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Detail of one of the hold-down posts on the aft skirt.

There are four such hold-down posts on each SRB. The entire Shuttle stack was attached to the mobile launcher platform by hold-down bolts passed through these hold-down posts. A frangible nut screwed onto each bolt, fastening the SRB to the bolt. At launch, igniters in the nuts were detonated, releasing the space vehicle.