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James Webb is fully deployed

nasa.gov

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Re: James Webb is fully deployed

#131

Post by WalterBright »

terramex wrote: post #82
WalterBright wrote: post #72

What puzzles me is the maneuvering fuel. When that fuel runs out, the telescope can no longer orient itself. This ended the life of the Kepler telescope. But aren't there other ways to orient in space? 1. use pressure from the solar wind 2. have 3 electric motors on 3 axis. Wouldn't spinning those motors rotate the craft? Electric power to do it would come from solar panels, giving it plenty of fuel.

Pressure from solar wind will constantly put some torque onto telescope as its center of pressure is offset from center of mass. This torque will be counteracted by reaction wheels but they have maximum rotation speed and need to be unloaded using thrusters periodically.

A rudder would work to counter any torque from the solar wind, and can be used to orient it.

Re: James Webb is fully deployed

#132

Post by [deleted] »

loonster wrote: post #107
WalterBright wrote: post #75

Since 90% of the cost is probably in R+D of the telescope, one could build and deploy another for another 10%. Why isn't this done? Why is every space telescope completely unique?

The Hubble telescope wasn't unique. It was simply the first one that was pointed away from earth. For the JWST, is there even room for a 2nd telescope at L2?

[deleted]

Re: James Webb is fully deployed

#133

Post by lisper »

WalterBright wrote: post #119
sailingparrot wrote: post #102

Earlier quoted context omitted.

Because it's a wrong assumption. Significant cost come from the assembly and testing of all the components itself, that you would have to redo entirely. Also if you were to build just 2 or 3 of them, you can't expect any economy of scale. On top of that, the operational cost of JWST is expected to be around 1B$ for it's lifetime, you could expect that to be similar for every single replica you have. And finally, you…

Assuming your figures are correct, you'll get double the data for another 30% cost. If you wait a month before launching #2, if problems appear in #1 (like the telescope mirror was ground improperly) it can be fixed in #2. The operational cost will not double. The same ground facility, equipment, and staff can manage both. If you're buying two identical launches, you can get a quantity discount. > the benefits of a s…

> double the data

Yes, but the data has diminishing returns as well.

The real value in a second unit is having a backup in the event of a total failure of the first. But that's a hard sell to the taxpayers who fund these things.

Re: James Webb is fully deployed

#134

Post by WalterBright »

oakwhiz wrote: post #76
WalterBright wrote: post #72

What puzzles me is the maneuvering fuel. When that fuel runs out, the telescope can no longer orient itself. This ended the life of the Kepler telescope. But aren't there other ways to orient in space? 1. use pressure from the solar wind 2. have 3 electric motors on 3 axis. Wouldn't spinning those motors rotate the craft? Electric power to do it would come from solar panels, giving it plenty of fuel.

IIRC the thrusters are also used for station-keeping since practical Lagrange orbits are unstable.

Being a telescope, what matters is where it is pointing, not where it is.

Re: James Webb is fully deployed

#135

Post by irdc »

WalterBright wrote: post #129
irdc wrote: post #81

Earlier quoted context omitted.

They've already mentioned having more than the of amount of manoeuvring fuel (or, as we should really be calling it, delta-v) they had planned to have left at this point. Space craft have a limited life time anyway (CCD's wear out, semiconductors are subject to electron migration, solar panels degrade), so having a limited amount of fuel to stay at Sol-Earth L2 is just part of the whole lifetime equation.

Right, but NOVA said that running out of fuel ended Kepler.

It worked for 9 years and 7 months, whilst being planned to operate for 3,5 years, so beyond it's expected lifetime then.

Re: James Webb is fully deployed

#136

Post by BobbyJo »

WalterBright wrote: post #119
sailingparrot wrote: post #102

Earlier quoted context omitted.

Because it's a wrong assumption. Significant cost come from the assembly and testing of all the components itself, that you would have to redo entirely. Also if you were to build just 2 or 3 of them, you can't expect any economy of scale. On top of that, the operational cost of JWST is expected to be around 1B$ for it's lifetime, you could expect that to be similar for every single replica you have. And finally, you…

Assuming your figures are correct, you'll get double the data for another 30% cost. If you wait a month before launching #2, if problems appear in #1 (like the telescope mirror was ground improperly) it can be fixed in #2. The operational cost will not double. The same ground facility, equipment, and staff can manage both. If you're buying two identical launches, you can get a quantity discount. > the benefits of a s…

Double the data may not be worth 30-50% more money to NASA. Going from no data of type X to 1 data of type X is worth a lot more than going from 1 to 2, and so the leap from 1 to 2 might just not be worth it, even if it is 50% cheaper than the leap from 0 to 1.

Re: James Webb is fully deployed

#137

Post by BurningFrog »

sailingparrot wrote: post #102
WalterBright wrote: post #75

Since 90% of the cost is probably in R+D of the telescope, one could build and deploy another for another 10%. Why isn't this done? Why is every space telescope completely unique?

Because it's a wrong assumption. Significant cost come from the assembly and testing of all the components itself, that you would have to redo entirely. Also if you were to build just 2 or 3 of them, you can't expect any economy of scale. On top of that, the operational cost of JWST is expected to be around 1B$ for it's lifetime, you could expect that to be similar for every single replica you have. And finally, you…

> the benefits of a second one would be marginal compared to the benefits of the first one.

This can't be true.

The benefit of the JWST is the observations it can make Two JWSTs can make twice as many observations as one, so it provides twice the benefit.

In some sense there is a diminishing return in that the most important observations will be attempted first, and and over time the average observation will be less and less important. But surely there is many decades of pent up very important research!

Re: James Webb is fully deployed

#138

Post by henrydark »

WalterBright wrote: post #75

Since 90% of the cost is probably in R+D of the telescope, one could build and deploy another for another 10%. Why isn't this done? Why is every space telescope completely unique?

Reminds me of the line in Contact: "why build one when you can have two at twice the price?"

Re: James Webb is fully deployed

#139

Post by WalterBright »

funklute wrote: post #85
WalterBright wrote: post #69

Earlier quoted context omitted.

Airliners are designed so that no single failure can bring it down.

That's not entirely true. Any reasonably complex system will almost certainly have single points of failure, including airliners. Those single points of failures might be very unlikely, but they are still there. E.g. what if the front fell off?

It is entirely true. Source: I worked on 757 flight controls design for 3 years.

> what if the front fell off?

All structural components are redundant and have redundant attachments. The wing spars are doubled, for example.

Re: James Webb is fully deployed

#140

Post by davesque »

Crash0v3rid3 wrote: post #124
davesque wrote: post #91

Earlier quoted context omitted.

I agree with the sentiment, but I think some people might take issue with the suggestion that this was entirely NASA's achievement.

Who else was involved?

ESA and CSA to name a couple. Probably also lots of other uncredited, small cross-institutional interactions. Not to mention all the details of the launch. I was partly trying to speculate as to why the comment appeared to be getting down-voted. But I guess it's also my opinion that it's kinda weird to claim that no one else could have done it. Who else are we talking about other than earthbound humans?
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