> Challenger: no gasses were supposed to make it past the o-rings no matter what,

> but when it became clear that gasses were escaping and the o-rings were being

> damaged, there was a push to suggest that it's an acceptable level.

Interestingly, the article<https://docs.google.com/document/d/1ddi792xdfNXcBwF8qpDUxmZz...> by heat shield expert and Shuttle astronaut Charles Camarda, the former Director of Engineering at Johnson Space Center, asserts that it was *not* the O-rings:

"The Challenger accident was not caused by O-rings or temperature on the day of launch; it was caused by a deviant joint design which opened instead of closed when loaded. It was caused by mistaking analytical adequacy of a simplified test for physical understanding of the system. The solution, post Challenger, was the structural redesign of the SRB field joint and the use of the exact same O-rings."

I find that highly surprising, because "it was the O-rings" explanation seems universally believed and sanctified by no lesser authority than the Nobel prize laureate Richard Feynman.

It's the same explaination. When the SRB joints flexxed the o-rings were meant to stay in place, but the joints were defective and NASA knew the o-rings were moving. However NASA also believed the o-rings could still take the abuse, because although they were moving they were getting shoved deeper into the joint, in a way that wasn't intended but was nonetheless at least marginally effective at stopping exhaust blow-by shortly after it began. But when the o-rings were cold and stiff... they didn't move the same way, exhaust blew by them longer and cut right through. At that point the SRB turns into a cutting torch (the SRBs didn't actually explode until after the shuttle broke up and range safety sent the signal to kill the boosters.

>>I find that highly surprising, because "it was the O-rings" explanation seems universally believed and sanctified by no lesser authority than the Nobel prize laureate Richard Feynman.

Essentially you are mischaracterizing what Feynman did or say, although this is also Feynman fault :-), by doing the famous public demonstration, with the ice water in a glass [2], although even there he only said it has "significance to the problem...". In other words, we should not simplify, even for the general public, what are complex subtle engineering issues. This is also the reason why current AI, will fail spectacularly, but I digress...

Feynman documented the joint rotation problem in his written Appendix F, but his televised demonstration became the explanation...[3]

Camarda is correct here. There was a fundamentally flawed field joint design, meaning the tang-and-clevis joint opened under combustion pressure instead of closing. This meant the O-rings were being asked to chase a widening gap something the O-ring manufacturer explicitly told Thiokol O-rings were never designed to do. Joint rotation was known as early as 1977, a full nine years before the disaster.

The cold temperature made things worse by stiffening the rubber so it could not chase the gap as quickly, but O-ring erosion and blow-by were occurring on flights in warm weather too and nearly every flight in 1985 showed damage.

The proof is how they fixed. NASA redesigned the joint metal structure with a capture feature to prevent rotation, added a third O-ring for redundancy, and installed heaters but kept the exact same Viton rubber. If the O-rings were the real problem, you would change the O-rings. They did not need to.

The report [1] is public for everybody to read...but not from the NASA page... who funnily enough has a block on the link from their own page, so I had to find an alternative link...

[1] - https://www.govinfo.gov/content/pkg/GPO-CRPT-99hrpt1016/pdf/...

[2] - https://youtu.be/6TInWPDJhjU

[3] - https://calteches.library.caltech.edu/3570/1/Feynman.pdf

My recollection is that a rocket design was scaled up from one that worked, by people who didn't consider how an o-ring should be loaded in order to function properly. They inadvertently changed the design rather than simply scale it. I don't think Feynman got this wrong either. His demo was because the justifications for flight were based on the fact that failure had a temperature correlation, and they had a model representing how damaged the o-rings would be.

The o-ring failure was a measurable consequence of the joint design failure. The data behind the model didn't go down to temperatures as low as that at Challenger's launch date.

For more inappropriate extrapolation to justify a decision: the data for the heat shield tile loss model was based on much less damage than sustained by Columbia (3 orders of magnitude IIRC).

Now they are looking at the same style of fallacy and don't even have a model based on damage sustained in flights.

Another parallel I haven't seen discussed here yet, though I haven't read all comments: I recall Feynman feeling like he was on the investigation panel as a prop, that the intention of the investigation was to clear NASA of any wrongdoing. They used a model, considered risks, etc. Feynman recognized the need for a clear and powerful visual to cut through an information dump and pull it to front page news. The invitation of Camarda to a presentation with a pre-determined conclusion has the same feeling. I don't know what Camarda can do to put it on a (non-HN) front page today.

Without being too familiar with the subject - another commenter referred to the "swiss cheese model": the O-ring design, the temperature etc. weren't the single cause, they were contributing factors, and the more contributing factors you eliminate, the more certain you can be that you won't have a repeat accident. AFAIK there weren't any more Shuttle launches at such low temperatures after that anymore either?

Both things can be true. A better O-ring with the same joint might have prevented the disaster. A better designed joint with the same O-ring might also. Feynman knew that a little theater would go a long way. The O-ring explanation, albeit a partial explanation, made for good theater.

Using the same o-rings afterwards is surprising, I've heard that the manufacturer was surprised that they were being used for that purpose because they weren't rated for that.

Also I'm not sure the assertion is correct. If the sealant and O-Rings were adequate, the joint would not have failed. It was suboptimal, and increased risk, sure, but it in itself wasn't the reason for the accident. It was the joint and the o-rings in combination. The holes in the swiss cheese model lined up that day, and a lot of small problems combined into one big problem