T-20 minutes and holding
This built-in hold typically lasts 10 minutes.
NASA Test Director conducts final launch team briefings
Complete inertial measurement unit preflight alignments
T-20 minutes and counting
Transition the orbiter's onboard computers to launch configuration
Start fuel cell thermal conditioning
Close orbiter cabin vent valves
Transition backup flight system to launch configuration
T-9 minutes and holding
This is the final built-in hold, and varies in length depending on the mission.
Final launch window determination
Activate flight recorders
Final "go/no-go" launch polls conducted by NASA Test Director, Mission Management Team and launch director
T-9 minutes and counting
Start automatic ground launch sequencer
Retract orbiter access arm (T-7 minutes, 30 seconds)
Start auxiliary power units (T-5 minutes, 0 seconds)
Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)
Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)
Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)
Crew members close and lock their visors (T-2 minutes, 0 seconds)
Orbiter transfers from ground to internal power (T-50 seconds)
Ground launch sequencer is go for auto sequence start (T-31 seconds)
Activate launch pad sound suppression system (T-16 seconds)
Activate main engine hydrogen burnoff system (T-10 seconds)
Main engine start (T-6.6 seconds)
T-0
Solid rocket booster ignition and liftoff!