Image above: The four reusable solid rocket motor segments are joined with the solid rocket booster assemblies to form the flight configuration SRBs.
The reusable solid rocket motor, or RSRM, is assembled and tested by ATK Thiokol Propulsion near Promontory, Utah.
Each motor consists of four steel tubes, or segments, lined with 1.1 million pounds of solid fuel propellant. An igniter is installed in the forward segment and a nozzle in the aft segment.
The RSRM segments are shipped by rail to NASA's Kennedy Space Center in Florida.
At Kennedy, the RSRM segments are joined with subassemblies built by United Space Alliance, including the forward assembly, aft skirt, frustum and nose cap.
These structures contain the booster guidance system -- the hydraulics system that steers the nozzles, booster separation motors and parachutes.
The combination of the reusable solid rocket motor segments and solid rocket booster subassemblies make up the flight configuration solid rocket boosters, or SRBs.
The SRBs come to life when the thrust vector control, or TVC, system is activated 28 seconds before launch.
After the main engines are running, the boosters are ignited by an electrical spark that sends flames from the igniter down the center of the propellant.
The boosters go to full power in two-tenths of a second. At the same time, the hold-down nuts are severed, and the shuttle lifts off.
The propellant in the forward segment of the RSRM is designed to provide fast acceleration, burning out 50 seconds after launch.
The remaining propellant is shaped to burn at a slower rate to reduce stress on the vehicle and the crew during the period of maximum dynamic pressure, or max Q.
After about two minutes, all of the propellant is consumed and the boosters burn out and separate at an altitude of 28 miles and speed of 3,100 mph.
The boosters coast upward to a 41-mile altitude, then parachute back to the Atlantic Ocean about 140 miles from the launch site.
After splashdown, NASA's recovery ships, Liberty Star and Freedom Star, tow the boosters back to Cape Canaveral for disassembly and recycling.
The motors are returned to Utah for final cleaning, inspection and propellant reloading.
The booster subassemblies (the frustum, forward skirt and aft skirt) go to the United Space Alliance Assembly and Refurbishment Facility at Kennedy.
The parachutes are refurbished at Kennedy's Parachute Refurbishment Facility.
Reusable Solid Rocket Motor and Solid Rocket Boosters