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When a space shuttle shuts down in the last seconds before liftoff, the launch team has its most important work to do.

"The last command we give the astronaut flight crew is at two minutes and 30 seconds," says Bartolini. At that point the shuttle begins running solely on internal power, and the OTC tells the astronauts to close and lock their visors and initiate oxygen flow. "He usually gives them a little send-off, and then it gets real quiet. The only talking that's being done is by the ground launch sequencer engineer calling out the different milestones as we go on down and the NTD, who starts calling at one minute, then 45 seconds... on down. Other than that the firing room is extremely quiet. Everybody is looking at their data hoping that they don't get an anomaly."

In today's simulation, everyone gets plenty of anomalies. Data systems engineer Robert Pierce and two math modeling colleagues have loaded the computers with a variety of virtual emergencies. "We're really taking a polished team and putting a high gloss on it," Pierce says. "We plan for things that have a

likely occurrence of happening. 'Likely' for us space nuts is less than one percent. We don't like surprises."

Many of the problems Pierce and his gremlins throw at the launch team occur during a pad abort. After the orbiter's computers command a main engine cutoff, they grind through the procedures for safing the vehicle: starting a spray of water to disperse unburned hydrogen exhausted from the main engines, for example, sealing the hydrogen and oxygen valves to the engines, disarming the explosive bolts on the solid rockets. Progress is reflected on computer screens filled with blue, green, yellow, and--to show exceeded limits or other trouble--red or flashing numbers. The red numbers require engineers to respond according to well-documented procedures.

In one simulated emergency, engineers begin to see temperatures in red because the shuttle's ground cooling unit fails. "You don't want to cook your equipment," Pierce explains. The NTD issues an order to activate backup systems, then another to shut down a series of electronic systems on the shuttle that produce heat. An engineer at the environmental control console manually flips a switch to turn on a chilled-water heat exchanger. Others activate radiators on the inside of the payload bay doors. At another environmental control console, a team lowers the temperature of air being pumped into the payload bay by a purge system.

Next, the NTD orders staff at the LOX and liquid hydrogen consoles to prepare to drain the external tank, a precaution in case power to the shuttle must be turned off. The next step is to reestablish power from the ground in order to shut down the onboard fuel cells, which are major heat generators.

"Then we have a decision point," Pierce says. "Are we still hot?" If so, members of the launch team will continue to turn off the shuttle's various systems. Throughout the process, the NTD is getting updates on temperatures from environmental control engineers. If the temperature doesn't drop to an acceptable range, he will order an emergency power-down and get the crew off the shuttle. Without electrical power on the shuttle, the launch crew no longer sees data from its systems, a situation that would require an emergency egress for the flight crew. "They open the hatch, jump out, run across the arm, and do the slide wire thing," Pierce says.

"The slide wire thing" is the astronauts' escape system: seven flat-bottom baskets that slide down 1,200-foot wires to safety. Each basket is made of steel and heat-resistant fiber surrounded by netting and can carry up to three persons. They slide down wires into catch nets, which drag chains to stop them near a bunker designed to withstand the force of a shuttle explosion.

In a real emergency, the astronauts would take a brisk walk--no more than 50 feet--across the shuttle access arm and fixed service structure to the baskets. Their trip would be complicated by a steady stream of water being sprayed to protect them from flames or heat. To ensure that no one gets lost, crew members are trained to grab a mitt full of each other's spacesuits. A crew of five, for example, splits into groups of two and three. They would follow a "yellow brick road"--gold and black chevrons painted on the metal grate floor--aiming them toward the baskets.

Riding the slide wires has its own risks--ones serious enough that during the abort simulations NASA fills the baskets with weights and dummies rather than people. But the agency has man-rated the system. George Hoggard, a training officer on the pad rescue team, is one of only three people who have ever ridden in a slide-wire basket at the launch pad. The ride began 195 feet above the ground and ended 21 seconds later. The basket reached 53 mph before striking the net.

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