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Your paraphrasing of Feynman's bridge quote is inaccurate. From Appendix F[1] of the report: [..] In spite of these variations from case to case, official
by avar 10y ago
Your paraphrasing of Feynman's bridge quote is inaccurate. From
Appendix F[1] of the report:
[..] In spite of these variations from case to case, officials behaved as
if they understood it, giving apparently logical arguments to each
other often depending on the "success" of previous flights. For
example. in determining if flight 51-L was safe to fly in the face of
ring erosion in flight 51-C, it was noted that the erosion depth was
only one-third of the radius. It had been noted in an experiment
cutting the ring that cutting it as deep as one radius was necessary
before the ring failed. Instead of being very concerned that
variations of poorly understood conditions might reasonably create a
deeper erosion this time, it was asserted, there was "a safety factor
of three." This is a strange use of the engineer's term ,"safety
factor." If a bridge is built to withstand a certain load without the
beams permanently deforming, cracking, or breaking, it may be designed
for the materials used to actually stand up under three times the
load. This "safety factor" is to allow for uncertain excesses of load,
or unknown extra loads, or weaknesses in the material that might have
unexpected flaws, etc. If now the expected load comes on to the new
bridge and a crack appears in a beam, this is a failure of the
design. There was no safety factor at all; even though the bridge did
not actually collapse because the crack went only one-third of the way
through the beam. The O-rings of the Solid Rocket Boosters were not
designed to erode. Erosion was a clue that something was wrong.
Erosion was not something from which safety can be inferred.
His point about NASA's nonsensical use of the "safety factor" is not that you could drive over a bridge, and look, it didn't break, so it must be OK!
It's even worse, you drive a truck over it, afterwards 1/3 of the steel is cracked, so you conclude that it must be able to safely accept 3x the weight. Nonesense! This is the sort of moronic engineering that killed the crew of the Challenger.
1. http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/rogers-commission/Appendix-F.txt http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/roger...
- masklinn 10y ago> This is the sort of moronic engineering that killed the crew of the Challenger. Note that it wasn't even engineering, reading the story in full (it's great, and covers the software side for which Feynman had nothing but praise) Feynman repeatedly noted that engineers were fairly realistic[0] and had been ringing alarms pretty much all along, this was entirely manglement mangling. [0] unless the spectre of manglement was involved, at least for some of them
- avar 10y agoIt's been some time but I've read the entire report cover-to-cover. While yes, the general conclusion is that NASA's dysfunctional management structure and institutional optimism driven by moneyed interests is the primary culprit. The report never really tries to perform a root cause analysis on how something like the O-ring "safety factor" problem arose. Something which, as summarized by Feynman's quote above, should be patently obvious to any engineer as bullshit. Feynman's appendix is the only part that even tries, but it doesn't go far enough through no fault of Feynman's, he had no resources to pursue this line of inquiry. It was a struggle just to get that appendix into the report. They should have interviewed every single person in any way remotely involved in that O-ring decision, find out if they objected to it, and if they didn't what money/institutional/social obstacles there were to prevent that. Did some engineer actually sign off on the aforementioned "safety factor"? We don't know, but somehow I doubt that's language management came up with on their own, and if they did that there was no way for an engineer to spot that and report "wtf? The system doesn't work like that!". Reading between the lines some engineer actually did come up with that estimate, but likely that engineer was where he was because NASA had a culture of promoting mindless yes-men.
- masklinn 10y ago> It's been some time but I've read the entire report cover-to-cover. I meant Feynman's later recounting of the whole affair (in "What do you care what other people think"), rather than just the report. > Did some engineer actually sign off on the aforementioned "safety factor"? We don't know, but somehow I doubt that's language management came up with on their own That doesn't mean they were fed that by an engineer, only that they'd encountered the term before. > and if they did that there was no way for an engineer to spot that and report "wtf? The system doesn't work like that!". And then what? Upper-management uses "safety factor" in a completely bullshit manner, and engineer spots that (because they're masochistic and read management reports?), tells their direct manager it's inane, and then what, you think it's going to go up the chain to upper-management which will fix the issue? Because IIRC (I don't have my copy of What Do You Care on me so I can't check) Feynman noted that engineering systematically got lost somewhere along management ladder as one middle-manager decided not to bother their manager with a mere engineer (or worse, technician!)'s concern or suggestions. > Reading between the lines some engineer actually did come up with that estimate, but likely that engineer was where he was because NASA had a culture of promoting mindless yes-men. That's really not what I read behind the lines considering engineers had failure estimates in the % range and management had estimates in the per-hundred-thousand range.