Frangible nut

The Frangible nut, commonly confused with an explosive bolt, is a component used in many industries, but most commonly by NASA to sever mechanical connections, including the solid rocket boosters (SRB) of the space shuttle, which are bolted to the mobile launcher platform (MLP) until liftoff. It is, by definition, an explosively splittable nut. The bolt remains intact while the nut itself is split in two or more parts [http://www.collectspace.com/collection/artifacts_sts_sts31fnut.html] . On the shuttle, this is accomplished by using a NASA standard detonator or NSD. The space shuttle uses 2 NSDs for the top nut on each of the four 3.5" diameter bolts holding each SRB to the MLP. Once detonation occurs, the shuttle lifts free of the MLP. Shrapnel and the broken nut are captured by energy absorption material, such as metal foam, to ensure damage to the shuttle does not occur. In case of NSD failure, or incomplete clearance of the nut from the bolt, the SRB has ample thrust to break the bolt itself and launch unhindered.

Frangible Nuts are also used for separation of the two aft structural attachments of the external tank prior to orbital insertion. The attach bolts are driven by the explosive force of the NSDs and a spring into a cavity in the tank strut. The Nuts and all residual pieces of the NSDs are caught in a cover assembly within the shuttle.

Spacecraft-stub