Ronald Reagan Building and International Trade Center
September 8-10, 2004
STS-86 USA Simplified Aid for EVA Rescue (SAFER) Failure
MODULE LEADER: - Keith E. Van Tassel , Group Lead, NASA JSC Pyrotechnics
MISHAP DATE: October 1,1997
FAILURE AND MAIN CONTRIBUTING FACTOR
The NSI in the SAFER (serial #1005) did not fire. Therefore, the pyrotechnic propellant isolation valve did not open and nitrogen gas was not sent to the SAFER’S thrusters.
The NSI did not fire because there was a change in the NSI resistance as the NSI “fire” current pulse was applied to the NSI by the avionics circuit. This caused the NSI “fire” current level (designed at 4.1 amps) to drop (to 2.8 amps) below the “all fire” (3.5 amps) NSI current specification. The NSI resistance was measured at 1.09 ohms before installation into the SAFER. During application of the “fire” pulse, the resistance changed due to “ohmic heating” to approximately 1.6 ohms. The change in resistance caused the 4.1 amp NSI “fire” pulse to drop to 2.8 amps because of the avionics circuit constant voltage design. At 2.8 amps, the probability of firing the NSI is approximately 60%.
PRESENTATION: safer_main, safer_backup
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