3 ms·

I hope people are aware that the O-rings are the last line of defense. Blaming the O-rings misses the fact that the entire joint design and the testing methodol

I hope people are aware that the O-rings are the last line of defense. Blaming the O-rings misses the fact that the entire joint design and the testing methodology were the problem. You can see people in the theregister commentsection say that the solution is to add more O-rings. But in reality you don't want redundancy in a joint like this at all. You want the metal of the joint itself and the putty applied on the interior side of the joint to form the seal. The O-ring is the redundant part! The last line of defense! Adding more O-rings just adds more joint rotation, making it worse than having even just a single O-ring.

jordanb 2y ago
No the O-rings were essential to prevent blowby. If the seal the o-rings provided was lost, then exhaust gasses could exit the rocket not via the nozzle. This would cause a cascading failure where the exhaust gasses would erode the structure of the booster and make the hole bigger until eventual failure. In the case of Challenger, the booster structure didn't fail completely due to blowby because the fuel tank exploded first. What you're thinking of is the fact that they used multiple O-rings to prevent blowby, but in reality only one was needed. Blowby appearing on the primary O-rings was ignored since the secondaries prevented catastrophic failure on earlier flights, but any blowby erosion on any O-ring should have been recognized as a dangerous anomaly. O-rings were the best way to seal this joint because they had a high ability to resist temperature and pressure, while remaining flexible. There's no "putty" that could be applied that would have worked better.
buildsjets 2y ago
Not really. The 2-ring design was designed so the second ring was a "backup", but as you note, this only increases loads that act in the direction of aiding "Joint Rotation" and hot gas leakage. The third O-Ring was added on the opposite side of the joint from the original O-rings, and the tang was re-designed with an additional clevis socket so that pressure loads are balanced to prevent "Joint Rotation". A comparison of the designs can be seen at https://www.semanticscholar.org/paper/Axisymmetric-shell-analysis-of-the-Space-Shuttle-Nemeth-Anderson/56e940967229d37eaf1256962671e3ba1c5e9ea6/figure/0 https://www.semanticscholar.org/paper/Axisymmetric-shell-ana... . McDonald's book has FEA showing the predicted clevis deflections for both configurations, and the 3 O-ring design significantly reduces the "Joint Rotation." But still, since they were continuing to the same use Viton O-rings, a resistance heater and temperature monitoring was added to address low-temperature performance. (Edit - I think this was McDonald's source for the FEA diagrams in his book - https://web.archive.org/web/20210717050737/https://ntrs.nasa.gov/api/citations/19910005305/downloads/19910005305.pdf https://web.archive.org/web/20210717050737/https://ntrs.nasa...) I've never really liked the description of the failure mechanism as "Joint Rotation" anyway. It's only rotating when you look at it in a 2D cross section. In 3D I'd call it "tang outward deflection." It's increasing in diameter, not rotating. The problem, at it's root cause, was a failure of requirements management. On the night of the launch, no one was able to identify what temperature either the SRBs or the Orbiter had been designed and qualified to. Within the specification for the SRBs, there are several low temperature requirements that conflict with each other, and were misapplied both in design, in testing, and on the night of the launch. Literally no one could say whether it was safe to launch the shuttle that night, because the requirements were not clearly defined. NB- I've personally designed and specified numerous O-ring installations in both static and dynamic aerospace applications, and done all the required DO-160 environmental testing to demonstrate that they perform as required throughout the thermal envelope. I am literally at a supplier/vendor right now doing a salt fog exposure test per DO-160.

jordanb 2y ago

No the O-rings were essential to prevent blowby. If the seal the o-rings provided was lost, then exhaust gasses could exit the rocket not via the nozzle. This would cause a cascading failure where the exhaust gasses would erode the structure of the booster and make the hole bigger until eventual failure. In the case of Challenger, the booster structure didn't fail completely due to blowby because the fuel tank exploded first. What you're thinking of is the fact that they used multiple O-rings to prevent blowby, but in reality only one was needed. Blowby appearing on the primary O-rings was ignored since the secondaries prevented catastrophic failure on earlier flights, but any blowby erosion on any O-ring should have been recognized as a dangerous anomaly. O-rings were the best way to seal this joint because they had a high ability to resist temperature and pressure, while remaining flexible. There's no "putty" that could be applied that would have worked better.

buildsjets 2y ago

Not really. The 2-ring design was designed so the second ring was a "backup", but as you note, this only increases loads that act in the direction of aiding "Joint Rotation" and hot gas leakage. The third O-Ring was added on the opposite side of the joint from the original O-rings, and the tang was re-designed with an additional clevis socket so that pressure loads are balanced to prevent "Joint Rotation". A comparison of the designs can be seen at https://www.semanticscholar.org/paper/Axisymmetric-shell-analysis-of-the-Space-Shuttle-Nemeth-Anderson/56e940967229d37eaf1256962671e3ba1c5e9ea6/figure/0 https://www.semanticscholar.org/paper/Axisymmetric-shell-ana... . McDonald's book has FEA showing the predicted clevis deflections for both configurations, and the 3 O-ring design significantly reduces the "Joint Rotation." But still, since they were continuing to the same use Viton O-rings, a resistance heater and temperature monitoring was added to address low-temperature performance. (Edit - I think this was McDonald's source for the FEA diagrams in his book - https://web.archive.org/web/20210717050737/https://ntrs.nasa.gov/api/citations/19910005305/downloads/19910005305.pdf https://web.archive.org/web/20210717050737/https://ntrs.nasa...) I've never really liked the description of the failure mechanism as "Joint Rotation" anyway. It's only rotating when you look at it in a 2D cross section. In 3D I'd call it "tang outward deflection." It's increasing in diameter, not rotating. The problem, at it's root cause, was a failure of requirements management. On the night of the launch, no one was able to identify what temperature either the SRBs or the Orbiter had been designed and qualified to. Within the specification for the SRBs, there are several low temperature requirements that conflict with each other, and were misapplied both in design, in testing, and on the night of the launch. Literally no one could say whether it was safe to launch the shuttle that night, because the requirements were not clearly defined. NB- I've personally designed and specified numerous O-ring installations in both static and dynamic aerospace applications, and done all the required DO-160 environmental testing to demonstrate that they perform as required throughout the thermal envelope. I am literally at a supplier/vendor right now doing a salt fog exposure test per DO-160.