NASA-LLIS-1184

Abstract

<strong>Description of Driving Event:</strong> <br>On August 12, 1991, starting at approximately 3:58 a.m. EDT, two of the three fuel cells installed on Orbiter Atlantis, OV104, were possibly damaged when inadvertently left connected to the orbiter main busses for 16 hours and 36 minutes without water removal capability. The accumulation of water has the potential for severe damage to the fuel cells. Atlantis landed at the Kennedy Space Center (KSC) on August 11, 1991, at 8:24 a.m. EDT after completing the STS-43 mission. The vehicle was undergoing deservice and safing operations in the Orbiter Processing Facility (OPF) High Bay 2 as part of the turnaround activities for its next mission, STS-44. The work effort was being accomplished by a NASA contractor. <br>The mishap occurred when helium was inadvertently ingested into the fuel cell oxygen (02) supply causing a sudden performance loss by Fuel Cells 2 and 3 resulting in the loss of power to orbiter busses B and C. This necessitated the implementation of the Emergency Power Down Procedure. The Emergency Power Down Procedure consisted of 6 crew module switch actions and was intended to electrically isolate the fuel cells from the orbiter main busses. The Emergency Power Down Procedure did not address the loss of multiple fuel cells with no vehicle ground power and, therefore, did not accomplish the required isolation of Fuel Cells 2 and 3 from the orbiter main busses. Emergency power down procedures have been used many times during shuttle processing, most often due to loss of orbiter cooling. The Emergency Power Down Procedure did not include the provision that the only functioning bus should be the last commanded off. Since it did not address this, there was no power available to drive the Fuel Cell 2 and 3 to main bus motor switches to the open (isolate) position. <br>During first shift and early second shift on August 12, 1991, there were reports of occasional alarms and noises, such as fans operating, emanating from the "powered down" orbiter. These reports, at first discounted, were investigated and determined to be resulting from the power generation of Fuel Cells 2 and 3 which were still connected to the Orbiter Main Bus B and Main Bus C respectively. The fuel cells were removed from the busses at approximately 8:34 p.m. EDT August 12, 1991 by use of ground power through the Orbiter ground umbilicals.