Hold-Down Posts
Each
solid rocket booster has four hold- down posts that fit into corresponding
support posts on the mobile launcher platform. Hold- down bolts
hold the SRB and launcher platform posts together. Each bolt has
a nut at each end, but only the top nut is frangible. The top nut
contains two NASA standard detonators, which are ignited at solid
rocket motor ignition commands.
When the two NSDs are
ignited at each hold-down, the hold-down bolt travels downward
because of the release of tension in the bolt (pretensioned before
launch), NSD gas pressure and gravity. The bolt is stopped by
the stud deceleration stand, which contains sand. The SRB bolt
is 28 inches long and is 3.5 inches in diameter. The frangible
nut is captured in a blast container.
The solid rocket motor
ignition commands are issued by the orbiter's computers through
the master events controllers to the hold- down pyrotechnic initiator
controllers on the mobile launcher platform. They provide the
ignition to the hold- down NSDs. The launch processing system
monitors the SRB hold- down PICs for low voltage during the last
16 seconds before launch. PIC low voltage will initiate a launch
hold.
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