GO for Launch
At T-20 minutes the LCC was locked down and the launch crew made its first run through a set of 2300 Launch Commit Criteria. These included weather, all tracking stations up, an acceptable level of hydrogen gas on the pad, and hundreds more. If the Launch Director got a GO from everyone, the countdown proceeded to T-9 minutes, at which time a built-in hold initiated. During this time, the Launch Director polled the team again to ensure the Launch Criteria were met. If they were, the Ground Launch Sequencer (GLS) was started, and the countdown proceeded under computer control.
The GLS monitored every system on the shuttle. If it detected an incorrect reading, it automatically stopped the countdown.
At T-9 seconds, the orbiter’s main engines were put through a gimble test by the GLS. If that is successful, a pyrotechnic device just beneath the orbiter’s main engines ignited. At that time the pad sound suppression system was initiated. This consisted of a spray of 900,000 gallons a minute of water into the flame deflectors through the holes in the Mobile Launcher.
Sound suppression is necessary because the shuttle at liftoff produced one of the loudest sounds ever heard on Earth—215dB. To put that in perspective, a sound level of 111 dB causes a human so much pain it is unbearable and will cause permanent hearing loss. Without suppression, that level would damage the launch platform, and the reflected sound would damage the shuttle. The thousands of gallons of water spraying into the flame deflectors reduce that to 142 dB—still a very high level of sound but not enough to damage the pad or shuttle.
At T-6.6 seconds fuel had begun pumping into the main engines and they ignited. By T-3 seconds they have built up to 90 percent thrust and were put into liftoff position.
At T-0 the explosive holddown bolts securing the shuttle to the launch platform were severed. The SRBs were ignited by a pyrotechnic device inside the rocket motor that fires down the entire length of the propellant grain. As soon as the SRBs ignite the shuttle lifted off for another trip into space. The white smoke billowing up around the pad was not from the SRB exhaust. It was from the sound suppression water pouring into the flame deflectors—it was steam.