of the thread before you start designing your own spacecraft.

[catdlr]

Just background info....

James Webb Telescope Delayed Due to "Incident"

NASA announced that the James Webb Space Telescope's launch was postponed to no earlier than December 22nd due to an incident that occurred during launch prep. The anomaly was due to a clamp band that shook the telescope unexpectedly. NASA is verifying that JWST is safe to fly before proceeding.

00:00 Clamp band anomaly
01:37 Prior shaking events
02:29 Anomaly review and testing
03:29 Delay
04:00 Next Steps

[LouScheffer]

[Edit: That said, potential repair missions on that scale seem off-topic for this thread. Even discussing a MEV life-extension seems to be pushing the edges too far. Creating a new topic for "JWST life-extension and repair mission concepts and speculation" is just a matter of clicking a button.]

OK, new thread created.  If mods could move over the existing life-extension posts, that would be great.

[daveglo]

Webb telescope confirmed undamaged during clamp band release incident by Arianespace:

https://www.arianespace.com/corporate-news/testing-confirms-webb-telescope-on-track-arianespace-launch/

Key bit: "On Wednesday, Nov. 24, engineering teams completed these tests, and a NASA-led anomaly review board concluded no observatory components were damaged in the incident. A “consent to fuel” review was held, and NASA gave approval to begin fueling the observatory. Fueling operations will begin Thursday, Nov. 25, and will take about 10 days."

[SPKirsch]

https://twitter.com/ChrisG_NSF/status/1465735852982685702

With #JamesWebb's MIRI instrument, scientists will observe the universe in the mid-infrared. But MIRI requires a very cold temperature, just 6K. How will it get this cold & why is MIRI so important?

@haygenwarren & I talked with the NASA MIRI lead

https://www.nasaspaceflight.com/2021/11/jwst-miri-instrument/

[bolun]

Webb fuelled for launch

https://www.esa.int/ESA_Multimedia/Images/2021/12/Webb_fuelled_for_launch

Webb’s propellant tanks were filled separately with 79.5 l of dinitrogen tetroxide oxidiser and 159 l hydrazine. Oxidiser improves the burn efficiency of the hydrazine fuel.

These propellants are extremely toxic so only a few specialists wearing Self-Contained Atmospheric Protective Ensemble, or ‘scape’ suits, remained in the dedicated fuelling hall for fuelling which took 10 days and ended on 3 December.

The next steps will start soon for ‘combined operations’. This is when specialists working separately to prepare Webb and Ariane 5 will come together as one team. They will place Webb atop its Ariane 5 launch vehicle and encapsulate it inside Ariane 5’s fairing.

Then, no longer visible, Webb, joined with its Ariane 5 launch vehicle will be transferred to the Final Assembly building for the final preparations before launch.

Image credit: ESA/CNES/Arianespace