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I think this is reasonable, but there’s a couple important points: Aerospace quality is largely driven by reducing false negatives. You’re generally more ok th

• TeMPOraL 3y ago
  > Aerospace quality is largely driven by reducing false negatives. You’re generally more ok throwing out something good than accepting something bad. Reducing false negative is what becomes costly. Right. But the special case here is that if you can eliminate part of the process entirely, you no longer have to pay for reducing false negatives in it. > Hiding quality issues by increasing throughout is a classic fallacy in the process control world because it increases waste. Sorry if I misinterpreted your point here. All I'm saying is that trying to increase throughput will force you to simplify and streamline, which often will increase waste, but still significantly lower the total cost. Random example: CNC milling is incredibly wasteful compared to some more complex (and more manual) processes, but it's also much simpler and faster, and this saves you way more money across the board than you lose over wasted material. Same principle applies to QA: if you have two tests with equal false negative rate, with the second one having 2x the false positive rate but being 4x cheaper to run, it's worth it to switch to the second one - while this doubles the waste at this stage, it also lets you double the production (or just save you half the costs). Now, to give specific SpaceX / aerospace-relevant examples, I'd have to dig through some of the things I've read over the years that gave me this overall impression. Having not done that, all I'm saying is that there are strong synergies involved in scaling production up. You don't just get bulk rates and pump stuff out faster - you get better at it faster, which lets you optimize it further and do it quicker. Simplifying design to simultaneously cut costs of parts, assembly and QA, is just one specific case. > Is there some specific new NDT you’re thinking? Not really, not right now. I could look over my notes and some references, and perhaps point something out - but that would really be pretending I know more about manufacturing processes than I do.
  • bumby 3y ago
    >But the special case here is that if you can eliminate part of the process entirely, you no longer have to pay for reducing false negatives in it. I don’t disagree, I was just asking for a concrete example. In the abstract it’s a bit of hand-wavey marketing speak, but I’m not clear on what part of the process would be cut out. You’d still need traceability and pedigree regardless of how the part was made. That is what I pointed out drives so much of the cost. I think we have a fundamental difference of opinion on reducing costs through quality control. Having worked at different times in process design and quality control in both automotive manufacturing and aerospace, using excess inventory to try to drive down costs is generally a bad practice. But I may an old codger stuck in my ways, so I’ll leave that discussion alone. As a specific SpaceX example, they had a strut failure because of poor supplier quality control. Their remedy wasn’t to cut more out of the process but rather layer more quality control into it. I don’t know for certain, but I have to think that made it more expensive, not less.
  • panick21_ 3y ago
    > CNC milling is incredibly wasteful compared to some more complex (and more manual) processes, but it's also much simpler and faster, and this saves you way more money across the board than you lose over wasted material Interestingly SpaceX opted for the exact opposite. Instead of having huge CNC milling machines like CNC has. SpaceX just manually welds in stringers into the tank. And they were able to build the rockets far cheaper and arguably simpler. So it really depends what the manual process is and why you are doing it.
    • bumby 3y ago
      This and the parent comment has to constricted view of QA in aerospace. Yes, manufacturing quality is a big part. And yes, it’s possible that new manufacturing processes like additive manufacturing can streamline the QA. But the part being missed is all the downstream QA. Those welds still need to be inspected. Any soldering needs to be inspected to the J-STD. The materials still need to be kept in an environmentally controlled environment. The materials still need samples kept, for possibly decades, in a similar manner in case something is found wrong later. All this requires administrative overhead like a chain of custody etc. That isn’t cheap.