The shuttle had a switch on panel C3 which allowed the crew to trigger a manual separation of the SRB. This switch had to have a purpose when moved from Auto to manual mode. What would have happened if the manual separation sequence would have been triggered at 70 seconds into the flight with the SRB at full thrust. Keep in mind that on the 51L flight both SRBs separated from the stack even without manual separation being triggered. Would a manual separation at full thrust have any likelihood of success?

  • $\begingroup$ I'm not sure what 51L has to do with the question; "both SRBs separated from the stack" is an extremely strange way of describing what happened on that flight. $\endgroup$ Jun 14, 2017 at 4:42
  • $\begingroup$ Perhaps strange because of the catastrophic nature of the failure. The LH SRB sheared the ET attachment points and flew relatively stable until RSO destroyed it. The RH SRB leak destroyed the aft ET attachment and rotated into the stack on the FWD ET attachment before shearing it. After doing a loop it then proceeded relatively stable until RSO destruction. In fact the RSO triggered the destruct code only because the stable flight was unexpected. Had the SRB's been tumbling he would not have issued the destruct code. The question is really in the title. $\endgroup$ Jun 14, 2017 at 4:54
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    $\begingroup$ "Had the SRB's been tumbling [the RSO] would not have issued the destruct" I find that statement to be doubtful at best. A large rocket engine tumbling (uncontrollably) with no way of shutting down thrust is not what you want anywhere, let alone within seconds of flight to densely inhabited areas. Once the Orbiter had broken up, keeping the SRBs burning with no means of significant attitude control (only the separation thrusters) makes no sense; it's better to terminate thrust, and in the case of the SRBs that implies activating the self-destruct mechanism. $\endgroup$
    – user
    Jun 14, 2017 at 12:02
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    $\begingroup$ That was my initial assumption with regard to the function of this switch. After talking to NASA people I found that although this was the primary purpose, it could be triggered at any point in the flight. I was then told that they had in 1985 simulated twice the use of this switch during the SRB "boost" phase when a SRB pressure imbalance was detected. The source indicated that the belief in 1985 was that their was a 75% likelihood of separation without loss of vehicle. That number seems high to me, hence I am floating the question to this audience. $\endgroup$ Jun 14, 2017 at 21:42
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    $\begingroup$ With regard to Michael's comment, I quote the RSO Maj. Gerald F. Bieringer, USAF. "The IP displays PRI and ALT indications were jumping around wildly I was about to recommend we do nothing....when I observed what appeared to be an SRB [Solid Rocket Booster] stabilized and flying toward the upper left corner of the display. As it appeared stabilized I felt it might endanger land or shipping and as the ET [External Tank] had apparently exploded I recommended to the SRSO [senior range safety officer] we send functions. I sent ARM, waited about 10 seconds, and sent FIRE.'' $\endgroup$ Jun 14, 2017 at 22:22

2 Answers 2



The reason that the SRBs couldn't be "jettisoned" at any desired time is because the separation system as designed was not powerful enough to safely jettison the boosters if they were delivering a significant amount of thrust. The separation system simply severed the bolts holding the boosters to the External Tank and fired booster-mounted motors to provide clearance between them and the accelerating Orbiter/ET stack. If significant thrust loads had been present, then significant forces and moments would have been applied to the Orbiter/ET stack. It is possible that a more robust system could have been designed, but it wasn't incorporated in the system we had.

From my answer to this question (which see for references, etc).

Edit: the final report of the Rogers Commission on the 51-L accident explicitly states that the SRBs cannot be safely separated while producing thrust: (italics mine)


  1. The Space Shuttle System was not designed to survive a failure of the Solid Rocket Boosters.

There are no corrective actions that can be taken if the boosters do not operate properly after ignition, i.e., there is no ability to separate an Orbiter safely from thrusting boosters and no ability for the crew to escape the vehicle during first-stage ascent.

Reference: Rogers Commission Report, page 187.

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    $\begingroup$ Agreed that the system wasn't designed for early separation. I believe based on a conversation with a SIMSUP that they simulated a problem with a thrust difference of greater than 100psi between the boosters. The response to that simulated issue was to execute a manual separation. Based upon that information I am wondering what the sequence of events would likely be if a early separation had been done. $\endgroup$ Jun 14, 2017 at 13:40
  • $\begingroup$ I know that early in the program that there was discussion of manual seps, even including a "fast sep" where you jettisoned the ET along with the firing SRBs (!). Later in the program cooler heads prevailed / more analysis was done and the consensus was that these cases weren't survivable. Exactly what would happen would depend on the details of the situation but large forces and/or moments would have been applied to the stack, probably more than the flight control system could handle. $\endgroup$ Jun 14, 2017 at 13:43
  • $\begingroup$ By the way 100 psi is an enormous difference. I don't think the SRB chamber pressure ever topped 1000 psi, so you are talking about 10%. The flight control system might not have been able to handle that either. $\endgroup$ Jun 14, 2017 at 13:48
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    $\begingroup$ Not to get into chatroom debate but that is why I choose the 70 second moment. The ET was still intact. If a manual separation had been triggered at that moment on 51L, would the SRB's been able to clear the stack? ET sep was not a viable abort option but was SRB fast sep possible? $\endgroup$ Jun 14, 2017 at 14:13
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    $\begingroup$ I don't have ready references, but even successfully firing the separation system would not allow the SRBs to cleanly separate if they were still producing meaningful thrust. The forward attach point was a ball-and-socket joint with a pyro bolt inside. The thrust of the SRB alone would be enough to keep that joint engaged. Firing the pyro bolts would sever the aft attachment, allowing the SRBs to freewheel around the forward attach point, at which point you would have pretty much exactly what happened with the right SRB during the Challenger accident. $\endgroup$
    – Tristan
    Jun 16, 2017 at 15:06

The SRB separation sequence is designed to be used after thrust drops off enough that the SRBs will fall behind the orbiter. The separation rockets push the SRB nose out. If you separated the SRB while at full thrust, you'd expose the orbiter and ET to the exhaust of the SRB. At hundreds of tons of thrust, that's a lot of force.

The aft attachment points consist of 3 cylinders with "pistons", all hinged in a plane perpendicular to the thrust vector. At full thrust, the pistons would be wedged into the cylinders with a large force. This force may be enough to prevent separation.

I haven't found a NASA study on early separation. The standard abort modes have the SRBs attached to the vehicle until burnout.

If an event requiring an abort happened after SRB ignition, it was not possible to begin the abort until after SRB burnout and separation about two minutes after launch.

  • $\begingroup$ Does the inability to jettison the SRBs pre-burnout strike anyone else as a dumb design decision? $\endgroup$
    – Vikki
    Jun 13, 2018 at 19:45
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    $\begingroup$ Ask yourself what happens when you jettison the SRBs while they're running at full power. $\endgroup$
    – Hobbes
    Jun 14, 2018 at 7:13
  • $\begingroup$ Which would not have been a problem if the SRBs were independently controllable, allowing them to be safely guided away from the orbiter and ET. $\endgroup$
    – Vikki
    Jun 14, 2018 at 13:24
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    $\begingroup$ The SRBs have vastly more thrust than the remaining stack, so if you release them they will accelerate ahead of the stack and expose the ET to the full effect of the SRB exhaust. A RCS with enough power to prevent that would need to be very powerful, which makes it prohibitively heavy. Rough estimate: you need as much acceleration sideways as forwards, so you'd need 600 tons of thrust for ~10 seconds. $\endgroup$
    – Hobbes
    Jun 14, 2018 at 22:30
  • $\begingroup$ That 2nd reference is a nice find. $\endgroup$ Jun 14, 2018 at 22:58

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