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Countdown
For anyone who has stood in the middle of a Florida swamp and watched a space shuttle launch, there’s nothing quite like the last 10 seconds before the shuttle lifts off the launch pad on a tower of fire so brilliant it turns night into day.
On a grassy spit of land in front of the viewing stands at the Kennedy Space Center, a large digital countdown clock moves relentlessly to "T minus 0" ("T" for "time"). The voice of Launch Control echoes over the intercom, providing a running commentary on the shuttle’s status during these last few critical seconds.
The shuttle, awash in floodlights, appears to be sitting quietly, serenely waiting for the explosion of energy that will wrench it from the tenacious grasp of Earth’s gravity. But inside the vehicle an enormous number of events are occurring in a rigidly controlled sequence to make the liftoff a reality.
At T-10 seconds, Launch Control announces: "Go for main engine start." Then, just 6.6 seconds before launch, under control of the shuttle’s four on-board computers, the first of the three main engines ignites, followed at 120 millisecond intervals by the other two. Within three seconds, the engines throttle up to 90 percent power. If the computers detect any faltering in the engines, they will—and have—shut down the launch even this late in the count.
For several heartbeats, the vehicle strains against the bolts holding it to the launch pad, waiting for the final event—the ignition of the two solid rocket boosters at T-0. The solid rocket boosters belch orange-white flames, the hold-down bolts are blown and the shuttle commits itself to launch.
Lifting off the pad is excruciatingly slow in the first moments, as the Shuttle fights the pull of gravity on its massive weight (2 million kilograms). But within seconds, the Shuttle clears the launch tower, then rolls on its side so that it lies beneath the External Tank—the large dark-orange tank feeding fuel to the main engines—and begins its fiery and turbulent eight-and-a-half minute ride into orbit.
Soon, all that’s left is a ragged column of smoke rising from the empty launch pad into the sky.
It takes an enormous amount of effort to bring the Shuttle to those last 10 seconds. The countdown officially starts at T-43 hours, at which time the "firing room" (located in the control centre for shuttle launches) at the Kennedy Space Center is staffed around the clock by teams of engineers who continuously monitor both the Shuttle and the ground support equipment.
The countdown manual is five volumes of about 5000 pages. It contains thousands of detailed procedures not only for launching the Shuttle, but for "scrubbing" the launch and recycling the countdown, which can happen for any number of technical reasons or because of bad weather. The manual also outlines contingency procedures for emergencies—for example, quickly evacuating the flight crew if there’s an imminent danger to their lives.
T-43 and counting...
T-43 actually takes place about 69 hours—nearly three days—before launch because seven "holds" are built into the timeline. These holds last from a few minutes to as long as 24 hours or more, and give launch crews leeway to deal with unexpected delays or problems that occur during normal pre-launch activities.
During the early portions of the countdown, crews check the Shuttle’s computer systems and software, as well as the crew cabin, including the oxygen supply at each seat; they also stow crew equipment and prepare the fuel cells that provide the Shuttle’s electrical power in space.
Just before the longest hold of the countdown starts at T-11 hours, the launch team activates the Shuttle’s flight control, navigation and communications systems.
The count resumes, and they close the Shuttle’s payload bay doors and roll back a large servicing structure that encloses the Shuttle, giving workers access to payloads in the Shuttle cargo bay while it’s on the launch pad. The crew escape pole, a device that would enable the astronauts to bail out of the Shuttle during an emergency, is also installed at this time.
When the countdown comes out of a two-hour hold at T-6 hours, the launch team implements the Launch Commit Criteria—an evaluation of elements affecting the safety and success of the mission that define whether the Shuttle is ready to fly. Some are designed to protect the Shuttle against damage; others deal with the safety of the astronauts. There are also criteria for suitable weather conditions.
T-6 marks the start of loading the External Tank with super-cold liquid hydrogen and liquid oxygen fuel. During Marc Garneau’s last mission, STS-97, the fueling process was delayed while engineers dealt with a loose bracket on the outside of the "White Room," a compartment at the end of an access arm extending from the launch tower through which astronauts enter the Shuttle. There was concern that the vibrations of launch might shake the bracket off entirely, creating debris that could damage the edge of one of the Shuttle’s wings during launch.
Fueling takes most of the time between T-6 hours and the next hold at T-3 hours. The launch team continues monitoring the Shuttle for any sign of trouble and conducts a series of checks on various systems—for example, the nozzles on the solid rocket boosters.
On Garneau’s mission, during the hold that stopped the clock at T-3 hours, a small grass fire was discovered in a field near the launch pad. Although it was not deemed a threat to the Shuttle or the launch, as a precautionary measure, a fire crew was sent to put out the fire.
During the two-hour hold at T-3 hours, the "ice team" springs into action. This group does a "walkdown" to inspect the vehicle and the pad for ice or debris that could damage the Shuttle during launch. Another ice team, located inside Launch Control, has been monitoring the exterior of the External Tank for any build-up of ice that might be caused by the loading of liquid hydrogen and liquid oxygen, both of which must be kept super-cold to remain in liquid form.
The "closeout crew" then goes into the White Room to prepare for the arrival of the astronauts. They inspect the crew cabin to make sure it’s properly configured for flight and run a series of checks of the communications and other systems.
The astronauts, who are sleeping in their quarters in the Operations and Checkout Building, are then awakened. They eat breakfast, receive a weather briefing and prepare to leave for the launch pad.
T-3 hours and counting...
As the count resumes at T-3 hours, the astronauts, dressed in their orange launch suits, board a van that takes them to the launch pad. They ride up the launch tower to the White Room. The closeout crew helps them put on their helmets, adjust and secure their equipment and climb into the cockpit. Once the astronauts are safely strapped into their seats, the closeout crew shuts and seals the entry hatch. The entry process takes roughly an hour.
The flight crew aboard the Shuttle goes through a series of pre-flight procedures and checklists, including verifying voice communications with the ground controllers. One task involves configuring the Shuttle to return for a landing at the Kennedy Space Center runway if the flight must be aborted for any reason.
Meanwhile, the launch team checks procedures to deal with emergencies that might occur shortly after launch while the Shuttle is climbing out over the Atlantic Ocean. This includes verifying a system for sending a command to destroy the Shuttle if it goes seriously off its planned trajectory.
The launch team also ensures that the airspace and ocean areas near to and "down range" from the launch pad are clear, including the region where the reusable solid rocket boosters will drop into the ocean after being jettisoned from the Shuttle about two minutes after launch.
They also test a computer system responsible for firing explosive charges that release the hold-down bolts on the Shuttle at T-0, as well as those that separate the External Tank and the solid rocket boosters from the Shuttle after their fuel has been exhausted a few minutes after launch.
A series of checks is done to ensure the Shuttle’s crew cabin is completely sealed and isn’t leaking air. As the next hold arrives, the closeout crew leaves the pad.
The clock stops again at T-20 minutes for 10 minutes, then resumes for 11 minutes. It’s now less than an hour before launch, with just one hold left. Events are escalating rapidly. The Shuttle’s onboard computers are configured for launch. The commander and co-pilot go through a series of last-minute procedures and checklists, most involving verification of computer, fuel and propulsion systems.
The launch team verifies that emergency aircraft and support personnel that might be needed in an emergency are ready.
At T-9, the clock stops for the last time. During the 46-minute hold, the entire launch team is polled to ensure there are no violations of the Launch Commit Criteria and everything is ready to go. If all agree, the countdown moves into the final minutes, known as the "terminal countdown."
T-9 minutes and counting...
The final nine minutes are automatically controlled by a computer system called the Ground Launch Sequencer. At T-7 minutes, a command is issued to retract the Orbiter Access Arm, the passageway extending from the launch tower through which the flight crew entered the Shuttle. (If there’s an emergency requiring crew evacuation, the Orbiter Access Arm can be extended again in 15 seconds.)
Over the next few minutes, dozens of events occur in rapid succession that ready the Shuttle’s engines and solid rocket boosters for launch.
At T-31 seconds: the Shuttle’s on-board computers take over the count; if they shut things down within this last half-minute, the countdown must be recycled to at least T-20 minutes and holding.
T-16 seconds: more than a million litres of water cascades onto the launch pad near the engines. (This "sound suppression" system protects the Shuttle from damage by acoustic energy created by the noise of the engines—reducing the decibel level to acceptable limits. It also protects the Shuttle from a pressure pulse (created when the solid rocket boosters ignite) that stresses the Shuttle’s wings and control surfaces. The water deluge blocks the pressure wave and reduces its intensity. This "overpressure" problem was discovered on the first Shuttle flight when it damaged some of the black heat tiles that protect the Shuttle during re-entry into the Earth’s atmosphere.)
Then, finally, the moment of truth:
- T-6.6 seconds: Main engine 3 start command
- T-6.48 seconds: Main engine 2 start command
- T-6.36 seconds: Main engine 1 start command
- T-0: Solid rocket boosters ignition/ solid rocket
Boosters holddown release command
