Crew Emergency Evacuation Area, Launch Pad 39B, Kennedy Space Center, October 2015
About 900,000 pounds each of LOX (liquid oxygen) & hydrogen were stored at launch areas during the Space Shuttle program. Hydrogen is the fuel used by the Shuttle's rockets and LOX was the oxidizer -- the substance that made combustion occur. If LOX & Hydrogen were to combine too soon, a large, uncontrolled explosion would happen. The LOX and hydrogen were stored on opposite ends of the launch area and pumped to the Shuttle's external tank just prior to launch. The external tank was actually two tanks in one, separating the LOX and hydrogen until the very last moment. When the fuel and oxidizer were combined, the result was a controlled explosion that launched the spacecraft. Detailed evacuation plans and systems were put in place to offer crew members a possibility of escape in case of any number of emergencies that could occur during launch.