Flown and retrieved shuttle Solid Rocket Booster Separation Motor (BSM) from the aft-side skirt from the mid 1980s. This booster separation motor (BSM) measures 12″ high, with the nozzle having a 9″ diameter and weighing 28.3 pounds. The BSM on the shuttle is a relatively small rocket motor that separates the reusable solid rocket boosters from the shuttle before the shuttle orbiter leaves the atmosphere.
About 2+ minutes into a space shuttle flight, 16 of these small, but powerful, motors are fired simultaneously for 1.2 seconds. This provides the precise thrust required to safely separate the spent boosters from the space shuttle’s ET and manned-Orbiter. Altogether, there are 8 such BSMs attached to each of the twin reusable SRBs, four on the forward skirt and 4 on the aft skirt. The BSMs in each cluster are ignited while traveling through the atmosphere at more than 3,000 mph with an altitude of about 24 nautical miles.
The BSMs are produced by ATK Launch Systems Group, part of Alliant Techsystems (ATK) Inc., at their production plant in Brigham City, Utah.
You can see the separation with the naked eye. Here’s a photo I took from 
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