21

Slightly different Challenger question - what could have happened if the SRB breach had faced outboard, away from any attachment fittings or the ET? Would the SRB have failed completely prior to separation (leading to a similar outcome)? Would it have caused significant guidance issues? Would they have had more time to execute some emergency maneuver?

Would abort to orbit have been possible?

I realize this is impossible to answer definitively, but I find myself curious as to what could have happened.

DrSheldon
  • 47,946
  • 12
  • 162
  • 329
John Bode
  • 2,300
  • 12
  • 13

2 Answers2

33

Do we get to pick our assumptions? I don't see how to answer this otherwise.

I'll assume the hole kept growing at a "slow rate" and the booster didn't come apart. (If the booster came apart, the result would not be much different from what actually happened). (Note many people were suprised back then that the booster didn't come apart with that size of hole in it.)

This plot shows the SRB chamber pressures starting to diverge which would cause the thrust mismatch to rise. (The certification limit for steady state flight thrust mismatch was only 85 klbf) You can pick your extrapolation of the curve here. I'm choosing to believe that it would continue to drop as the leak got bigger.

enter image description here

Plot from Rogers Commission Report

Given that, I would guess that vent force + thrust mismatch would overcome the vehicle control limits pretty quickly (gimbals hitting the hard stops) and it would have started to rotate uncontrollably with a result not much different from what actually happened.

Organic Marble
  • 181,413
  • 9
  • 626
  • 815
  • 3
    Yes, feel free to make any assumptions necessary, I'm asking from a position of almost pure ignorance. – John Bode May 22 '20 at 00:16
  • Interesting — is that plot saying there was a stable pressure differential of a bit over 1% for a while before things really started to diverge? Was that common on other launches? If thrust vs pressure is near linear, that would be around 30klb of the 85klb limit? – Russell Borogove May 22 '20 at 02:50
  • 2
    @RussellBorogove the tables on (document page) 621 here makes it look like 1% is in spec https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19850008628.pdf I will look for some examples from other flights but it will be a while, I have to cut holes in my walls today :) I don't know much about SRM calculations but thrust proportional to PC sounds reasonable to me. – Organic Marble May 22 '20 at 11:12
  • 1
    I’m having a little trouble reading the X axis on that graph - that’s a span of about 30 seconds, right, with the more severe divergence occurring over 15 seconds? – John Bode May 22 '20 at 12:00
  • @JohnBode Yes, the start and stop times at the top of the graph agree that it covers 30 seconds. – Organic Marble May 22 '20 at 12:58
  • It might work out that the extended 'warning time' as pressure readings went bad as well as trajectory errors would have been just enough for all abort/eject systems to activate. – Carl Witthoft May 22 '20 at 13:23
  • 7
    @CarlWitthoft there were no aborts available in first stage and the ejection seats were not installed in Challenger – Organic Marble May 22 '20 at 13:30
  • 1
    @OrganicMarble Thanks for the additional info – Carl Witthoft May 22 '20 at 13:53
  • @CarlWitthoft: SRB Jett has to be triggered manually, and carries a high chance of LOV if attempted on a healthy vehicle. With the gimbals pivoted to the stops, that risk goes way up. – Joshua Sep 03 '20 at 04:21
  • @Joshua SRB Sep was automatic, not 'triggered manually'. See https://space.stackexchange.com/q/21949/6944 – Organic Marble Sep 03 '20 at 12:28
  • @OrganicMarble: https://space.stackexchange.com/questions/21972/was-a-method-available-to-save-the-challenger-crew The analysis has gone both ways whether throwing that switch with the SRBs still burning would even jettison at all. – Joshua Sep 03 '20 at 14:44
  • 1
    @Joshua Russell Borogove's answer is correct, at least for the shuttle in its final days. There was no man sep of the SRBs then. The button might have done something in 1986 -maybe- but it would have been 'committing suicide to keep from getting killed` – Organic Marble Sep 03 '20 at 15:43
5

The correct response to this question, is that it could not happen.

The only location in which a Challenger type leak could occur is within 10 degrees of the ET attachment. The interaction between the ET attachment and the SRB joint dynamics played a role in the 51L leak. The failure mode requires the locking effect of the ET attachment on the SRB wall.

In Appendix C (https://history.nasa.gov/rogersrep/v1appc.htm) a failure mode is setout which ends with the statement:

"If the very tight tang-to-clevis assembly gap did persist to time of launch, it could have resulted in near maximum compression of the O-rings. Such compression, in conjunction with cold temperatures, joint dynamics, and the variable performance of the insulating putty has been shown to have detrimental influences on the joint's ability to seal."

It was 52 days between the stacking error of 12/7/86 and launch. In that time frame the joint would have been subject to movement to the pad, heating and cooling in the Florida sun and the natural tendency of a "tight" joint to move to it's lowest energy state. A flat on flat stack, if missed by the stacking team, would not "persist to time of launch" on any location on the joint where the clevis could move outward and change dimensions. Only if the flat on flat occurred very near the ET attach point, where the clevis would need to move the mass of the external tank to reduce the tight joint could a flat on flat persist to launch. Therefore, a flat on flat stack, which was a necessary part of the 51L failure mode, could only occur within 10 degrees of the ET attachment.

Like most accidents, the failure mode of STS-51L is not simply cold temperature (which was a constant across all O-ring locations) but a unique combination of factors which occur only once and at a single location and are wholly unanticipated.

Challenger Truth
  • 767
  • 8
  • 19