Countdown 101
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The countdown clock is one of the most-watched timepieces in the world. On this page, you’ll learn how the countdown operates, and what milestones to watch for during our live launch coverage.
Image above: Spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA
Pauses in the countdown, or “holds,” are built into the countdown to allow the launch team to target a precise launch window, and to provide a cushion of time for certain tasks and procedures without impacting the overall schedule. For the space shuttle countdown, built-in holds vary in length and always occur at the following times: T-27 hours, T-19 hours, T-11 hours, T-6 hours, T-3 hours, T-20 minutes, and T-9 minutes.
Here are some of the key events that take place at each milestone after the countdown begins. Note: Event times and lengths are approximate and subject to change.
T-43 hours and counting
The Shuttle Test Director performs the traditional call to stations and the countdown clock is activated.
Begin final vehicle and facility close-outs for launch
Check out backup flight systems
Review flight software stored in mass memory units and display systems
Load backup flight system software into the orbiter’s general purpose computers
Remove middeck and flight deck platforms
Activate and test navigational systems
Complete preparation to load power reactant storage and distribution system
Complete flight deck preliminary inspections
T-27 hours and holding
This is the first built-in hold and typically lasts four hours.
Clear launch pad of all non-essential personnel
Begin loading cryogenic propellants into orbiter’s power reactant storage and distribution (PRSD) system
Image above: After rollback of the rotating service structure on Launch Pad 39A, space shuttle Endeavour waits for launch on the STS-127 mission. Credit: NASA/Kim Shiflett
T-27 hours and counting
Begin operations to load cryogenic reactants into the orbiter’s fuel cell storage tanks
T-19 hours and holding
This built-in hold typically lasts four hours, but may be extended if PRSD offload is required.
Demate the orbiter’s midbody umbilical unit
Clean and vacuum crew module
External tank nosecone purge
T-19 hours and counting
Begin final preparations of the orbiter’s three main engines for main propellant tanking and flight
Fill launch pad sound suppression system water tank
Resume orbiter and ground support equipment close-outs
Close out the tail service masts on the mobile launcher platform
T-11 hours and holding
This built-in hold varies between 13 to 14 hours.
Weather and engineering briefings
Pad debris inspection and closeout
Flight crew equipment late stow
Move rotating service structure to “park” position
Activate the orbiter’s inertial measurement units and communications systems
Perform ascent switch list
T-11 hours and counting
Activate the orbiter’s fuel cells
Clear the blast danger area of all nonessential personnel
Switch the orbiter’s purge air to gaseous nitrogen
Image above: Sparks fly beneath space shuttle Discovery as engines ignite for the liftoff on mission STS-124. Credit: Photo courtesy of Scott Andrews
T-6 hours and holding
This built-in hold typically lasts two hours, or one hour for a 24- or 48-hour scrub.
Mission Management Team and launch director receive weather update
Launch team verifies no violations of launch commit criteria before loading the external tank with propellants
Clear pad of all personnel
Chill-down of propellant transfer lines
Begin loading the external tank with about 500,000 gallons of cryogenic propellants
T-6 hours and counting
Finish filling the external tank with its flight load of liquid hydrogen and liquid oxygen propellants
T-3 hours and holding
This built-in hold typically lasts two-and-a-half hours.
External tank loading enters stable replenish
Perform inertial measurement unit preflight calibration
Align Merritt Island Launch Area (MILA) tracking antennas
Final Inspection Team proceeds to the launch pad to conduct a detailed analysis of the vehicle as the team walks up and down the entire launch tower
Closeout Crew proceeds to the launch pad to configure the crew module for countdown and launch and assist the astronauts with entry into the orbiter
Televised weather briefing
Flight crew weather briefing
Astronaut Support Person enters crew module and begins comm checks
T-3 hours and counting
Crew departs for the launch pad and, upon arriving at the pad, begins entry into the orbiter via the White Room
Complete close-out preparations in the launch pad’s White Room
Check cockpit switch configurations
Astronauts perform air-to-ground voice checks with Launch Control (Kennedy Space Center) and Mission Control (Johnson Space Center)
Close the orbiter’s crew hatch and check for leaks
Complete White Room close-out
Close-out crew retreats to fallback area
A fish-eye view captures space shuttle Endeavour just after liftoff on mission STS-111. Credit: NASA
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!
Source:NASA’s John F. Kennedy Space Center / Space Shuttle Mission Page
Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer
2012-12-04 08:05:44
Source: http://www.dearastronomer.com/2011/02/22/countdown-101/
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