Up to 20 men could seek refuge in the blast chamber for up to 24 hours, though with more men, things became problematic due to the rise in carbon dioxide levels. The room was equipped with carbon dioxide scrubbers that came with spare filters and a store of oxygen candles - a type of chemical oxygen generator containing a mix of sodium chlorate and iron powder that burns to produce 6.5 man-hours of oxygen per kilogram of the gas mixture.
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At the time, on the wall was a detailed schedule outlining exactly when oxygen candles had to be lit and filters had to be changed. With less than six men in the blast room, they could all breathe normally for a full day while the air above them cleared. With up to 10 men in the room, things got a little more complicated. Additional methods of providing oxygen became imperative if everyone inside was going to survive.
As evidence that men could last for a while in the blast room, there was even a toilet. But barely tucked away behind one of the chairs, using it in such a small space wouldn't have been an appealing prospect.
To get into this fortress of safety, the astronauts and pad crews had to take a ride. Elevators would carry them from any level on the gantry to the base of the mobile launch platform where, on the north side, was a square door with rounded edges. It opened to a slide, 200 feet (60 meters) long, that would send astronauts and pad crews on a winding ride to a point 40 feet (12 meters) under the launch pad. They landed in the rubber room, so called because it was padded entirely with bouncy rubber. A six inch steel door admitted them through a short tunnel and into the blast chamber.
Once the air around the launch pad had cleared and it was safe to leave, astronauts and pad crews could take one of two long, narrow, and winding tunnels to the western edge of the launch pad area. There, they could open a door and step outside.
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After the Apollo program ended, the rubber rooms and blast chambers were abandoned in place. There were no circumstances under which shuttle astronauts would use this underground shelter; the preferred method beginning in the 1980s were the gondolas on cables that led from the top of the gantry to a safe site on the ground.
The rubber room and blast chamber, at least the one under Pad A, is still there. It's off limits to the public and preserved as a historic site, but if you can finagle your way in (which involves knowing the right people) it's definitely a piece of history worth seeing.
To see NASA's rubber room in all its glory, watch a clip from a BBC documentary on the Apollo emergency escape procedure.
Image: Top: The door to the rubber room. Middle: The rubber room with fire blankets in the center. Credit: NASA