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I like this part: It seems that NASA's been willing to accept a fair amount of it, given the tragedies that have bedeviled our space program. NASA has been te
by juiceandjuice 16y ago
I like this part:
It seems that NASA's been willing to accept a fair amount of it, given the tragedies that have bedeviled our space program.
NASA has been terrible on this. Not because of how much risk they're willing to accept, and not because they don't do the work to understand it. They always know what the probability of failure is. But, historically at least, they haven't been honest and forthright in taking to the public about it. In the early '80s, before the Challenger accident, they would say—and this is where I think they actually lied, I don't think that's too strong a word—they would say, "Flying the shuttle is like flying a 727 to Disney World."
That's absurd. Not only are you more likely to get killed in the shuttle than in an airplane; you're more likely to get killed going up once in the shuttle than if you had flown combat missions for two years in Vietnam. That's a statistical fact, but NASA doesn't make it clear. They might tell the House [of Representatives] that there's a 1.5 percent failure rate, but most Americans don't understand what that means. I mean, 1.5, what is that? Is that a lot? You have to relate it to something that means something to somebody. Otherwise, people have the perception that space flight is safe, and when there's an accident, they're shocked. It's like, "We gotta stop flying." If we want to add additional safeguards because now we're feeling emotional about it, okay, we can do that. But if we're still meeting our design specs for loss, why would we stop flying?
I like this approach.
- eliasmacpherson 16y agoIf you haven't seen it here's R. P. Feynman's Personal observations on the reliability of the Shuttle. I think the reason that additional safeguards were added was not because they begun to feel emotional about it, but rather NASA had underestimated the failure rate in the first instance. The emotional factor is what prompted the investigation. 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...
- gvb 16y agoAppendix F is a must read. The tl;dr summary is: Engineers at Rocketdyne, the manufacturer, estimate the total probability [of catastrophic failure] as 1/10,000. Engineers at Marshal estimate it as 1/300, while NASA management, to whom these engineers report, claims it is 1/100,000. An independent engineer consulting for NASA thought 1 or 2 per 100 a reasonable estimate. — Personal observations on the reliability of the Shuttle by R.P. Feynman Empirical post script (as of STS-133 24 February 2011) Total launches 132 Failures 2 Successes 130 Failure rate: 1.5% Edit: add opinions: See also http://en.wikipedia.org/wiki/Rogers_Commission_Report#Role_of_Richard_Feynman http://en.wikipedia.org/wiki/Rogers_Commission_Report#Role_o... The reason for appalling and wildly inaccurate estimate of the failure probability is because the failure probability was a top-down "estimate." Someone Important apparently said "the space shuttle needs a reliability of 1/100,000". This mandate was pushed down on all the suppliers. The result was that the probability of failure was divided out onto all the systems, subsystems, subsubsystems... ad nausium all the way down to the bolts. The result was that all the subsystems had reliability numbers that could not be refuted (e.g. bolts at 1/10^12 IIRC), but could not be realistically substantiated either. Unfortunately, some of the probabilities of the major subsystems (SRB and ice damaging the heat shield tiles) were not accurate. The most accurate estimate was the engineer with a lot of experience in extremely complicated systems who gave an experience-based estimate, not a top-down (dictated) numerically derived estimate.
- dmfdmf 16y agoI think Feynman also identified how rationalizations due to schedule or political pressure were creeping in to safety assessments. When o-ring erosion was initially found they examined it and claimed that since the burn through was x%, that implied they still had y% of "margin" and used that to justify continuing flights. But this was never how the o-rings were designed to operate, 0% burn through is what was expected and now they were using engineering safety margin as operational margin, on a process (burn through) that was not fully understood.