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The Space Shuttle mission sequence is divided into phases. I'll start with the one that involves the most preparation, tension, timing, effort and prayers - the Terminal Count. This is the part of the countdown that proceeds from T minus 20 minutes through liftoff (time T). Other nodes may follow, but let's start here. I'll use times in 'T minus' format and offer some of the activities or major checkpoints that occur. Note that there are thousands of people and millions of components working during this time; this is just the very tip of the iceberg. I'll be adding more detail to it as I come across it.

For all acronym expansion that isn't done inline, just follow the hardlinks.

Sources: NASA websites, the Space Shuttle Operator's Manual, launch footage and commentary

Terminal Count

T Minus 20 minutes

Up to this point, control over the countdown and all launch systems resides with a set of computers on the ground, known as the Launch Processing System (LPS). At T-20:00, the Orbiter general purpose computers (GPCs) are loaded with their first set of operations software (OPS1) and formed into a cluster of four GPCs with a fifth online as backup and 'tiebreaker'. From this point on, launch commands pertaining to the vehicle are triggered by the LPS issuing commands to the onboard GPCs and awaiting their report. All backup flight systems are commanded to 'flight' status. Fuel cell thermal conditioning - essentially warmup - begins, and all orbiter cabin vent valves are closed.

The navigation systems come further online; the orbiter's Inertial Measurement Units (IMUs) are initialized. An external bias to compensate for the Earth's rotation is fed into them until just before launch as the onboard IMUs won't show any of the Earth's rotation, due to gravity.

T Minus 9 minutes

At this point, there is a pre-planned hold in the countdown of ten minutes. Any pre-launch actions deferred from earlier phases that are not mission-critical are performed here (final checks, missed verifications, etc.) The NASA Test Director gets the 'go for launch' verification from the launch team. The LPS is augmented by the activation of the Ground Launch Sequencer (GLS) computer, located in the Firing Room.

T Minus 7 minutes 30 seconds

The orbiter crew access arm is retracted. In the event of emergency, the arm can be re-extended in approximately 15 seconds to allow crew egress.

T Minus 5 minutes 15 seconds

The Mission Control Center (MCC) at the Johnson Space Center in Houston transmits a command that starts all shuttle onboard operational instrumentation flight data recorders (black boxes). The data from these is analyzed after landing.

T Minus 5 minutes

Shuttle crew activates the Auxiliary Power Units (APUs) onboard the Orbiter to power the Main Propulsion System (MPS) and Space Shuttle Main Engine (SSME) hydraulics as well as the flight control aerosurfaces. A final mechanical safety switch for the Solid Rocket Boosters ignition system is engaged and the range safety package is enabled by a motor-driven system.

T Minus 4 minutes 55 seconds

Pressure relief valve on the External Tank liquid oxygen storage is closed. This marks the beginning of bringing the external tank up to flight pressurization levels. The GLS automatically checks that the Range Safety System and Solid Rocket Motor ignition system are set to 'Arm' (as opposed to 'Safe').

T Minus 4 minutes 30 seconds

Main fuel valve heaters are turned off

T Minus 4 minutes

Final purge of the SSMEs using liquid helium is performed preparatory to main engine start.

T Minus 3 minutes 55 seconds

The elevons, speed brakes and rudder execute a preprogrammed motion sequence to run a final check and bring them to launch configuration.

T Minus 3 minutes 30 seconds

Ground power transition: ground power is cut out of operations, leaving the Shuttle internal fuel cells carrying the full load.

T Minus 3 minutes 25 seconds

All three SSMEs are run through a preprogrammed gimbaling sequence to verify readiness for launch.

T Minus 3 minutes 03 seconds

Gimbal test complete, the SSMEs are commanded to start position; the main body flap (below the main engines) is verified to be in launch position.

T Minus 2 minutes 50 seconds

The 'beanie cap' - external tank oxygen vent hood - is retracted. At this point, the external tank oxygen stores should be at flight pressure.

T Minus 2 minutes 35 seconds

Piping of (gaseous) oxygen and hydrogen into the vehicle ceases. Fuel cells onboard the Shuttle switch to burning only internal reactant stores.

T Minus 1 minute 57 seconds

Pressure relief valve on the external tank hydrogen storage is closed.

T Minus 1 minute 46 seconds

The PRSD brings up the tank heaters on the liquid hydrogen (LH2) tanks, bringing the hydrogen stores up to flight pressure.

T Minus 55 seconds

The SRB Multiplexer-DeMultiplexer (MDM) components, which manage command communication within the SRB, are locked out to prevent spurious commands and the critical command set is verified.

T Minus 55 seconds

Ground power is removed, leaving the STS on internal power from its fuel cells.

T Minus 48 seconds

Legacy of STS-51L, the SRB joint heaters are deactivated.

T Minus 31 seconds

Sequencing and control of the launch process is handed off from the LPS to the GPCs aboard the Orbiter. The GPCs assume control of all launch activity; however, they will still respond to 'hold', 'resume count' and 'recycle' (re-do part of the countdown) commands from the LPS. Note that at and after this point, any 'hold' or anomalous events will cause the countdown to recycle to the T minus 20 minute mark. After this, only one ground command is required to launch.

During this phase, the detonation controllers for the explosive bolts holding the orbiter and SRBs to the pad, as well as those holding the launch umbilical to the orbiter, are flagged to arm. NASA calls the actuators Pyro Initiator Controllers, or PICs. The computers set arm and fire time flags for these PICs as well as for those used to ignite the SRBs. All PIC voltages are monitored continuously from this point on; deviation in voltage out of the acceptable range will trigger an automatic hold (you'd hate to ignite and have the SRBs still bolted to the launch pad).

T Minus 28 seconds

A command from the GLS starts the SRB hydraulic power units, which provide power to the SRB systems.

T Minus 21 seconds

A gimbaling test sequence on the SRB motors is begun.

T Minus 16 seconds

The SRB nozzles complete a preprogrammed gimbaling sequence to verify that they are prepared to launch. Water begins to spray onto the deck of the Mobile Launch Platform and pad areas to protect the Shuttle from acoustic damage at liftoff - the water disrupts and absorbs echos and vibrations from the noise of launch, preventing sound waves from damaging the Shuttle.

T Minus 12 seconds

The earth rotational compensation bias is removed from all onboard IMUs.

T Minus 11 seconds

The SRB Range Safety Package is activated.

T Minus 10 seconds

The GLS issues the 'Go for Main Engine Start' command. This causes flares to ignite under the SSMEs (visible during launch footage as a shower of sparks) to burn away any excess hydrogen that might have collected under or in the bells; a half-second after that, valves are opened in the orbiter allowing propellant and oxidizer to flow to the main turbopumps.

T Minus 8 seconds

The navigation systems onboard transform the known location of the launch facility (long/lat/attitude) into a positional vector containing the Earth's rotational speed and activate inputs from onboard sensors, preparing to integrate vehicle motion into the state vector. Trivia: Launch complex 39-A at the Kennedy Space Center is at 28 degrees 36 minutes 29.7014 seconds north and 80 degrees 36 minutes 15.4166 seconds west. The shuttle is always positioned with the tail or vehicle 'top' oriented directly south.

T Minus 6.6 seconds

The Main Propulsion System (SSMEs), which has been monitored by the GPCs and LPS throughout, is sent its start command. The three SSMEs are ignited at 120-millisecond intervals (Engine-3, Engine-2, Engine-1). They have three seconds after start to reach 90% thrust levels as monitored by the GPCs; if any engine fails to reach this level, the engines are automatically shut down, the countdown is halted and a 'safing' procedure kicks in.

T Minus 3 seconds

If all three SSMEs have reached the requisite 90% thrust level by this point, they are commanded to liftoff position (thrust position, away from their 'startup' position). At the same time, the fire 1 arming command is issued to the SRBs. All aerosurfaces on the orbiter are commanded to 'launch' position and locked by the ascent digital autopilot.

Note that since the SSMEs are not under the STS center of gravity, the vehicle will begin to 'bend' inward towards the external tank, still locked into launch position by explosive bolts. The computers monitor this as well; a base bending load mode checks to make sure that the nose of the external tank doesn't move more than 25.5 inches towards vehicle 'down.' This motion is called the 'twang.' The vehicle will reach a maximum deviation and then move back towards vertical, which it will reach right around T Minus zero.

T Minus 1 second

The four active onboard GPCs issue the fire command for the SRBs at T minus 0.3 seconds. This is a commit; the SRBs cannot be shut down once ignited. At this point, the following occurs:

  • The ignition pyrotechnics in the SRBs fires, igniting the boosters
  • The four explosive bolts per SRB fire, releasing the vehicle from the launch pad
  • The two launch umbilicals (one per side of the orbiter) are retracted
  • The onboard master timing unit, event timer and mission timer are all started
  • The three main engines reach 100% of rated thrust
  • The ground-based launch sequence is terminated, and all control is transferred to the four active GPCs.

T Minus zero - LIFTOFF

…and like that, you're gone.

On to First Stage Ascent

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