T-minus 31 seconds: At this moment in the countdown, if there are no technical issues, the "go" command will be given for "auto sequence start," which means that the Ground Launch Sequencer will hand off primary control of the countdown to the shuttle's onboard computers. In these final minutes leading up to launch, the Ground Launch Sequencer is still in automatic control of the countdown.
T-minus 2 minutes: At this point, the shuttle's commander will advise his or her crewmates to close the visors of their launch and entry suits. As the clock ticks down, the GLS will also perform checks of the fuel and space shuttle main engines. At the T-minus 4 minute, 30 seconds mark, the Ground Launch Sequencer program will switch the main fuel valve heaters off. Once the APUs are powered up, ground teams will analyze the system, and if they detect any glitches, this could halt the countdown. There are three separate onboard APUs, and their fuel systems are located in the aft fuselage of the orbiter. T-minus 5 minutes: Barring any technical or weather concerns, the commander of the shuttle will be given the "go" to start the orbiter's auxiliary power units (APU), which produce pressure for the shuttle's hydraulic system. The arm, which is 65 feet (20 m) long and five feet (1.5 m) wide, can be mechanically or manually repositioned in about 15 seconds if there is an emergency or contingency situation. The orbiter access arm remains in its extended position until seven minutes, 24 seconds before launch to serve as an emergency escape route for the flight crew.
This structure allows people to enter the shuttle's crew compartment. T-minus 7 minutes, 30 seconds: At this point, the command will be given to retract the orbiter access arm, which is the lowermost scaffolding arm located 147 feet (about 45 meters) above the surface of the launch pad. The GLS is typically started at about the T-minus 45 minute mark. This program will continue to monitor the vehicle's parameters and will be able to halt the countdown if a problem is detected. The Ground Launch Sequencer (GLS), which is an automated program that controls all activity during the final portion of the countdown, will assume automatic control of the countdown at the T-minus 9 minute mark. These technicians closely monitor computer displays and gauges that show the performance of the shuttle's systems at the launch pad.Īfter the T-minus 9 minute built-in hold, the countdown will resume. During the hold, NASA officials on consoles will be polled for their "go/no go" decisions.
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