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[i]Due to electromagnetic interference problems with the vac-ion pumps on cryogenic tank domes in earlier Apollo spacecraft, a modification was introduced and a decision was made to replace the complete oxygen shelf in SM 106. An oxygen shelf with approved modifications was prepared for installation in SM 106. On October 21, 1968, the oxygen shelf was removed from SM 106 for the required modification and installation in a later spacecraft.[/i]
[i]One shelf bolt was mistakenly left in place during the initial attempt to remove the shelf; and as a consequence, after the front of the shelf was raised about 2-inches, the fixture broke, allowing the shelf to drop back into place. Photographs of the underside of the fuel cell shelf in SM 106 indicate that the closeout cap on the dome of oxygen tank no. 2 may have struck the underside of that shelf during this incident. At the time, however, it was believed that the oxygen shelf had simply dropped back into place and an analysis was performed to calculate the forces resulting from a drop of 2 inches. It now seems likely that the shelf was first accelerated upward and then dropped.[/i]
[i]These tests... would not disclose fill line leakage within oxygen tank no. 2.[/i]
[i]The probability of tank damage from this incident, therefore, is now considered to be rather low, although it is possible that a loosely fitting fill tube could have been displaced by the event.[/i]
[i]It was decided that if the tank could be filled, the leak in the fill line would not be a problem in flight, since it was felt that even a loose tube resulting in an electrical short between the capacitance plates of the quantity gage would result in an energy level too low to cause any other damage. Replacement of the oxygen shelf in the CM would have been difficult and would have taken at least 45 hours. In addition, shelf replacement would have had the potential of damaging or degrading other elements of the SM in the course of replacement activity. Therefore, the decision was made to test the ability to fill oxygen tank no. 2 on March 30, 1970, twelve days prior to the scheduled Saturday, April 11, launch, so as to be in a position to decide on shelf replacement well before the launch date.[/i]
[i]Further [post-flight investigation] tests have shown that the temperatures on the heater tube may have reached as much as 1000 degrees F during the detanking. This temperature will cause serious damage to adjacent Teflon insulation, and such damage almost certainly occurred. None of the above, however, was known at the time and, after extensive consideration was given to all possibilities of damage from a loose fill tube, it was decided to leave the oxygen shelf and oxygen tank no. 2 in the SM and to proceed with preparations for the launch of Apollo 13.[/i]
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