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The first of Discovery's two rocket boosters is beginning to take shape in preparation for Return to Flight. Section by section, the booster components are being hoisted into place at NASA's Kennedy Space Center in Florida. Each Solid Rocket Booster - or SRB - is made up of four propellant sections, a lower aft skirt and nose frustum. The Space Shuttle SRBs stand nearly 150 feet tall and are the world's largest reusable rocket boosters. The start of stacking followed the fitting of the aft skirt to the first propellant section to form the aft booster. This procedure was completed at the Center's Rotation, Processing and Surge Facility. After the work was finished, the new aft booster was rolled over to the Vehicle Assembly Building aboard the SRB transporter. Inside the building, this first segment was fastened to a mobile launch platform where it will remain anchored until launch. Once locked in place, a crane began lifting additional SRB sections from the building's transfer aisle and stacking them atop the aft booster. When crews finish stacking both Shuttle SRBs, the next step will be securing the External Tank between them. Following installation of the tank, the final phase of assembly will begin with the attachment of Discovery.