6 ms·
Oh dear! It looks like someone accidentally divided by 2 for the cylindrical hoop stresses, forgetting that they're not the same equation as for spherical stres
by NamTaf 7y ago
Oh dear! It looks like someone accidentally divided by 2 for the cylindrical hoop stresses, forgetting that they're not the same equation as for spherical stresses. :)
Less facetiously, a thin-walled cylinder's stress is a function of pressure, radius and thickness with the equation:
sigma = P*r/t
whereas for a sphere (including the spherical end caps on a cylindrical pressure vessel), it follows the equation:
sigma = Pr/(2t)
To me, it looked like it failed around the join between the spherical end cap and the cylindrical body, where this transition between stress regimes occurs, which is a common weak spot in the above analysis. This is a canonical early engineering statics problem.
It's worth noting, though, that all of the above is almost certainly a gross simplification and likely has very little bearing on the actual problem. It's fun to simplify and hypothesise, though! Also, unplanned failures make for great viewing :)
- taneq 7y agoThat's why you take your maximum design stress and double it, right? /s
- namirez 7y agoI don't know what happened here, but if I remember correctly, in rockets, safety factor is typically smaller than 2 or the design would be too heavy. But I guess the safety factor for pressure testing is 4. Source: article 6.8.2.2 of this document; https://www.faa.gov/about/office_org/headquarters_offices/ast/reports_studies/library/media/Operational_Guidelines_for_Spaceflight_Pressure_Vessels.pdf https://www.faa.gov/about/office_org/headquarters_offices/as... Edit: I didn't see the "/s" before writing this comment.
- dehrmann 7y ago> in rockets, safety factor is typically smaller than 2 or the design would be too heavy Safety factors feel like p-values. Why .05? Because. What if it's not good enough? We'll move the goal post.
- NamTaf 7y agoThey are, somewhat. You can theoretically make a safety factor 1.0 if you're really absolutely certain of the input criteria. Safety factor is really just a (somewhat blunt) method of managing risk. It's just admitting that we don't know the true load spectra that a design is exposed to, so we take what we believe is the max load and then slap a multiplier on it to manage how much uncertainty we expect, be it from estimates of that load, or dynamic factors, etc. etc. We then also consider the consequence (since risk = likelihood x consequence). If the consequence of a failure is that our balsa-wood model bridge falls over, we shrug and keep it low. If it's that our pressure vessel undergoese a BLEVE [1] in the middle of a population centre then you jack it right the hell up. There's nothing that specifically requires a safety factor. We could spend millions of dollars and thousands of hours understanding exactly the load spectra a design experiences, but that may be prohibitively expensive in the case of designing a bridge, so we instead accommodate more risk by overdesigning the item. In a space application where every kg of launch mass represents big $$$, then spending that extra time and money to understand the load specifics in more detail makes sense. [1]: https://en.wikipedia.org/wiki/BLEVE https://en.wikipedia.org/wiki/BLEVE
- TeMPOraL 7y agoSafety factors also mitigate stupidity (your bridge is rated for load X, but someone drives 1.1 X through it because they're in a hurry) and cascade failures (a component upstream of yours fails and sends more load down your way).
- jefftk 7y agoThose both sound like kinds of ways the real maximum load could be higher than what you've called your maximum load
- TeMPOraL 7y agoOnly as long as your "real maximum load" != "announced maximum load". For the type of stupidity mitigation I mentioned, you want the real maximum load to be greater than the max load you announce to the customer/users. For mitigating cascade failures, you want the real maximum load to be greater than the maximum load value meant to be used internally by people designing other components in the system (though here, a smarter way would be to do a system-wide analysis of load flows to prevent cascade failures under user stupidity; however, here we're rapidly approaching the point at which I just talk out of my ass, having no real expertise on the topic).
- d-sc 7y agoTypically the factor of safety is not that high in the aerospace industry.
- cdumler 7y agoHow else do you think I keep my reputation as a miracle worker? -- Scotty
- LeoPanthera 7y ago"The tank can't handle that much pressure." "Where'd you get that idea?" "What do you mean, where did I get that idea? It's in the impulse engine specifications." "Regulations 42/15: 'Pressure Variances in IRC Tank Storage'?" "Yeah." "Forget it. I wrote it… A good engineer is always a wee bit conservative, at least on paper." - La Forge and Scott, Star Trek TNG "Relics"
- soapboxrocket 7y agoIt's been a while since I did pressure vessels but it's usually 2.5 times for proof and 3.5 for burst. Then with added safety you usually need about 4x to actually see a "planned" failure. This is most likely a construction fault.
- ew6082 7y agoThey left the nose cone off to save money. I wonder if the designer counted on some portion of the cone weight to support the cylinder head in their design?
- deleted 7y ago[deleted]
- dsfyu404ed 7y agoI know that SpaceX is trying to make space flight cheap so they have to figure out how to do things like use cheap pressure vessels but this mode of failure is exactly why aerospace pressure vessels have long been made from a hot rolled body (as opposed to cold worked and welded). https://www.youtube.com/watch?v=MvADC43EHmE https://www.youtube.com/watch?v=MvADC43EHmE
- IshKebab 7y agoI'm fairly sure SpaceX's engineers know 1st year mechanics.