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In a YouTube "Short", Dr. Becky explains that a problem with ten washer nuts wound up adding 800 days and $800 million to the JWST. The problem is that they came loose in the shake test, and I assume there were cascading problems that she didn't detail.

But I can think of several ways to keep nuts tight. Lok-tite, a spot-weld at the nut-bolt interface, cotter pins,

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wire them on like the bolts on an airplane propeller,

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I assume the engineers know all that stuff, too. So... what was the $800 million problem, and how was it solved?

Greg
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  • perhaps threading itself failed? – zephyr0110 Dec 31 '22 at 12:41
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    This is why I don't like those youtube shorts. It of course did not cost NASA $80 million each ($800 million total) to replace those ten failed washers. – David Hammen Dec 31 '22 at 12:47
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    @DavidHammen: That's not what the video claims. The video claims that knock-on effects from the test failure delayed the project by 800 days, costing about $1 million in labor each. That's a very different claim than saying each nut costs $80 million, which the video never does. – Jörg W Mittag Dec 31 '22 at 14:29
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    As I understand it, the safety wire locking you show above for airplanes is standard procedure for spacecraft too. – Greg Miller Dec 31 '22 at 16:24
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    Bolt locking can be surprisingly problematic. For example, castle nuts have a tendency to just rotate anyway and shear the cotter pins off. Additionally, space and other ultra-high-reliability engineering tends to require applying obsessive analysis to even the smallest change. – ikrase Jan 01 '23 at 05:30
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    Cotter pins and locking wire on airplanes are often being replaced by nylon locking nuts @Greg, they are easier to use and more reliable. In any case I suspect the answer to this is not the nuts, but that there was more vibration coming through the structure than expected. That's just a guess though. – GdD Jan 01 '23 at 17:42
  • Well I didn't know that a cotter pin meant that in the USA! In the UK it's one of these: https://en.wikipedia.org/wiki/Cotter_(pin) – Dave Gremlin Jan 04 '23 at 18:01

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That those washer nuts failed to hold during the shake test was a telltale sign that the JWST might well not have been capable of withstanding the incredible shaking that happens during launch. Those washer nuts should not have failed. The failures indicated that something was wrong deep inside the system, well beyond those failed washer nuts. This in turn meant the entire system had to be reinvestigated and, in some key places, reengineered.

David Hammen
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    "The failures indicated that something was wrong deep inside the system": more precisely, that the process used to develop the system had failed. If they failed to catch basic mistakes with those fasteners, what else got through? – Christopher James Huff Dec 31 '22 at 15:33
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    @ChristopherJamesHuff That was essentially my point. Maybe those ten washer nuts were just a problem with those fasteners, and nothing else. Or maybe they weren't; maybe the contractor had missed something else. When parts fall off during a shake test or acoustic test, it is a sign that the process has failed, and it might have failed elsewhere. – David Hammen Jan 01 '23 at 09:25
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    It's a coffee stain on the flip-down trays kind of thing. (Don Burr, founder of long-gone People Express Airlines, flipped out on seeing coffee stains on the flip-down trays during an inspection: “Coffee stains on the flip-down trays in our airplanes mean to passengers that we do our engine maintenance wrong.”) – David Hammen Jan 01 '23 at 09:28
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    I've heard tales of doing quick sanity-checks on fasteners holding together important aerospace vehicle bits before tests, and finding that they weren't even finger-tight. In my own experience (the FASTRAC college nanosat), on one project the torque we were given to tighten some bolts to was many times the appropriate torque value, so fasteners were yielding easily/early because they were already under considerable stress just from improper installation. sometimes we just aren't qualified to fasten things i guess – Erin Anne Jan 02 '23 at 01:31
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    @ErinAnne On an expensive and complex spacecraft such as the JWST, those fastenings are supposed to be checked, checked again, and checked yet again prior to the vibration / acoustic testing. Those last integrated tests are supposed to be a pass-thru, but just in case ... – David Hammen Jan 02 '23 at 05:56
  • @DavidHammen nah, not that. Those are totally unrelated things. There can be airlines with stained trays and perfect maintenance because they know what really matters. There can't be manufacturers who make spacecraft parts with nuts that fall off at launch, but get everything else right. – user253751 Jan 02 '23 at 18:42
  • this sounds like a place - perhaps the only place on space.se.com! - to reference Van Halen's "no brown M&Ms" contract clause ... – davidbak Jan 03 '23 at 01:46