The last minutes of Endeavour’s countdown to launch is marked by several steps, including the retraction of the orbiter access arm and the gaseous oxygen vent arm, which holds the “beanie cap” over the external tank.
The ground launch sequencer manages the countdown during the last nine minutes and monitors the shuttle’s systems until the final moments before launch. At the 31-second mark, Endeavour’s onboard computers will take control and ignite the solid rocket boosters and orbiter main engines.