These are a pair of the nuts used to hold the explosive bolts in place on the Space Shuttle during launch. They're common across spaceflight, as a handy way of holding a rocket down or together until the right moment for it to separate. Stages are held together with explosive bolts. Rockets are affixed to the pad with explosive bolts. They're used because they're extremely sturdy while intact, but pop *very* quickly, near-instantly freeing the rocket from the pad or the spent stage from the rest of the stack.
On the Space Shuttle, they played an especially important role, for a twofold reason: The Shuttle's unique geometry, and the use of Solid Rocket Boosters.
Solid Rocket Boosters are two things: Very powerful, and completely uncontrollable once lit. As soon as a solid rocket is ignited, it cannot be turned off or throttled down until the fuel is spent.
They used SRBs for the Shuttle, and continue using them for the SLS, because they're powerful, simple, reliable, and dead cheap (relatively). This is the same reason that rockets like the Ariane 5, Atlas V, and Delta IV use SRBs. This is also why, with both the Space Shuttle and the SLS, the main engines are ignited at about T-5 to T-6, but the SRBs are only ignited at the moment of launch. It gives launch control a precious few seconds to see if there's anything wrong with the main engines before the unstoppable SRBs are lit and the rocket is inevitably going. If the engines fail to reach 100% power and stabilise there in three seconds, the entire launch is automatically cut off and the engines are shut down.
This is also where the Space Shuttle's geometry comes into the mix. Because of the way it's built, the Space Shuttle has asymmetrical thrust at launch. It has three RS-25s mounted to the side of the stack. Now, the SRBs have six times the thrust of the RS-25s, but the thrust from the SSMEs isn't inconsequential.
This is the cause of the famous "Twang." When the SSMEs ignite, the offset thrust causes the entire stack to pitch down/forward about two metres, then sway back to vertical. The ignition sequence is timed so that the Shuttle will be back to perfectly vertical at the moment of booster ignition/launch.
The eight bolts, which are holding down the SRBs, have to withstand the force of the RS-25s throttling up, so as to not tip the rocket stack over. The SRBs themselves flex- they consisted of four (on the Shuttle) or five (on the SLS) segments, joined together with rubber O-rings.
Now, in the event that the bolts failed to detonate, that would not stop the SRBs. Once a solid rocket is lit, it's going. The only question is where. If it happened, the SRBs would tear themselves free of the pad. The severity of the damage this would cause depends heavily on the amount that failed. There was never an incident where all eight bolts failed to detonate, but there were instances where one didn't pop, and in those situations, the Shuttle launched without issue and the damage caused by the SRBs pulling themselves free was negligible, a bit of extra repair work to do upon booster recovery and refurbishment.
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