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. It is, by definition, an explosively-splittable nut. The bolt remains intact while the nut itself is split into two or more parts.[1]
Frangible nuts secure the solid rocket boosters (SRB) of the space shuttle, which are bolted to the mobile launcher platform (MLP) until liftoff. On the shuttle, they are separated using a NASA standard detonator or NSD. The space shuttle uses two NSDs for the frangible nut atop each of the four 28" long, 3.5" diameter bolts holding each SRB to the MLP. Once detonation occurs, the shuttle lifts free of the MLP. The broken nut and any shrapnel from detonation is captured by energy absorption material, such as metal foam, to prevent damage to the shuttle. 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.