g gaseous oxygen into the vent line to push the liquid oxygen out through the fill line. This worked perfectly for Tank No.1, but not for Tank No. 2.Ground crews thought there was a loose fitting that allowed the gaseous oxygen being pumped in the vent line to escape through the fill line with out pushing out much liquid oxygen in the tank. Later, the possibility that the fitting had been loosened when the tank was dropped at North American months before was considered to be a possible cause of the problem. To empty the tanks, the ground crew turned on the heaters and fans to try to boil the oxygen from the tank. On March 27 and 28 the heaters and fans were turned on by applying 65 volts of direct current from the ground power supply for extended periods of 6 to 8 hours at a time. Unknown to the ground crew, this had set stage for the accident. The 65 volts was far too much for the thermostatic switches that controlled the heaters. They were designed to operate on 28 volts from the spacecraft fuel cell generators. Although the switches would carry 65 volts when closed, they would fail in the closed position if they started to open to interrupt the load. At one point, the switches did start to open and were then welded shut during the long period when the heaters were operating with 65 volts to boil the oxygen out of the tank. The failure of the thermostatically controlled switches allowed temperatures in the heater assembly to reach 1000F instead of shutting the heaters off at 80F. The fact that the safety switches had been welded shut and hence were not operating could have been detected at the Kennedy Space Center if someone had been watching heater current readings on Tank No. 2. Sensors would have shown that the heaters had exceeded that safety switch temperature limit."The blueprint for the accident was finally drawn by the Review Board. Because of a bump one day in the fall of 1969, a fitting might have been loosened. ...
