Comment by mikkupikku
6 days ago
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.
> 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,
Why would they be "shoved deeper," when the problem is that the joint opens wider under load?
See here: https://www.researchgate.net/profile/Scott-Post/publication/...
What would happen "normally" (i.e. the normalization of deviance) was that the rotation (from the SRB joints bowing--essentially "ballooning") would create a gap, and the O-rings would get blown into that gap and ultimately seal in there
With Challenger, it was too cold, so the O-ring rubber was not malleable enough to seal into that space (like the O-ring towards the right of the diagram), so the hot gases were allowed to blow by and erode the O-ring. If they had sealed in (like the one on the left) it would have just taken the pressure but not worn away
> What would happen "normally" (i.e. the normalization of deviance) was that the rotation (from the SRB joints bowing--essentially "ballooning") would create a gap, and the O-rings would get blown into that gap and ultimately seal in there
But data from previous Shuttle flights showed that even that wasn't happening, at temperatures up to 75 F. And the Thiokol engineers had test stand data showing that it wasn't happening even at temperatures up to 100 F. In short, that joint design was unacceptably risky at any temperature.
It is probably true that the design was somewhat more unacceptably risky at 29 F. But that was a relatively minor point. The reason the cold temperature was focused on by the Thiokol engineers (who were overruled by their own managers in the end, as well as NASA managers) in the call the night before the launch was not that they had a good case for increased risk at cold temperature; it was that the cold temperature argument was the only thing they had to fight with--because NASA had already refused to listen to their much better arguments the previous summer for stopping all Shuttle flights until the joint design could be fixed.
Two directions.
Let's examine a slice of the booster. Going vertically you have one segment, then the joint, then the next segment. The O-rings were in that joint and had some ability to move horizontally.
As designed the joint would always be in compression, the O-rings sandwiched between two big pieces of metal. If they moved horizontally in the space they had it made no difference, their job was simply to keep the 1000psi inside the booster inside it. Going inward there was a layer of putty that could stand up to the heat but was useless for sealing.
Unfortunately, when the engines lit the whole booster stack twanged a few inches. A joint meant to always be in compression was suddenly for a moment in tension--the two pieces of metal moved slightly apart--gas could now go above/below the ring. If the rings were pliable enough they got slammed against the outside of their groove where the pressure against the joint stopped the escape of gas--examination of the boosters showed blow-by but it cut off soon enough that the mass of metal was enough to absorb enough heat to avoid catastrophe.
But that night was very cold. And it was very calm--the boil-off from the LOX tank was simply dumped overboard and the booster that failed was downwind. The point of maximum chilling was between the booster and the tank, the lowest segment joint got the worst of it. And that's where it failed.
When the stack twanged the ring didn't slam against the outside quite fast enough--some exhaust leaked past and tore up the ring. But the gas still had to go out the joint--and the shuttle fuel used aluminum. The ring wasn't sealing the joint but enough aluminum solidified out against the still-cold metal of the joint that it sealed the gap and Challenger roared into the sky. But as it went faster and faster the vibrations grew stronger--and eventually the really sloppy weld let go. Even that didn't doom the mission, there was enough fuel to tolerate the pressure loss. But the leak was pointing at a strut and the tank with a whole bunch of LH2 in it. Neither was designed to stand up to that.
There was also a second failure that got little attention: the putty. As intended, it should have covered the entire gap, the force would have been evenly applied and it probably would have made it. But the putty was spread and the segments placed together--in atmosphere. Air was trapped and compressed--and the putty gave way letting it out. What had been an even layer now had holes in wherever the weakest spots were--and that concentrated the escaping gas from the booster. And why wasn't that caught? Because in the static testing someone had gone inside and made sure the putty job was good. Easy enough in a booster laying on it's side, but the Shuttle was stacked vertically.