Author Topic: Why do contractors make so much money but aren't responsible when things go wrong?  (Read 7589 times)

Online speedevil

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This is the real issue. Not a serviceable design.

It is not the design, it is the type of telescope.  IR cooled with sunshield.   Neither like thruster plumes

A very large cold mirror is a mission requirement - warming and cooling it back down is probably a bad idea, and would add additional cost.

The very fragile sunshield is a design choice imposed by the mass, packaging, and delta-v constraints.

There is no particular reason helium or hydrogen cold gas couldn't be used for final approach of a servicing mission for a clean-sheet designed telescope, even with a fragile sunshield and cold mirror.

Cold gas vernier thrusters with tiny thrust would need to be added if you're going to be servicing something with a fragile heatshield with the same constraints as JWST that was serviceable.

Yes, approach may take a week.

Yes, it may mean that you need a considerably more capable launch system to do the servicing than a hubble-like instrument.


Offline FinalFrontier

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Although it's OT you raised an interesting couple of points here.

First of all how well is that sun-shield going to do with MMOD I wonder. Especially since in ground testing so far the thing apparently ripped just trying to deploy it.

Second, to me this is another good example of a contractor issue with no accountability. I find it hard to believe there was not some alternative to this particular design element even in spite of the mass restrictions. If there truly wasn't then it's an example instead of a contractor being hurt by poor NASA mission planning and poor LV selection. Of course to be fair when JWST was first conceived we did not have/see more powerful wider fairing LV's coming any time soon.
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Online speedevil

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Second, to me this is another good example of a contractor issue with no accountability. I find it hard to believe there was not some alternative to this particular design element even in spite of the mass restrictions. If there truly wasn't then it's an example instead of a contractor being hurt by poor NASA mission planning and poor LV selection. Of course to be fair when JWST was first conceived we did not have/see more powerful wider fairing LV's coming any time soon.

At the current (as of JWST design) launch options, you would have either had to live with a smaller sun-shield compromising science, do multiple launches with assembly, or had fewer layers and active cooling which raises its own issues.

The choice to do it in one launch - perhaps largely due to concerns about in-orbit assembly was NASAs, and with existing rockets once that choice was made, pretty much everything - foldable mirror, large fragile sunshield - flows from that.

Unfolding large structures in space, especially complex ones, was always going to be really expensive and risky. Who bears the risk is a contracting decision.

For such large projects, spending $300M or whatever in a couple of launches to reduce risk of in orbit assembly may actually have made sense.
 

Offline SWGlassPit

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First of all how well is that sun-shield going to do with MMOD I wonder. Especially since in ground testing so far the thing apparently ripped just trying to deploy it.

Well, to start with, there's no OD out there whatsoever, since it's not in earth orbit.  That cuts down a lot.

That leaves the meteoroid aspect.  On average, the omnidirectional flux for 0.1 mm meteoroids around earth is in the neighborhood of 10 per square meter per year, with a median encounter velocity around 25 km/s.  At that size and speed, you'll get a tiny pinhole in the first layer of the shield, if you penetrate it at all.  The quantity of particles drops off dramatically as you go up in size, and at those velocities, nearly all of the energy of impact will go into vaporizing the incoming particle.  You'll get pinholes, not rips, in the sunshield.

The bigger concern would be critical bus elements -- attitude and stationkeeping hardware, as well as science instruments.  In this environment, though, even very light shielding is likely plenty adequate.

Offline Jim

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 I find it hard to believe there was not some alternative to this particular design element even in spite of the mass restrictions. If there truly wasn't then it's an example instead of a contractor being hurt by poor NASA mission planning and poor LV selection. Of course to be fair when JWST was first conceived we did not have/see more powerful wider fairing LV's coming any time soon.


This is just wrong.  There is nothing wrong with the basic design.
The sun shield would still have to be deployable with larger fairings.

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