From one of the upper levels of the Fixed Service Structure (FSS) on launch pad 39-A, this view looks down on the Solid Rocket Booster's (SRB) Aft Skirt Hold Down Posts (HDPs). The HDPs hold the vehicle to the launch pad during rollout and after Space Shuttle Main Engine (SSME) start at T-6 seconds. The pyrotechnics in the HDPs are fired at T-0, releasing the vehicle from the pad.