Somewhere on this site I read that in NASA, the rocket leaves the launchpad (through release of explosive bolts) only after the engines have attained full power.

Assuming in the scarce seconds between SRB ignition and launch a critical failure occurs, say, main engines switch off, could the shuttle just "wait it out" letting the SRBs burn out without leaving the launchpad? Or would that result in yet different disaster?

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    $\begingroup$ I suppose there's a good reason why the SSMEs were given time to get up to rated thrust before SRB ignition. See also RSLS abort. $\endgroup$ – a CVn Apr 4 '17 at 15:13
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    $\begingroup$ It wasn't possible in Spacecamp :) $\endgroup$ – Steve Apr 4 '17 at 15:33
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    $\begingroup$ For as much crap as that movie gets, I loved it when I was a kid. I think it probably had a not-insignificant influence on my eventual career path. $\endgroup$ – Tristan Apr 4 '17 at 16:49
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    $\begingroup$ @TaylorOstberg your comment greatly oversimplifies the situation. Depending on the time of failure, different aborts were possible. space.stackexchange.com/questions/10600/… $\endgroup$ – Organic Marble Apr 4 '17 at 17:19
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    $\begingroup$ "Booster Systems Engineer Jenny M. Howard acted quickly to inhibit any further automatic SSME shutdowns based on readings from the remaining sensors, preventing the potential shutdown of a second engine and a possible abort mode that may have resulted in the loss of the vehicle and crew." I'd argue that an abort mode that results in the loss of vehicle and crew kinda defeats the purpose. "Oh. no. The engine might blow up. Better turn it off so that we plummet to our deaths..." $\endgroup$ – Shane Apr 5 '17 at 16:50

No. Compare Space Shuttle Launch Countdown.

The SRB fire command and the hold-down fire commands are issued at the exact same time. The delay of about 6 seconds (note: each SSME is started at a different time, all 0.2 seconds apart) is to ensure that each of those engines are up to full throttle without errors prior to launch (which is called a Redundant Set Launch Sequencer abort). This has happened 5 times in the program history:

  • STS-41-D (at T-6 seconds due to a stuck LOX valve)
  • STS-51-F (at T-3 seconds, due to a coolant valve malfunction....also note the subsequent successful launch of this mission was the first and only Abort-to-Orbit of the Shuttle program)
  • STS-51 (at T-3 seconds, due to a faulty fuel flow sensor)
  • STS-55 (at T-3 seconds, due to incomplete ignition of #3 because of a LOX check valve leak)
  • STS-68 (at T-1.9 seconds, due to a LOX turbopump overtemp reading on #3)

Russell is correct in that the exhaust of the APCP in the SRB is too hot (it boils steel) for anything to survive it for more than a few seconds. In fact, the Ares 1-X launch damaged a "substantial" section of the fixed service structure on Pad 39B when its exhaust plume briefly torched the above-ground structure of the pad after liftoff (due to a pad avoidance maneuver that it had been programmed to execute). While it didn't matter much since NASA was planning on disassembling this structure anyhow, it's proof that a pad can little survive a continuous SRB burn.


I don't have an immediate citation handy, but the answer is no.

The SRBs were powerful enough to overcome the hold-down studs. That said, the same signal was used to blow the nuts on the hold-down studs and ignite the SRBs.

Once the SRBs light, the vehicle is going somewhere.

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    $\begingroup$ Weight of shuttle stack at liftoff: ~ 4.4 million lbs; Thrust of 2 SRBs: ~ 6.2 million lbs $\endgroup$ – Organic Marble Apr 4 '17 at 17:41
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    $\begingroup$ An astronaut said to me, only half-joking, that once the SRBs light, the question wasn't if you were going, but how much of Florida is going with you. $\endgroup$ – Adam Apr 4 '17 at 17:50
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    $\begingroup$ Not only was the trust-to-weight ratio of the Orbiter plus SRBs (minus thrust from SSMEs, but including the ET) about 1.27:1 in favor of going, the TWR was also climbing rapidly as the engines burned through their fuel. The SRBs last two minutes and change, massing a combined 1,142,000 kg each. Even when you consider their dry weight, that's a metric crapton (how I wish that was a real unit of mass) of fuel burned per second. I looked up the numbers for my answer to How far would the STS get without the SRBs. $\endgroup$ – a CVn Apr 4 '17 at 18:37
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    $\begingroup$ @MichaelKjörling the TWR would not mandatory climb, because SRBs thrust may vary during flight. You have some examples of thrust profiles in this answer $\endgroup$ – le_daim Apr 5 '17 at 7:13
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    $\begingroup$ From Wayne Hale: "A real fear – alleviated by a million software verification checks – was that somehow the solid rocket boosters would be ignited and something – an RSLS Abort command for example – would stop the launch sequencer in the last milliseconds. That would be a disaster. If the hold down posts and T=0 umbilical panels and the GUCP arm did not separate, or the liquid engines were commanded to shut down, the consequences would be immediate and devastating. So that software was tested over and over again with all the variations of inputs that could be devised." $\endgroup$ – Tristan Apr 7 '17 at 14:05

Could the shuttle wait out the whole burn of SRBs on launchpad in case of a problem on launch?


The question seems based on the assumtion that something (the hold-down bolts perhaps) could hold the stack down while the SRBs burn out, while being immersed in the 3300 degree Celsius exhaust for about 128 seconds. Note that during this time, while the hold-down bolts are melting, the stack is getting lighter due to the fuel burned and thus easier for the now-rogue SRBs to lift.

In fact, there were two incidents in which two of the hold-down bolts did not fire, and over twenty more in which a single bolt did not fire. The SRBs tore them apart with no effect on the mission, just as expected. However, the failure of four or more frangible nuts to fire may in fact exceed the space shuttle load-carrying capability, i.e. hold down the stack. Thank you to reirab for linking to the relevant PDF in the comments!

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    $\begingroup$ There are redundant pyrotechnics on the bolts that fire simultaneously for just this scenario. I did not hear of instances of the pyros not firing, but I do remember a couple cases of the fired bolts not staying in their capture devices, which posed a debris impact hazard to the orbiter on liftoff. I can't find a source at the moment though. $\endgroup$ – ereisch Apr 5 '17 at 13:36
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    $\begingroup$ As I understand it, it wasn't that the nut pyros didn't fire at all, it's that they didn't successfully separate the nut, resulting in what they called a "stud hangup". They later redesigned the frangible nuts to include a pyrotechnic bridge to improve reliability. $\endgroup$ – Tristan Apr 5 '17 at 14:41
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    $\begingroup$ The phrase "you seem to assume" is a bit rude. I suggest avoiding it unless the asker is so confused that the question makes no sense. $\endgroup$ – Tor Klingberg Apr 6 '17 at 16:08
  • $\begingroup$ @Tristan Yes, you're right. $\endgroup$ – reirab Apr 7 '17 at 16:40
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    $\begingroup$ @TorKlingberg: Thank you, I've reworded the part in question. $\endgroup$ – dotancohen Apr 9 '17 at 6:52

The existing answers are correct that the same signal that ignites the SRBs also blows the nuts that hold the restraining bolts in place, so the answer is generally, no, it's not possible.

If, for some crazy reason, the nuts didn't blow, but the SRBs did ignite, things are a little different.

According to this engineer on reddit, who apparently helped design the aforementioned frangible nuts, the nuts are made of Inconel-718. Each SRB is held down by 4 of these.

According to the same engineer, the nuts and studs are actually already preloaded with about 750,000 pounds of force each. About an eighth of the (thrust - weight) of the stack would then be added to this on each post if the SRBs ignited without blowing the nuts. The aforementioned engineer performed a back-of-the-envelope calculation showing that the nuts would have a failure load around 1.65 million pounds each. Since each bolt would only have in the ballpark of 1 million pounds of force on it (including the preloaded tension,) the bolts should be able to initially hold the stack down when the SRBs light.

However, there are a couple of remaining problems.

First, the weight of the stack is rapidly decreasing due to fuel burn, meaning that the force on the bolts is rapidly increasing.

Second, and probably more importantly, the exhaust of rocket motors is really hot. While I haven't seen an exact source for the SRB exhaust specifically, NASA lists the combustion temperature of the main engines at up to 3,300 C! However, Inconel-718 will melt around 1,300 C. So, while the nuts and studs should initially hold the stack down, they'll almost certainly start heating up very quickly from the SRB exhaust and fail soon afterwards.

While I'm not sure about Inconel-718 specifically, in general, metals lose a lot of their strength well before hitting their actual melting point, so it's not likely that the bolts would be able to hold the stack down for very long. Even if they withheld all of their strength prior to melting, given the amount of energy and temperatures involved, it's quite likely that the nuts would melt well before the SRB burnout and the stack would leave the stand. No guarantees about how or in what direction it leaves the stand, but it will leave it (provided the stack doesn't disintegrate first.)

Of course, it's also entirely possible that some other failure mode will destroy the stand and/or stack before the nuts fail. The stand is designed to withstand the energy of several seconds of the SSMEs burning and a very brief encounter with the exhaust of the SRBs, but it's not designed to be able to withstand and quickly dissipate the energy of the entire 2-minute-long SRB burn.

  • $\begingroup$ The SRB exhaust does not impinge on the bolts.' $\endgroup$ – Organic Marble Apr 7 '19 at 1:35
  • $\begingroup$ @OrganicMarble No, but it's close enough that I can't imagine it wouldn't be heating rather rapidly... Direct impingement isn't necessary for that. $\endgroup$ – reirab Apr 7 '19 at 1:53
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    $\begingroup$ The parts of your answer about the bolts melting are pure speculation. $\endgroup$ – Organic Marble Apr 7 '19 at 1:56
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    $\begingroup$ @OrganicMarble Being in close proximity to very large amounts of exhaust that is 2,000 C above their melting point for nearly 2 minutes, I tend to disagree. Of course, they'd likely fail before that, anyway. $\endgroup$ – reirab Apr 7 '19 at 2:00

The hold down bolts are not designed or meant to hold the shuttle stack down for the complete burn of the SRBs. In Mike Mullane's book "Riding Rockets" he says that the shuttle stack would rip itself apart if the hold down bolts didn't release. They have had trouble with some bolts not falling out of the post after the nut is blown. But even one bolt not releasing would cause the loss of the shuttle and most likely the crew, as that booster would be ripped apart and separated from the shuttle stack. The stack with one SRB would cartwheel destroying it.

There is fractions of a second between the bolts being blown and the ignition of the SRBs.

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    $\begingroup$ As other answers mention, there have actually been several incidents of a nut not firing. The force of the stack's thrust ripped it apart with no effect on the mission. The failure load of the nuts is well below the stack's (thrust - weight.) $\endgroup$ – reirab Apr 7 '19 at 1:50
  • $\begingroup$ @reirab There were no incidents of a "nut not firing". There were some incidents where one of the redundant pyro systems didn't fire. $\endgroup$ – Organic Marble Jan 12 at 19:34
  • $\begingroup$ @OrganicMarble Fair. I should have said that there have been several incidents of the nuts hanging up. They were ripped off by the launch force anyway. The NASA document linked in an above answer said that there were concerns that 4 or more hang-ups could cause a launch to fail. There were several incidents of a single nut hanging, 2 incidents of 2 hanging, and no incidents of more than 2. In all of the incidents where at least one hung, the launch force of the shuttle ripped it/them off anyway and the mission continued normally. $\endgroup$ – reirab Jan 13 at 8:15
  • $\begingroup$ @reirab It wasn't the nuts that were hanging up, it was the bolts, or "studs". The buzz phrase for this problem in the program was "stud hang-ups". If you want to read about how this system really worked and the actual problem with it, read my answer to this question: space.stackexchange.com/questions/34072/… $\endgroup$ – Organic Marble Jan 13 at 13:38

When the SRBs light nothing will stop them short of propellant exhaustion. The hold down bolts cannot hold the stack and with no main engines it won't reach any kind of altitude in two minutes of burn and have no directional control. This opens a whole range of ugly scenarios the engineers have had many sleepless nights over. This is a situation that simply cannot happen.

  • $\begingroup$ Hi Harley Foster! Welcome to Space Exploration SE! Can you quote some sources: i.e. for the fact that hold down bolts cannot hold the thrust of SRBs? Also, about directional control, I think SRBs had gimbals - couldn't they achieve directional control just with that? $\endgroup$ – BlueCoder Jan 10 at 13:58

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