Apollo: The Race To The Moon

by Catherine Bly Cox

On all the Apollo flights up through Twelve, the switches had not had to open. When the tanks were pressurized with cryogens hundreds of degrees below zero, the switches remained cool and closed. When, for the first time in the history of the cryo tanks, the temperature in the tanks rose high enough to trigger the switch—as O2 Tank 2 emptied—the switch was instantaneously fused shut by the 65-volt surge of power that it had not been designed to handle. For the eight hours that the heaters remained on, the Teflon insulation on the wires inside the cryo tank baked and cracked open, exposing bare wires. On the evening of April 13, when Liebergot ordered the cryo stir, some minute shift in the position of two of those bare wires resulted in an electrical short circuit, which in turn ignited the Teflon, heating the liquid oxygen. About sixteen seconds later, the pressure in O2 Tank 2 began to rise. The Teflon materials burned up toward the dome of the tank, where a larger amount of Teflon was concentrated, and the fire within the tank, fed by the liquid oxygen it was heating, grew fierce. In the final four seconds of this sequence, the pressure exceeded the limits of the tank and blew its dome off, jarring the shelf above it with a force of 86 g’s for about eleven microseconds, and slamming shut the reactant valves on Fuel Cell 1 and Fuel Cell 3. Then the Teflon insulation between the inner and outer shells of the tank caught fire, as did the Mylar lining in the interior of the...