Purges - to prevent contamination in the engine - water, air on H2 side froze (~AC-71?) Valve for prelaunch He is spring loaded closed. A positive flow out of the engine was to be maintained with He to keep air out. AC-70 and AC-71 Tolerances were tighter on some new valves. Valve failed to close and air got into the fuel pump which operates at a temperature of -400 F. The nitrogen in the air froze. C1 started but C2 did not. In AC-70 a pump started to rotate then seized. They thought some foreign matter had gotten into the engine.