NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.


2004 MAPLD International Conference

Ronald Reagan Building and International Trade Center
Washington, D.C.

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.

SUMMARY

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%.

MISHAP REPORT

PRESENTATION: safer_main, safer_backup

Return to 2004 MAPLD Seminar: Aerospace Mishaps and Lessons Learned


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