21
$\begingroup$

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,

enter image description here

wire them on like the bolts on an airplane propeller,

enter image description here

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

$\endgroup$
7
  • 2
    $\begingroup$ 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. $\endgroup$ Dec 31, 2022 at 12:47
  • 8
    $\begingroup$ @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. $\endgroup$ Dec 31, 2022 at 14:29
  • 2
    $\begingroup$ As I understand it, the safety wire locking you show above for airplanes is standard procedure for spacecraft too. $\endgroup$ Dec 31, 2022 at 16:24
  • 3
    $\begingroup$ 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. $\endgroup$
    – ikrase
    Jan 1, 2023 at 5:30
  • 2
    $\begingroup$ 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. $\endgroup$
    – GdD
    Jan 1, 2023 at 17:42

1 Answer 1

32
$\begingroup$

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.

$\endgroup$
7
  • 39
    $\begingroup$ "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? $\endgroup$ Dec 31, 2022 at 15:33
  • 2
    $\begingroup$ @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. $\endgroup$ Jan 1, 2023 at 9:25
  • 11
    $\begingroup$ 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.”) $\endgroup$ Jan 1, 2023 at 9:28
  • 3
    $\begingroup$ 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 $\endgroup$
    – Erin Anne
    Jan 2, 2023 at 1:31
  • 2
    $\begingroup$ @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 ... $\endgroup$ Jan 2, 2023 at 5:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.