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The sign accompanying the hold-down bolt. It reads

Shuttle Solid Rocket Booster Hold-Down Bolt

Eight hold-down bolts are used on every mission to keep the shuttle stack stable and in an upright position on the launch platform. Each solid rocket booster has four bolts attached to its base and the launch platform. Attached to the top of each bolt is a frangible nut. The nut has two explosive charges installed, which detonate upon command from the shuttle's computers.

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