WI: STS-51-L is delayed

Yep, that inspection arrangement might have been able to prove the Thiokol engineer on site was correct to call for a scrub, but every inspectable point looking good would not have been able to prove he was incorrect. It would be worthwhile to order the inspection in case it delivered proof positive the launch was compromised and teardown was mandatory, but having it pass with flying colors still does not prove the launch is therefore safe in the face of countervailing evidence such as a temperature record. Ordering inspection and launching if it passes would have been a compromise with real risk factors.

Why would it not be feasible to withdraw the assembly into the VAB (having drained any LH, LOX, and Orbiter hypergolic propellant first of course), remove only the SRBs, set the fully assembled compromised ones aside, assemble a fresh new pair from untouched segments and attach them to the tank, without detaching Orbiter from Tank or messing with the integrated payload or otherwise setting the readiness of the non-SRB elements back any? If this could have been done would not an abbreviated countdown to the point at which the assembly was scrubbed before be acceptable, to verify stuff liable to come "unstuck" over time but otherwise quickly verify that stuff expected to stay put has done so?

To be sure this is a quibble--at the end of the day a full integration count might not take much more time and labor than trying to design an ad hoc shortcut one.

Anyhow the old stack of SRB elements can be tackled after the launch, with the elements being candidates for reuse.
 
As far as I know, didn't the SRBs already have a partial burn through of the O-Ring on a number of Columbia and Challenger missions, but the fail safe mechanism in that part kept sealing in a way that kept the fuel from igniting?

The pre-Challenger SSRMs (Shuttle Solid Rocket Motors) had two o-rings that sealed the segments together. There were known instances of erosion on the o-rings on several flights including STS-2, 11/41B, 13.41C, 14/41D, 20/51C and 30/61A. Actual blow-by of hot gasses past the primary happened on 20/51C and 20/61A. In both instances the secondary o-ring sealed, but suffered from some damage.
 
Why would it not be feasible to withdraw the assembly into the VAB (having drained any LH, LOX, and Orbiter hypergolic propellant first of course), remove only the SRBs, set the fully assembled compromised ones aside, assemble a fresh new pair from untouched segments and attach them to the tank, without detaching Orbiter from Tank or messing with the integrated payload or otherwise setting the readiness of the non-SRB elements back any? If this could have been done would not an abbreviated countdown to the point at which the assembly was scrubbed before be acceptable, to verify stuff liable to come "unstuck" over time but otherwise quickly verify that stuff expected to stay put has done so?

This would not work because the SRBs are the only thing holding the stack up. The SRBs are placed on the MLP in a series of eight lifts (four segments for each SRB), and then the ET is hung between them (the SRBs transfer thrust through their top into a large beam inside the ET intertank), and the shuttle is hung off of the ET with no load being taken up by the T-Zero umbilical plates.
 
The pre-Challenger SSRMs (Shuttle Solid Rocket Motors) had two o-rings that sealed the segments together. There were known instances of erosion on the o-rings on several flights including STS-2, 11/41B, 13.41C, 14/41D, 20/51C and 30/61A. Actual blow-by of hot gasses past the primary happened on 20/51C and 20/61A. In both instances the secondary o-ring sealed, but suffered from some damage.
Wait, really? I thought it was one before, and the two ring design you describe was the alternative rejected as too heavy and labor-intensive and all round expensive, until it was instated by the Challenger Board afterward.

So are you saying that the original design was two ring and the post-Challenger resurrection of the older more failsafe design rejected for economy was a three ring design, or what?

Because from what I understood, actual jets of core gases, albeit feeble ones, were getting past the single ring of the old system, and the second ring as you describe checked such leaks completely, at the cost of being disposable. That would be quite good enough provided nothing got past the second ring.

Were the O-rings as well as the metal segments viewed as something that needed to be reused, or were they not regarded as cheap enough to just use once and then discard?
 
Why would it not be feasible to withdraw the assembly into the VAB (having drained any LH, LOX, and Orbiter hypergolic propellant first of course), remove only the SRBs, set the fully assembled compromised ones aside, assemble a fresh new pair from untouched segments and attach them to the tank, without detaching Orbiter from Tank or messing with the integrated payload or otherwise setting the readiness of the non-SRB elements back any? If this could have been done would not an abbreviated countdown to the point at which the assembly was scrubbed before be acceptable, to verify stuff liable to come "unstuck" over time but otherwise quickly verify that stuff expected to stay put has done so?
This would not work because the SRBs are the only thing holding the stack up. The SRBs are placed on the MLP in a series of eight lifts (four segments for each SRB), and then the ET is hung between them (the SRBs transfer thrust through their top into a large beam inside the ET intertank), and the shuttle is hung off of the ET with no load being taken up by the T-Zero umbilical plates.

As @TimothyC says, the SRBs were the structural backbone of the stack during mating and rollout. You can see here how there's nothing holding the ET up other than the SRBs (and then nothing holds the Shuttle up other than the ET).
050228_05pd0348.jpg
 
OK, three rings was the final design, I will strive to remember that!

I would think it would be possible to deploy a frame inside the VAB to support the attached Orbiter and brace the tank, remove the SRBs and put on another set, and hustle that out while disassembling the two suspect SRBs at leisure--but it would not be necessary to have such a frame during normal assembly so it would be an extra design and maintenance cost to build it and store it against a contingency that is not supposed to happen--"the Shuttle has reliability comparable to a jetliner! Anyone suggesting otherwise can JUST SHUT UP! It's our DC-3 to space! Don't diss the Gooney Bird, look what it did during the Berlin Airlift!" I suppose no scrubbed launch ever did involve disassembly of an SRB, so the frame would be useless.

Also the tank is not designed with hard points to attach an auxiliary frame to I suppose. If NASA faced the contingency in planning at all they planned to eat the cost of reversed assembly steps I suppose. Not to mention I would guess you could show why keeping the Orbiter on ice would not work either; I guess it would have to have payload bay emptied and other stuff (oxygen-hydrogen tanks for the fuel cells for instance) stripped out before removing the Orbiter from the tank. They run the whole process in reverse, in that rare contingency that never actually happened in over 100 launches OTL, and just one where they should have.

Success Oriented Management for the Win!
 
OK, three rings was the final design, I will strive to remember that!

It's actually a bit more complicated:

Solid_Motor_Designs.jpg


I would think it would be possible to deploy a frame inside the VAB to support the attached Orbiter and brace the tank, remove the SRBs and put on another set, and hustle that out while disassembling the two suspect SRBs at leisure--but it would not be necessary to have such a frame during normal assembly so it would be an extra design and maintenance cost to build it and store it against a contingency that is not supposed to happen--"the Shuttle has reliability comparable to a jetliner! Anyone suggesting otherwise can JUST SHUT UP! It's our DC-3 to space! Don't diss the Gooney Bird, look what it did during the Berlin Airlift!" I suppose no scrubbed launch ever did involve disassembly of an SRB, so the frame would be useless.

Shevek, prior to STS-33/51L, the SSRMs were considered the most reliable part of the stack. After all, they were large segmented solids, and those had been flying for the last twenty years on Titans! All of the focus was on the SSMEs, the APUs, and the Fuel Cells (IE the places where lots of energy was happening near liquids). To make it even more complicated, the process of stacking the SSRMs limited what else could be done inside the VAB, as they were big tubes of pre-mixed fuel and oxidizer that were exposed to the air prior to the forward segment being stacked. Furthermore, you have crane limit issues - the crane in the VAB can only lift so much, and can only lift loads that are so offset (you'd have to lift from the orbiter, which means you'd have a big 30 metric ton weight hanging off of the side. Even worse, all you are doing is moving from 20 lifts (removing the OV, the ET, and all 8 SSRM segments, and stacking up 8 new segments, followed by the ET and then the OV) with 18 (ET-OV move, 8 SSRM destacks, 8 SSRM stacks, followed by the ET-OV), and as noted above, the stacks you've consolidated are the ones that don't shut down the VAB for other work.
 
OK, three rings was the final design, I will strive to remember that!
To help elaborate on the full evolution from the snippet from Jenkins (the true holy text for Shuttle details) helpfully provided by @TimothyC:
Solid_Motor_Designs.jpg

  • The original design had two O-rings, one on the sidewall and one on an axial flange (leftmost). These wouldn't have been subjected to the same pressures and the flange would have helped seal from the force of the motor firing. Thus, it would have been truly redundant.
  • The "productionized" SSRM as flown until Challenger changed to an easier-to-build design with two O-rings on the sidewall, one after the other (second from left). These were now subject to the same loading, so issues that could weaken one could weaken both, but they were easier to build and to position correctly for sealing axially (you can see if the pin was positioned a little low or high, the O-rings in their grooves on the wall would still seal against a different part of the other wall, as opposed to if the O-ring was on a sealing flange as in the original design. Thus axial position was less critical to a correct seal than in the original design--enhancing ease of manufacturing and assembly. In flight, @TimothyC has already transcribed how it worked out--several issues were seen with damage to the first O-ring or even burn-through of the first and damage to the second prior to 51L.
  • The middle design was part of the filament-wound composite core designs, which introduced a "tang" to wrap over the joint and force gasses to turn a corner before reaching the O-rings. This was before 51L, and paired with the original design's fully redundant design is why it's said that NASA knew the joint had problems.
  • The initial redesigned joint was to just adopt the FWC version, but eventually they added a third O-ring before the tang on the other side of the sidewall, among other modifications including heaters for the joints.
 
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