3 ms·

This is the best explanation I've seen yet. The summary is that you want a buffer of air between the hot gasses of the running motor and the o-ring seal such t

This is the best explanation I've seen yet. The summary is that you want a buffer of air between the hot gasses of the running motor and the o-ring seal such tat the air gets compressed but remains between the hot gasses and the o-ring thus insulating the o-ring from the hot gasses. But to avoid a pressurization test NASA went with a two-o-ring scheme where the space between them is pressurized, which forces the inner o-ring to be on the wrong side of where it should be, thus leaving little or no air buffer between that o-ring and the hot gasses. That in turn can cause point failures in the inner o-ring which will result in concentrated jets of hot gasses impinging on the outer o-ring which then cannot hold (because the pressure isn't uniform across the o-ring's circumference, instead it's concentrated on a point). Add the cold o-ring brittleness and boom. Is SLS still using this o-ring design?? I sure hope not.

KennyBlanken 2y ago
It was revised after the Challenger disaster. It's not the "best explanation yet" - it's outright historical revisionism that directly contravenes reams of documentation, test results, and testimony as part of the Rogers Commission. The author repeatedly claims NASA created the design of the inter-section seal, that they were incompetent in solid rocket motor design, insisted on the design, etc because Those Silly Government Workers blah blah. In reality, NASA engineers saw Thiokol's initial bore/face seal design (which was also a double seal...) and objected to it: "The initial Thiokol design proposal was changed before the production motors were manufactured. Originally, the joint seal design incorporated both a face seal and a bore seal.16 (Figure 1.) However, the motor that was eventually used had double bore O-rings. The original bore seal/face seal design was chosen because it was anticipated that it "provides [better] redundance over a double bore ring seal since each is controlled by different manufacturing tolerances, and each responds differently during joint assembly. " 17 Because the early design incorporated tolerances similar to the Titan and it also incorporated a face seal, Thiokol believed it possessed "complete, redundant seal capability." > Nevertheless, as the Solid Rocket Motor program progressed, Thiokol-with NASA's concurrence-dropped the face/bore seal design for one using a double bore seal (Figure 1). NASA engineers at Marshall said the original design would have required tapered pins to maintain necessary tolerances and assure enough"squeeze" on the face-sealing O-ring.19 However, design analysis determined that motor ignition would create tension loads on the joint sufficient to cause the tapered pins to pop out. Solving that would have meant designing some type of pin-retainers. Moreover, the rocket assembly was much easier with the dual bore seals. Because inspections and tests had to be conducted on the Solid Rocket Motor stack, horizontal assembly was required. The seal design was still not adequate and NASA engineers objected more. Testing showed NASA engineer's concerns to be valid. During both static pressure testing and actual test-firings there was documented leakage, and Thiokol's response was to revise what was considered an acceptable amount of leaking. Numerous NASA engineering staff, and even the o-ring suppliers, all objected. Thiokol repeatedly ignored those objections, as did NASA management. You can read it right here: https://www.nasa.gov/history/rogersrep/v1ch6.htm https://www.nasa.gov/history/rogersrep/v1ch6.htm The entire premise of the blog post is contravened by early tests. Whereas the author claims that the seal was doomed to fail because the o-ring would leak until it was forced upward to where it would properly seal, the actual problem was: > Although the test was successful in that it demonstrated the case met strength requirements, test measurements showed that, contrary to design expectations, the joint [123] tang and inside clevis bent away from each other instead of toward each other and by doing so reduced-instead of increased-pressure on the 0-ring in the milliseconds after ignition.26 This phenomenon was called "joint rotation." Testifying before the Commission, Arnold Thompson, Thiokol's supervisor of structures, said, > "We discovered that the joint was opening rather than closing as our original analysis had indicated, and in fact it was quite a bit. I think it was up to 52 onethousandths of an inch at that time, to the primary O-ring."27 > Thiokol reported these initial test findings to the NASA program office at Marshall. Thiokol engineers did not believe the test results really proved that "joint rotation" would cause significant problems,28 and scheduled no additional tests for the specific purpose of confirming or disproving the joint gap behavior.
- csours 2y ago
  It seems like their main complaint is with NASA management - though it did sound like it bled over onto engineering as well. That feels like a very natural human response - to paint with a wider brush than is needed.

KennyBlanken 2y ago

It was revised after the Challenger disaster. It's not the "best explanation yet" - it's outright historical revisionism that directly contravenes reams of documentation, test results, and testimony as part of the Rogers Commission. The author repeatedly claims NASA created the design of the inter-section seal, that they were incompetent in solid rocket motor design, insisted on the design, etc because Those Silly Government Workers blah blah. In reality, NASA engineers saw Thiokol's initial bore/face seal design (which was also a double seal...) and objected to it: "The initial Thiokol design proposal was changed before the production motors were manufactured. Originally, the joint seal design incorporated both a face seal and a bore seal.16 (Figure 1.) However, the motor that was eventually used had double bore O-rings. The original bore seal/face seal design was chosen because it was anticipated that it "provides [better] redundance over a double bore ring seal since each is controlled by different manufacturing tolerances, and each responds differently during joint assembly. " 17 Because the early design incorporated tolerances similar to the Titan and it also incorporated a face seal, Thiokol believed it possessed "complete, redundant seal capability." > Nevertheless, as the Solid Rocket Motor program progressed, Thiokol-with NASA's concurrence-dropped the face/bore seal design for one using a double bore seal (Figure 1). NASA engineers at Marshall said the original design would have required tapered pins to maintain necessary tolerances and assure enough"squeeze" on the face-sealing O-ring.19 However, design analysis determined that motor ignition would create tension loads on the joint sufficient to cause the tapered pins to pop out. Solving that would have meant designing some type of pin-retainers. Moreover, the rocket assembly was much easier with the dual bore seals. Because inspections and tests had to be conducted on the Solid Rocket Motor stack, horizontal assembly was required. The seal design was still not adequate and NASA engineers objected more. Testing showed NASA engineer's concerns to be valid. During both static pressure testing and actual test-firings there was documented leakage, and Thiokol's response was to revise what was considered an acceptable amount of leaking. Numerous NASA engineering staff, and even the o-ring suppliers, all objected. Thiokol repeatedly ignored those objections, as did NASA management. You can read it right here: https://www.nasa.gov/history/rogersrep/v1ch6.htm https://www.nasa.gov/history/rogersrep/v1ch6.htm The entire premise of the blog post is contravened by early tests. Whereas the author claims that the seal was doomed to fail because the o-ring would leak until it was forced upward to where it would properly seal, the actual problem was: > Although the test was successful in that it demonstrated the case met strength requirements, test measurements showed that, contrary to design expectations, the joint [123] tang and inside clevis bent away from each other instead of toward each other and by doing so reduced-instead of increased-pressure on the 0-ring in the milliseconds after ignition.26 This phenomenon was called "joint rotation." Testifying before the Commission, Arnold Thompson, Thiokol's supervisor of structures, said, > "We discovered that the joint was opening rather than closing as our original analysis had indicated, and in fact it was quite a bit. I think it was up to 52 onethousandths of an inch at that time, to the primary O-ring."27 > Thiokol reported these initial test findings to the NASA program office at Marshall. Thiokol engineers did not believe the test results really proved that "joint rotation" would cause significant problems,28 and scheduled no additional tests for the specific purpose of confirming or disproving the joint gap behavior.

csours 2y ago
It seems like their main complaint is with NASA management - though it did sound like it bled over onto engineering as well. That feels like a very natural human response - to paint with a wider brush than is needed.