ntical software, pull information from thousands of sensors, make hundreds of milli-second decisions, vote on every decision, check with each other 250 times a second. A fifth computer, with different software, stands by to take control should the other four malfunction.
At T-minus 6.6 seconds, if the pressures, pumps, and temperatures are nominal, the computers give the order to light the shuttle main engines — each of the three engines firing off precisely 160 milliseconds apart, tons of super-cooled liquid fuel pouring into combustion chambers, the ship rocking on its launch pad, held to the ground only by bolts. As the main engines come to one million pounds of thrust, their exhausts tighten into blue diamonds of flame.
Then and only then at T-minus zero seconds, if the computers are satisfied that the engines are running true, they give the order to light the solid rocket boosters. In less than one second, they achieve 6.6 million pounds of thrust. And at that exact same moment, the computers give the order for the explosive bolts to blow, and 4.5 million pounds of spacecraft lifts majestically off its launch pad.
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