4 ms·
>Making two adds little cost. I think you are overextending your experience in a production environment here. I’ve worked in both, and the type of builds in t
by bumby 5y ago
>Making two adds little cost.
I think you are overextending your experience in a production environment here.
I’ve worked in both, and the type of builds in these aerospace applications still have huge costs in subsequent runs. Hell, even rebuilding an existing component can be prohibitively expensive.
Much of the GSE was likely existing so that’s probably a non-issue.
- WalterBright 5y ago> huge costs in subsequent runs Often that's because they have to rebuild and reset the tooling. Within the run, though, the incremental cost should be minimal. The huge costs may be huge, but not huge compared to the cost of the prototype.
- bumby 5y agoIMO your logic fits with large production runs. In space applications costs can be exaggerated compared to actual production environments because the risks aren’t mitigated by something like the FAA, meaning they are often mitigated by some downstream process. Besides, a lot of the the designs already include critical spares, so there’s more than a single run, even in a one-off design
- WalterBright 5y agoSo build a copy out of the spares! If they're there, they're already paid for.
- bumby 5y agoNot every part meets the criteria to be a "critical" spare. You don't have a complete set of spares for every component.
- WalterBright 5y agoAre you suggesting that spares are only produced for the cheap parts?
- bumby 5y agoNo. Only for the parts deemed critical. An example may be a part with a long lead time and no redundancy. If it is found defective during testing, they need a replacement right away and don’t want the schedule to slip while it gets fabricated
- kragen 5y agoA lot of times when you make one of something, there's stuff you would make tooling for if you were going to do more than one, but which instead you do by hand. Sometimes this is even true in software, but not very often, because the advantages of being able to recompile something from source code are so huge.
- WalterBright 5y agoEven when doing things by hand, making a duplicate alongside the original is not much more time. That happens to me every time I do something by hand.
- kragen 5y agoThis is true for things like motor mounts and pipe flanges, but not for things like optical surfaces and lathe ways. Even in cases where it is true, if you didn't make the duplicate alongside the original, you have to do the setup again to make a duplicate.
- WalterBright 5y agoYes, make it alongside the original. I've been saying that all along.
- kragen 5y agoDoesn't help with telescope mirrors, as you likely know from experience.
- bumby 5y agoI think your mindset is stuck in thinking about this as an airplane production environment because that was your experience. And your points make sense in that type of environment. But research aerospace in which JWST operates is a very different scale, operating much more at the margins where the tech is less well understood and takes much longer to develop. Investing billions for redundancy on an unproven system is a big risk (besides the fact that it would likely be designed differently - and better - if started today). The sunk cost fallacy already drives a lot of these projects...imagine how much worse it would be (and how many other, competing projects wouldn't get funded) if the costs were higher.