James Webb is fully deployed • Hacker News Board

Re: James Webb is fully deployed

#81

Post by irdc » Sat, Jan 08, 2022, 8:31 PM UTC

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.

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.

Re: James Webb is fully deployed

#82

Post by terramex » Sat, Jan 08, 2022, 8:31 PM UTC

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.

Re: James Webb is fully deployed

#83

Post by kortilla » Sat, Jan 08, 2022, 8:31 PM UTC

jes wrote: post #70

ISL wrote: post #27

Earlier quoted context omitted.

If you're in a car being held above the ground by a chain, every single link is a potential single-point of failure.

In a real chain, only one link is actually the weakest, and it is at that link where the chain will break, in the hypothetical you offer.

That’s not correct, someone with a bolt cutter can get any link in the chain and it causes collapse.

Single point of failure doesn’t mean “weakest link”. It means if this one piece (“single point”) fails, the whole system will fail.

Re: James Webb is fully deployed

#84

Post by ctdonath » Sat, Jan 08, 2022, 8:32 PM UTC

jes wrote: post #70

ISL wrote: post #27

Earlier quoted context omitted.

If you're in a car being held above the ground by a chain, every single link is a potential single-point of failure.

In a real chain, only one link is actually the weakest, and it is at that link where the chain will break, in the hypothetical you offer.

But you don’t know which, and others may fail first due to various adverse events (say, random micrometeoroid strike). If any link fails, the whole chain fails.

Contrast a second chain, or double linked chain, so if any link fails the load is not lost.

Re: James Webb is fully deployed

#85

Post by funklute » Sat, Jan 08, 2022, 8:32 PM UTC

WalterBright wrote: post #69

ducktective wrote: post #26

Earlier quoted context omitted.

>344 single points of failure How can a system have more that one single point of failure?

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?

Re: James Webb is fully deployed

#86

Post by davesque » Sat, Jan 08, 2022, 8:34 PM UTC

This feels perhaps like a silly comment, but I have this intuition that the data collected by JWST could prove to be some of the most important ever collected.

Re: James Webb is fully deployed

#87

Post by Server6 » Sat, Jan 08, 2022, 8:34 PM UTC

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?

Why would we need two? I would think the next one 20+ years from now would hopefully be exponentially better.

Re: James Webb is fully deployed

#88

Post by lazide » Sat, Jan 08, 2022, 8:34 PM UTC

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?

When it takes so long to build, it’s already obsolete by the time it gets launched.

If you made a new one every year, it wouldn’t be so bad of course, but a decade+? Oof.

It was so bad they kept redesigning it mid way through to upgrade things as new discoveries were made, which caused even more delays.

Re: James Webb is fully deployed

#89

Post by macintux » Sat, Jan 08, 2022, 8:34 PM UTC

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?

I believe in this case it's not 90%.

Much of the cost here was (on the ground) assembly and testing, given the intricate nature of the setup.

Re: James Webb is fully deployed

#90

Post by scottlawson » Sat, Jan 08, 2022, 8:35 PM UTC

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?

Part of the reason is that each telescope has different scientific goals. For example, Hubble is a visible wavelength telescope and we learned a lot from the data it collected and continues to collect. JWST is an infrared telescope designed to see wavelengths that Hubble cannot see, and has a different set of science goals.