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The ignite command for the Solid Rocket Boosters (SRB) would not be issued unless all three Space Shuttle Main Engines (SSME) were at least 90% of thrust. The launch would have been aborted if one or more of the SSMEs failed to reach required performance before T-minus zero. Abort was possible because the SSMEs could be shutdown after starting.

The SRBs, of course, couldn't be shutdown after ignition. Once the SRB ignition command was issued, abort was impossible (unless maybe both SRBs failed to ignite?).

If only one ignited, what would have happened?

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    $\begingroup$ Cartwheel mode. But not for long. It's a pretty unlikely failure mode though. $\endgroup$ – Organic Marble Oct 31 '15 at 17:44
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    $\begingroup$ Very very bad things. $\endgroup$ – geoffc Nov 1 '15 at 12:22
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    $\begingroup$ "You will not go to space today" $\endgroup$ – Keith Thompson Nov 2 '15 at 22:28
  • $\begingroup$ @KeithThompson: Or probably ever. $\endgroup$ – Sean Jun 9 '18 at 23:10
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Everything I can find says the crew would have died. Depending on which one lit, the space shuttle would have either ended up in the ocean, or smacking the Launch Control Center. Of course, before either of those events would happen, the Flight Termination System would have been activated, which would have caused all 3 of the rocket stages to explode.

Even if somehow the SRBs could be ejected when thrusting, the orbiter didn't have enough thrust to lift itself up without SRBs at the launch pad. As a result, the orbiter would have crashed in to the ground, which while it might have been at least slightly more likely to survive than if the FTS had been activated, it still would have almost certainly resulted in a very bad day.

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    $\begingroup$ No. The self destruct were all linked as well, so... Even if you could dump the core, the SME couldn't lift off from the ground, which would place you in a really bad spot, where the dumped SRB would essentially explode. All around a bad situation... $\endgroup$ – PearsonArtPhoto Oct 31 '15 at 16:24
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    $\begingroup$ Found a discussion of thrust termination for solid rocket boosters here: everything2.com/title/thrust+termination ... Hypothetically, thrust from a burning SRB could be terminated so as not to topple the vehicle, but it would appear that no such system was fitted to the Shuttle SRBs. One could imagine all sorts of bad things happening even if such a system was fitted and activated. Best approach (as appears to have been employed): do everything possible to ensure that both engines always light. $\endgroup$ – Anthony X Oct 31 '15 at 17:21
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    $\begingroup$ How about blowing the top end open to cancel upwards thrust? Tearing the SRB length-wise? Separating it, let it fly away and then use FTS on it? ... $\endgroup$ – TildalWave Oct 31 '15 at 17:47
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    $\begingroup$ The SRBs were simply made not to fail. If they did fail, bad stuff would happen... $\endgroup$ – PearsonArtPhoto Oct 31 '15 at 18:12
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    $\begingroup$ There were four giant bolts holding down each SRB, which kept the thing on the pad while the three main engines were firing. The eight bolts were all cut with pyros at the same time the SRBs were ignited. I was thinking that maybe you could not cut the bolts and wait out one SRB burning. However it turns out that the bolts were designed to shear if the pyro didn't fire, so that wouldn't work. Even with just one SRB firing, the thing goes no matter what. So the answer is indeed a very bad day. $\endgroup$ – Mark Adler Nov 1 '15 at 6:44

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