Potential servicing missions for the Webb

[Lee Jay]

Am I the only one who has ever set up a tarp or tent?  They NEVER tear in the middle.  Loads across the area (from wind, usually) are concentrated at the corners and they tear at the grommets.  I've seen tarps 20 times thicker than 50 microns and 1/100th the size of JWST's sun shield tear (at the corners, where the loads are concentrated) in winds of 30mph.  Rocket exhaust, even from cold gas thrusters can be 10 times that velocity.  Yes, the density will be far lower but the loads could still add up at the corners and tear it.

2 mills is half the thickness of a standard sheet of printing paper and only the first layer is that thick. The rest are half that thick.  And the thing is the size of my entire back yard.

[redliox]

There could be some options, but not to the same extent as the Hubble-shuttle relationship.

Regardless of the Starship and Orion, a few companies are emerging to assist communication satellites. This however is limited to refueling, or perhaps more accurately acting like a jetpack to extend the life of a decent sat that simply ran out of gas. It is promising to see a series of small, pod-like satellites docking to large satellites and assisting.

Frankly I believe if the Webb screws up during deployment, it's dead. However I think mission extension is doable; something like a propulsion pod perhaps with gyroscopes. L2 would be further than usual, but it's not impossible. If Webb runs out of gas or gets an unwound gyroscope I'd advocate a minor servicing mission based off what's currently coming online. After all we've blown a few billion on it, may as well ensure the thing lasts two decades to get the most of it. Gold-plated beryllium doesn't grow on trees, even on Mars.

[Lee Jay]

Frankly I believe if the Webb screws up during deployment, it's dead.

Agreed.

However I think mission extension is doable; something like a propulsion pod perhaps with gyroscopes.

Maybe, but not with gyroscopes.  The JWST has special gyroscopes (they aren't what you'd normally think of when using that term) and those are integrated into the spacecraft pointing system (the gyroscopes are detectors, not actuators).  The external mission extension vehicle would have to integrate with the vehicle electrical system (probably not possible)  Further, it has 6 and needs 2.  Perhaps you are saying "gyroscopes" when you meant "reaction wheels" but I'd consider even that to be hard or impossible to do.  Propulsion for maintaining orbit (yes, JWST orbits) and desaturation of the reaction system might be possible, but it's not really designed for it.

I consider conserving fuel during operational use to be a more likely method of mission extension than attaching another vehicle.

[LouScheffer]

Maybe, but not with gyroscopes.  The JWST has special gyroscopes (they aren't what you'd normally think of when using that term) and those are integrated into the spacecraft pointing system (the gyroscopes are detectors, not actuators).  The external mission extension vehicle would have to integrate with the vehicle electrical system (probably not possible)  Further, it has 6 and needs 2.  Perhaps you are saying "gyroscopes" when you meant "reaction wheels" but I'd consider even that to be hard or impossible to do.  Propulsion for maintaining orbit (yes, JWST orbits) and desaturation of the reaction system might be possible, but it's not really designed for it.

I don't think a servicing craft would need to attach to the electrical system, even it has reaction wheels.  Webb obviously has a high-bandwidth radio system to communicate with Earth, and receiving this from a meter away through sidelobe leakage and scattering should be easy.  So Webb would just need to devote some packets to talking to the servicer.  The bandwidth requirements should not be large in any case, as Webb does detailed pointing with a tip/tilt mirror.
Talking the other way (servicer to Webb) would be lower bandwidth (space transponders are not built for high upload rates) but should be plenty fast.

I consider conserving fuel during operational use to be a more likely method of mission extension than attaching another vehicle.

I agree this is surely the first method that will be tried.  But I believe they already schedule observations to minimize usage, and it's not clear how much more can be done and still allow good science.

[Lee Jay]

Maybe, but not with gyroscopes.  The JWST has special gyroscopes (they aren't what you'd normally think of when using that term) and those are integrated into the spacecraft pointing system (the gyroscopes are detectors, not actuators).  The external mission extension vehicle would have to integrate with the vehicle electrical system (probably not possible)  Further, it has 6 and needs 2.  Perhaps you are saying "gyroscopes" when you meant "reaction wheels" but I'd consider even that to be hard or impossible to do.  Propulsion for maintaining orbit (yes, JWST orbits) and desaturation of the reaction system might be possible, but it's not really designed for it.

I don't think a servicing craft would need to attach to the electrical system, even it has reaction wheels.  Webb obviously has a high-bandwidth radio system to communicate with Earth, and receiving this from a meter away through sidelobe leakage and scattering should be easy.  So Webb would just need to devote some packets to talking to the servicer.  The bandwidth requirements should not be large in any case, as Webb does detailed pointing with a tip/tilt mirror.

I don't know the electrical system on Webb, but I've designed many control systems and none of them had the capability to externalize the control loop to the communications system with the outside world.  All the control stuff (data from sensors, data and power sent to actuators) was internal and the external communications didn't include the ability to integrate sensors or actuators into that stream.

[LouScheffer]

I don't think a servicing craft would need to attach to the electrical system, even it has reaction wheels.  Webb obviously has a high-bandwidth radio system to communicate with Earth, and receiving this from a meter away through sidelobe leakage and scattering should be easy.  So Webb would just need to devote some packets to talking to the servicer.  The bandwidth requirements should not be large in any case, as Webb does detailed pointing with a tip/tilt mirror.

I don't know the electrical system on Webb, but I've designed many control systems and none of them had the capability to externalize the control loop to the communications system with the outside world.  All the control stuff (data from sensors, data and power sent to actuators) was internal and the external communications didn't include the ability to integrate sensors or actuators into that stream.

I completely agree it's not designed to do this.  On the other hand, I'd be surprised if the attitude target and current gyro readings are not included in the downlink telemetry.  Given this, Webb could potentially turn off its own attitude control and let the attached package handle it.  Webb or the ground would also have to tell the pointing package when it's between observations and OK to desaturate. 
   However, I doubt that exporting reaction wheel control will be needed.   As you said, Webb has 6 and needs at most 3, and reaction wheels seem to be pretty reliable (now that the space-ion-charging problem is fixed, even the low cost wheels are doing well, and Webb likely has fancy no-contact wheels anyway.)

[Lee Jay]

I don't think a servicing craft would need to attach to the electrical system, even it has reaction wheels.  Webb obviously has a high-bandwidth radio system to communicate with Earth, and receiving this from a meter away through sidelobe leakage and scattering should be easy.  So Webb would just need to devote some packets to talking to the servicer.  The bandwidth requirements should not be large in any case, as Webb does detailed pointing with a tip/tilt mirror.

I don't know the electrical system on Webb, but I've designed many control systems and none of them had the capability to externalize the control loop to the communications system with the outside world.  All the control stuff (data from sensors, data and power sent to actuators) was internal and the external communications didn't include the ability to integrate sensors or actuators into that stream.

I completely agree it's not designed to do this.  On the other hand, I'd be surprised if the attitude target and current gyro readings are not included in the downlink telemetry.

Oh, I'm sure it can be.  However, not necessarily in real-time.  It probably just saves the data to internal storage and includes it in a stream to Earth when that's available, minutes, hours or days later.

The same is true in my systems.  The sensor and actuator data is included in the storage loop but the storage loop can't be included in the real-time control loop.  This is intentional so that the real-time loop is never held up by or dependent on the storage loop timing.

[Barley]

I don't know the electrical system on Webb, but I've designed many control systems and none of them had the capability to externalize the control loop to the communications system with the outside world.  All the control stuff (data from sensors, data and power sent to actuators) was internal and the external communications didn't include the ability to integrate sensors or actuators into that stream.

I've worked on many control systems and all of them had the ability to externalize control loops.  Very common to see vehicles driving around the lab being controlled by an external laptop through a serial port, USB port or Bluetooth.

On most modern systems a general purpose micro controller (or FPGA) reads all the inputs, does some calculations and then sets the outputs.  As long as the micro is reprogrammable and has some communications capability (which is almost always there for debugging) the only problem is if the comm system has too much delay for the needed frequency response.

The systems were not designed to do this.  But it's actually quite hard to design a system that prevents it, you've got to hardwiring things and not giving a micro controller access.  The hardcore EEs who believe all PID loops should be implemented in op amps balk when they realize that they won't be able to fix things in software.

[Lee Jay]

I don't know the electrical system on Webb, but I've designed many control systems and none of them had the capability to externalize the control loop to the communications system with the outside world.  All the control stuff (data from sensors, data and power sent to actuators) was internal and the external communications didn't include the ability to integrate sensors or actuators into that stream.

I've worked on many control systems and all of them had the ability to externalize control loops.  Very common to see vehicles driving around the lab being controlled by an external laptop through a serial port, USB port or Bluetooth.

When a control-system failure results in a multi-million dollar machine crumbling to the ground, you generally don't do that.  I can unplug my HMI and the system stays running.

[deadman1204]

Whats the point of this thead? The OP basically said "cons say its basically not possible", yet still wants to discuss it.
All of this also ignores that this is out in deep space at L2.

“Yet still wants to discuss it”.  Huh, I wonder if that’s the point.

I think it’s very interesting to ask “how could we” and perhaps the answer is “we really can’t in practical terms”, but gosh, it sure is interesting to talk about.  Lots to be learned along the way.

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

[LouScheffer]

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

OK, to improve the discussion, please state WHAT makes it improbable, and WHY the solutions proposed here cannot work.  If you can do so with numbers, or references to documents, that's even better.  That's the point of this discussion, to look at the obstacles and see if any workarounds or solutions can avoid them.   A statements of "it can't be done", without reasoning and (better yet) numbers, is not helpful here.

[Lee Jay]

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

OK, to improve the discussion, please state WHAT makes it improbable, and WHY the solutions proposed here cannot work.

Starship won't be ready in time for a mission to repair a problem with deployment.
No other spacecraft will either.
JWST may not be approachable due to limitations on loads on the sun shade.
JWST may not be approachable due to contamination from the visiting vehicle.
JWST's location, assuming it gets there, isn't an easy place to get to for humans (remember, it's 4 times farther away than the moon is, so much further than any human has ever been away from Earth before).
A repair to what is broken may not be possible in space (depends on what is broken).
JWST wasn't designed for in-space service like Hubble was.

[ulm_atms]

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

OK, to improve the discussion, please state WHAT makes it improbable, and WHY the solutions proposed here cannot work.  If you can do so with numbers, or references to documents, that's even better.  That's the point of this discussion, to look at the obstacles and see if any workarounds or solutions can avoid them.   A statements of "it can't be done", without reasoning and (better yet) numbers, is not helpful here.

Let me help.  You can't land a booster...it's impossible and not worth trying.  Or at least that was what was said ~10 years ago...

deadman1204 question...Why are you so worked up that you have to shut everyone else down?  If you think it is impossible that is fine...but these people want to work through the problems and discuss....which is what the forum is for.  I guess I am trying to understand why you keep coming back to this thread instead of just leaving it be and letting the people who want to discuss...um well....discuss. 

This topic is strange though here...you got two sides.  One side is 100% no...why bother discussing and the other going "This is a problem if a service mission was considered...so how could we get around it?"  Why are you no trying so hard to shut this convo down?  If you don't agree, state it, state why, and then go about your business.  Coming back over and over to state the same "it's impossible" is well......confusing...and will get this thread locked eventually....which I hope is not the reason as that is bad taste IMO.

[DanClemmensen]

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

OK, to improve the discussion, please state WHAT makes it improbable, and WHY the solutions proposed here cannot work.

Starship won't be ready in time for a mission to repair a problem with deployment.
No other spacecraft will either.
JWST may not be approachable due to limitations on loads on the sun shade.
JWST may not be approachable due to contamination from the visiting vehicle.
JWST's location, assuming it gets there, isn't an easy place to get to for humans (remember, it's 4 times farther away than the moon is, so much further than any human has ever been away from Earth before).
A repair to what is broken may not be possible in space (depends on what is broken).
JWST wasn't designed for in-space service like Hubble was.

Thank you for this concise list of objections, all of which may very well be valid. This thread has been attempting to address or at least discuss each of them. A careful preliminary engineering and programatic analysis may very well conclude that a repair mission should not be attempted, if in fact it is needed. I think the "advocates", including me, are suggesting that such an analysis should be done before the mission is dismissed out of hand.  Here is an attempt to summarize the "answers" to each of your points:

*Starship won't be ready in time for a mission to repair a problem with deployment.
No other spacecraft will either.
       This depends on the useful life after repair of the JWST and of the amount of time it can spend in an unrepaired state before it becomes unrepairable. If the mission could be mounted in 2024 and JWST can then operate for six years, the mission might be cost-effective.

JWST may not be approachable due to limitations on loads on the sun shade.
JWST may not be approachable due to contamination from the visiting vehicle.
      This thread has proposed several possible solutions that do not load or contaminate the shade or JWST at all. One of them might be feasible.

JWST's location, assuming it gets there, isn't an easy place to get to for humans (remember, it's 4 times farther away than the moon is, so much further than any human has ever been away from Earth before).
      It's energetically closer that Mars, and Starship is designed to go to Mars.

A repair to what is broken may not be possible in space (depends on what is broken).
JWST wasn't designed for in-space service like Hubble was.
      True. A careful analysis might determine that repair is impossible or highly likely to fail. If so the mission would not be mounted. The analysis should be done. Intelsat 603 was not designed for repair, but a repair process was designed and successfully implemented.

I am not advocating that a mission should be mounted. I am suggesting that a study should be funded as soon as possible if a failure occurs.

[LouScheffer]

Except the "discussion" ignores basically EVERYTHING that makes it improbable. It resets back to the start every couple days ignoring all the problems that were raised again. Its literally a couple people who are unhappy they are told its not possible just saying the same stuff over and over ad naseum.

OK, to improve the discussion, please state WHAT makes it improbable, and WHY the solutions proposed here cannot work.

Thank you for this list.  I'll discuss each objection in turn:

Starship won't be ready in time for a mission to repair a problem with deployment.  No other spacecraft will either.

Absolutely true.  A deployment failure is doom for the mission.  The missions being discussed here are life extension, assuming Webb works as designed, and the main problem is that it is running out of fuel.  Starship is not needed for for a mission like this; the servicing spacecraft can be launched just like JWST itself.  A recoverable Falcon 9 can put about 3300kg to L2; commercially available life extension vehicles mass less than 2900 kg.  Also, the running-out-of-fuel scenario gives several years of warning - enough to construct a JWST specific vehicle, as will be required.

JWST may not be approachable due to limitations on loads on the sun shade.

Needs detailed analysis, but controlled approach appears possible.  Only low relative speeds are needed, and thrusters of well under a Newton will suffice.  Assuming the thruster is designed with a diffuse exhaust (hurts efficiency, but not a concern here) this should put very survivable forces on the subshield.

JWST may not be approachable due to contamination from the visiting vehicle.

If the visiting vehicle uses cold-gas thrusters with helium, there should be no contamination problems.  Easier solutions may be possible as the subshield also shields the optics, but cold helium thrusters should work if nothing else does.

JWST's location, assuming it gets there, isn't an easy place to get to for humans (remember, it's 4 times farther away than the moon is, so much further than any human has ever been away from Earth before).

True, but why would you want humans?  The life extension already commercially available for comsats does not use humans.  It approaches, clamps on, then assumes attitude control.  These are all relatively simple tasks that do not require human presence.

A repair to what is broken may not be possible in space (depends on what is broken).

True, but a life extension mission looks very possible.

JWST wasn't designed for in-space service like Hubble was.

Neither were the comsats that are the current customers of life extension missions.  To my knowledge, MEV-1 and MEV-2 simply approach and clamp on to various sturdy points (such as the engine bell) that are guaranteed to have capable load paths for satellite maneuvering.  Webb has such locations, with two main maneuvering engines on the spacecraft body.

[Lee Jay]

JWST may not be approachable due to limitations on loads on the sun shade.

Needs detailed analysis, but controlled approach appears possible.  Only low relative speeds are needed, and thrusters of well under a Newton will suffice.  Assuming the thruster is designed with a diffuse exhaust (hurts efficiency, but not a concern here) this should put very survivable forces on the subshield.

Really? Show your work.  That "detailed analysis" you mentioned.

[su27k]

Some thoughts:
1. Why would JWST be lost if it fails during deployment? Why can't it be put into a safe state, waiting a few years for a servicing mission? Note Hubble waited 3.5 years for its first serving mission.
2. If a servicing spacecraft will be permanently docked to JWST similar to Northrop Grumman's MEV, how feasible it is for the servicing vehicle to bring its own sunshade (a more sturdy one hopefully, using the larger mass margin provided by Starship) to replace the existing one in case the existing one was torn or just failed to deploy? Note Skylab 2 mission brought a replacement sunshade to fix Skylab, so it's not unprecedented either.

[LouScheffer]

JWST may not be approachable due to limitations on loads on the sun shade.

Needs detailed analysis, but controlled approach appears possible.  Only low relative speeds are needed, and thrusters of well under a Newton will suffice.  Assuming the thruster is designed with a diffuse exhaust (hurts efficiency, but not a concern here) this should put very survivable forces on the subshield.

Really? Show your work.  That "detailed analysis" you mentioned.

OK.  Let's model a patch of the sunshield as a 1 meter square, constrained on all edges (to account for stresses being concentrated at corners). Assume the thruster impingement creates a force of 10N/m^2 (this should be quite conservative).  We know the membrane is 50u thick Kapton.
From Dupont, we know the tensile modulus of Kapton is 2.48 x 10^9 Pascal.

Then using the "Stress Analysis Manual," Air Force Flight Dynamics Laboratory, October 1986, section 7.5.3, we can find the deflection and stress on the membrane.  Here is the calculation in Python:

n1 = 0.35  # Coefficients for a square panel
n6 = 0.35
p = 10     # Pressure of 10 N/m^2
E = 2.48e9 # From Dupont Kapton spec
t = 50e-6  # From Webb description
a = 1      # Assumed membrane panel size (meters)
print(f"deflection (meters) = {n1*a*pow(p*a/(E*t), 1/3):5.3f}" )
print(f"stress  (Mpa) = {n6*pow(p**2*E*(a/t)**2, 1/3)/1e6:5.3f}" )

When we run this we get:

deflection (meters) = 0.015
stress  (Mpa) = 1.620

We compare this to the allowable stress of Kapton of 234 MPa and conclude we are in no danger of rupturing the membrane.  We can re-run this with a 7 meter square (not realistic, you would not get this much pressure over such a large area), and get:

deflection (meters) = 0.202
stress  (Mpa) = 5.929

Still well under the strength of the Kapton film.

So overall, the force of a small thruster should not rupture the sunshade.

[Lee Jay]

JWST may not be approachable due to limitations on loads on the sun shade.

Needs detailed analysis, but controlled approach appears possible.  Only low relative speeds are needed, and thrusters of well under a Newton will suffice.  Assuming the thruster is designed with a diffuse exhaust (hurts efficiency, but not a concern here) this should put very survivable forces on the subshield.

Really? Show your work.  That "detailed analysis" you mentioned.

OK.  Let's model a patch of the sunshield as a 1 meter square, constrained on all edges (to account for stresses being concentrated at corners).

I think the edge constraint is wrong. The edges are unconstrained and the resulting analysis is complex for two reasons. First, the resulting system is non-linear (wrinkles can and will form). Second, the impingement isn't steady, it's dynamic (the plume isn't steady and there will be flow over the edges).  If I'm right, your numbers are wrong by orders of magnitude.

[edzieba]

Taking the Gordian option: 'damn the torpedoes sunshield, full speed ahead!'. Send a vehicle (Starship or otherwise) to L2 with sufficient Delta-V to haul JWST back to an Earth orbit (or another one more conducive to a subsequent manned or real-time-remote-controlled servicing mission). Assume the sunshield will be torn to shreds during rendezvous and capture and will need to be replaced later during servicing. Assume JWST and its instruments and structure will heat up to unacceptable temperatures and assume they will need to go through recomissioning later (along with any new instruments installed).
The major stumbling block - beyond a vehicle with sufficient delta-V to shuttle JWST around, and cost/benefit of the dual mission architecture - is mirror and instrument contamination. A copious inert cold-gas supply for use during rendezvous minimises reaction product contamination, and the subsequent heating aids in boiling of any volatiles, but servicing (and especially any interaction that releases particulates like having to go poking around in the instrument section) is going to be the hard part whilst avoiding contaminating the optics and instruments and rendering the servicing pointless. A vacuum is not automatically a cleanroom especially once you start generating local contaminants.
If a clean handling system ends up of similar complexity to design, test, and build as a new telescope, a servicing mission is not an ideal option vs. a follow-up telescope unless there is some time-critical task it is needed for (it's a scientific instrument, so almost certainly not).