From non engineering point of view I'd have so many problems with it that I'd call o ring design stupidity.

  1. Why use rubber? The gases are under high pressure and extremely hot! it'll always burn through rubber. Even a child knows that. I know crash happened because rubber didn't expand. But even if it did Why does anyone expect Rubber to survive such a temperature anyway?

  2. Why use o rings to seal the joint rather than something more robust? Like seal it with silica or something? Hell it's a billion dollar rocket shuttle combo not your pressure cooker to use a $2 rubber gasket. Did nasa went so cheap?


closed as off-topic by Emilio Pisanty, sammy gerbil, StephenG, tfb, Chris Feb 9 '18 at 3:39

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    $\begingroup$ They knew this, they studied this extensively, and they decided that it was an acceptable flight risk. Obviously, they were wrong, but it wasn't easy at the time to see that they were being misled by contractors. Plenty of missions had been flown before with only minimal O-ring damage (so the gases definitively don't always burn through the rubber). The joints made at the factory were sealed with silica. It was only the joints made in the field that were sealed with rubber O-rings, most likely due to unavailability of equipment. $\endgroup$ – probably_someone Feb 8 '18 at 21:40
  • $\begingroup$ @probably_someone really? A rubber ring just barely surviving being burned by gas at thousands of degrees is an acceptable risk? What kind of idiots are there at nasa? Even a 3rd grader should know not to use something that burns to stop fire $\endgroup$ – user1062760 Feb 8 '18 at 21:53
  • $\begingroup$ The fire wasn't touching the O-rings, and there were two or three layers of failsafes preventing it from doing so. They're not idiots. They were just misled about the risk of all of their failsafes failing at once. $\endgroup$ – probably_someone Feb 8 '18 at 21:54
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    $\begingroup$ Read it and tell us the answer: spaceflight.nasa.gov/outreach/SignificantIncidents/assets/… $\endgroup$ – JEB Feb 9 '18 at 0:21
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    $\begingroup$ -1 This is a rant rather than a question. And it is a rant about Engineering not Physics. $\endgroup$ – sammy gerbil Feb 9 '18 at 1:17

The o-rings were not rubber, they were silicone, which has much higher temperature resistance than rubber. There was not a single o-ring, there were two, so if one burned through the second one was supposed to hold. They were sealed upon assembly in the field with mineral paste and the joint between the two booster sections was secured with clevis pins.

The joint failed because at cold temperatures, the silicone o-ring material lost its resilience and took a set. Then, when the the booster was lit and is casing expanded, the o-rings retained their cold-compressed shapes and allowed hot gas from inside the casing to spurt out the resulting gap in the seal. The engineers who specified the o-ring material had no idea that launches in Florida could occur in sub-freezing temperatures and so did not test the joint for proper performance at those temperatures.

When they did learn that that particular launch WOULD occur at those temperatures, they did get concerned about possible o-ring joint failure and warned management, but management overruled them.

Why use a field-assembled joint in the first place? Because the Morton Thiokol factory was in Utah, the launch pad in Florida, and the boosters had to be transported by rail, which set a limit on the maximum length of a railcar to guarantee it could negotiate the turns in the track and fit through the tunnels on the way. So the booster had to be shipped in sections and assembled to full length in the field.

The known problem of partial o-ring burn-through- present on almost every previous shuttle launch- was ignored by management because 1) they believed that each successful shuttle launch could be interpreted as a halving of the estimated probability of failure for each subsequent future launch and 2) the data on burn-through was never plotted as a function of ambient temperature at launch until after the accident, at which time the design flaw became glaringly obvious.

  • $\begingroup$ So what happened to mineral paste? The o rings alike be the last failsafe if the paste failed? If there was a ring burn on every launch then that means the sealing paste failed at every launch? $\endgroup$ – user1062760 Feb 9 '18 at 8:50
  • $\begingroup$ read the report linked above for answers to these questions. $\endgroup$ – niels nielsen Feb 9 '18 at 18:38

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