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Shuttle Countdown

Launch day -- the external tank is filled with liquid oxygen and hydrogen. Communications checks are made with elements of the Air Force's Eastern Space and Missile Center. Gimbal profile checks of the main engines are performed. Preflight calibration of the Inertial Measurement Units (IMU) is made, and tracking antennas at the nearby Merritt Island Tracking Station are aligned for liftoff.

T minus 5 hours, 20 minutes -- a 2-hour built-in hold occurs. During this hold, an ice inspection team goes to the launch pad to inspect the external tank's insulation. Closeout crew is preparing for the arrival of the flight crew. Resume count down at T minus 3 hours.

T minus 3 hours, Perform flight crew weather briefing, power up SRB tracking system.

T minus 2 hours, 55 minutes, the flight crew heads for the launch pad and proceed on board. Commander and pilot begin to conduct air-to-ground communications checks with the LCC and MCC. Soon after the orbiter hatch is closed the hatch seal and cabin leak checks are made. The white room is then evacuated and the closeout crew proceeds from the launch pad to a fallback area.

T minus 2 hours, 40 minutes, verify transatlantic landing abort (TLA ) sites

T minus 20 minutes a planned 10-minute hold begins. Computers on-board are commanded to their launch configuration and fuel cell thermal conditioning begins when the countdown is resumed. Cabin vent valves are closed and the backup flight system transitions into its launch configuration.

T minus 18 minutes, 30 sec, backup flight system to OPS 1 transistion is complete

T minus 16 minutes, perform backup flight system preflight, uplink loading, verify water spray boiler quantity, perform MPS Helium reconfiguration.

T minus 15 minutes, OMS/RCS crossfeed valves configured for flight, initiate SRB AFT skirt GN2 purge

T minus 13 minutes, perform fuel cell purge, adjust fuel cells for loads

T minus 9 minutes another planned 10-minute hold occurs. The NASA Test Director gets the "go for launch" verification from the launch team then terminal countdown begins. All countdown functions are now automatically controlled by the GLS computer located in the Firing Room Integration Console.

T minus 7 minutes, 30 seconds, the orbiter access arm is retracted.

T minus 6 minutes, 15 seconds, perform Auxiliary Power Units pre-start

T minus 5 minutes, 15 seconds the MCC transmits a command that activates the orbiter's operational instrumentation recorders.

T minus 5 minutes, the crew activates the Auxiliary Power Units to provide pressure to the Shuttle's three hydraulic systems. The firing circuit for SRB ignition and the range safety destruct system devices are mechanically enabled by a motor-driven switch called the safe and arm device.

T minus 4 minutes, 55 seconds, the liquid oxygen vent on the external tank is closed. Verify ET/SRB range safety system safing & arm device armed

T minus 4 minutes the final helium purge of the Shuttle's three main engines is initiated in preparation for engine start. Five seconds later, the orbiter's elevons, speed brakes and rudder are moved through a pre-programmed series of maneuvers to position them for launch.

T minus 3 minutes, 30 seconds , the ground power transition takes place and the Shuttle's fuel cells transition to internal power. Up to this point, ground power had augmented the fuel cells. Then, 5 seconds later, the main engine nozzles are gimballed through a pre-programmed series of maneuvers to confirm their readiness.

T minus 2 minutes, 55 seconds, preparations are made to bring the ET to its flight pressure.

T minus 2 minutes, 50 seconds, the external tank oxygen vent hood -- known as the beanie cap -- is raised and retracted.

T minus 2 minutes, 35 seconds , the piping of gaseous oxygen and hydrogen to the fuel cells from ground tanks is terminated and the fuel cells begin to use the on board reactants.

T minus 1 minute, 57 seconds , the external tank's liquid hydrogen is brought to flight pressure by closing the boil off vent, as was done earlier with the liquid oxygen vent.

T minus 31 seconds , the Shuttle's on-board computers start their terminal launch sequence. Any problem after this point will require calling a "hold" and the countdown recycled to T minus 20 minutes. However, if all goes well, only one further ground command is needed for launch. This is the "go for main engine start," which comes at the T-minus-10-second point. Meanwhile, the Ground Launch Sequencer (GLS) continues to monitor more than several hundred launch commit functions and is able automatically to call a "hold" or "cutoff" if a problem occurs.

T minus 28 seconds the SRB booster hydraulic power units are activated by a command from the GLS. The units provide hydraulic power for SRB nozzle gimballing.

T minus 16 seconds, SRB nozzles are commanded to carry out a pre-programmed series of maneuvers to confirm they are ready for liftoff. Sound suppression system is turned on and water begins to pour onto the deck of the MLP and pad areas to protect the Shuttle from acoustical damage at liftoff.

T minus ll seconds, the SRB range safety destruct system is activated.

T minus 10 seconds, the "go for main engine start" command is issued by the GLS. (The GLS retains the capability to command main engine stop until just before the SRBs are ignited.) At this time flares are ignited under the main engines to burn away any residual gaseous hydrogen that may have collected in the vicinity of the main engine nozzles. A half second later, the flight computers order the opening of valves which allow the liquid hydrogen and oxygen to flow into the engine's turbopumps.

T minus 6.6 seconds , the three main engines are ignited one after another. The engines throttle up to 90 percent thrust in 3 seconds.

T minus 3 seconds, SRB ignition sequence starts if the main engines are at the required 90 percent,. All of these split-second events are monitored by the Shuttle's four primary flight computers.

T minus zero , the holddown explosive bolts and the T-O umbilical explosive bolts are blown by command from the on-board computers and the SRBs ignite. The Shuttle is now committed to launch. The mission elapsed time is reset to zero and the mission event timer starts. The Shuttle lifts off the pad and clears the tower at about T plus 7 seconds. Mission control is handed over to JSC after the tower is cleared.

Space Shuttle Operations Guide