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Curious where the signal to detonate the pyro bolts and release the hold-down clamps comes from... The rocket about to lift off? Or ground control?

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  • $\begingroup$ Why would it have to be consistent between launch vehicles and operators? Some could do it one way, others another. $\endgroup$ Jun 7 at 2:31
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    $\begingroup$ @RossMillikan I wouldn't be surprised if there was an important reason that made it similar across platforms. For example, if rockets were often stolen, it would make sense for ground control to release the clamps, but if they were used for emergency evacuations, it would make sense for the vehicle to release them. Obviously neither example is the case, but rocket science is complex. $\endgroup$ Jun 7 at 15:16
  • $\begingroup$ Computers control everything - humans lack the precision to accurately launch a spacecraft by actuating anything manually. $\endgroup$
    – J...
    Jun 7 at 16:36
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For Apollo, the signal came from a small computer room built inside the mobile launch platform.

Giant holddown arms, whose name exactly describes their function, are positioned on the launcher surface to support and restrain the Saturn V. These arms hold the rocket during the first 8.9 seconds of ignition of its mighty engines while the computer beneath, communicat­ing directly with the computer in the Launch Control Center, verifies the performance of each of the 1,500,000 pounds thrust power plants of the first stage. When all five engines reach full thrust, and only then, the computers release the holddown arms which retract and allow the rocket to rise.

The Kennedy Space Center Story, p. 28

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  • $\begingroup$ Thank you for another insightful answer :) $\endgroup$
    – user39728
    Jun 7 at 3:07
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For shuttle it was commanded by the vehicle computers.

At T-31 seconds control of most remaining countdown events was handed over to the vehicle, including SRB ignition and blowing the hold-down post nuts.

Source: Countdown

See What holds the Space Shuttle orbiter itself stable on the launch pad? for details of the mechanism itself

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  • $\begingroup$ Thank you! Curious: was the shuttle held down both by hold-down clamps and pyro bolts? Or was it just the clamps? How much lag would there be between them if both were used? $\endgroup$
    – user39728
    Jun 7 at 15:27
  • $\begingroup$ The second link in your answer seems to suggest only pyro bolts/frangible nuts were used, and that there would have been eight of those on the SRBs. Were there no mechanical articulated hold-down clamps like those SpaceX is using? $\endgroup$
    – user39728
    Jun 7 at 15:31
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    $\begingroup$ It doesn't suggest, it states it. Bolts and nuts only for STS. no clamps $\endgroup$ Jun 7 at 16:12
  • $\begingroup$ It mentions the frangible nuts. It doesn't say a thing about the articulated hold-down clamps. All I can conclude from that particular link is that frangible nuts were used. I could assume hold-down clamps were not used, but that is why I asked---to confirm so I don't go spewing my sad assumptions across the web. Isn't this the very thing you mentioned in a separate question---that I seem to have gross misunderstandings about the space shuttle? Well, I'm trying to spare the world from those gross misunderstandings. The link in your answer does not mention hold-down clamps. $\endgroup$
    – user39728
    Jun 7 at 16:23
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    $\begingroup$ It doesn't say anything about clamps - moonbeams or elves either - because the shuttle did not use clamps I don't know how else to say this. From the linked answer "The entire weight of the stack was supported by four posts at the bottom of each Solid Rocket Booster (eight total). The boosters were attached to the posts by large bolts (aka "studs"). At liftoff time the nuts on the bolts were exploded apart and the bolts retracted downward." $\endgroup$ Jun 7 at 16:37

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