Likelihood Assessment of Wire-to-Wire Shorts Causing Uncommanded RCS Firing while the Orbiter Is Docked at the ISS (PSAM-0019)


The Columbia Accident Investigation Board (CAIB) report had provided NASA with a set of comprehensive recommendations to improve the safety of the Space Shuttle Program. At the same time, NASA also pursued an Agency-wide effort to take a fresh look at all aspects of the Space Shuttle Program, from technical requirements to management processes, and developed a set of internally generated actions that complement the CAIB recommendations. The resulting implementation plan is the path that NASA took to safely return to flight. One “return to flight” technical issue is the potential hazard of an uncommanded reaction control system (RCS) firing while the Orbiter is docked at the International Space Station (ISS).

This paper shows a quantification of the likelihood of an uncommanded RCS firing because of wire failures while the Orbiter is docked at the ISS. The fault tree methodology is based on known failure modes and operational considerations that potentially can lead to damage. The failure modes in the model are connector pin-to-pin shorts, exposed wires due to vibration damage (chafing), heat damaged wires, arc faults (undetermined cause), resistance heating from degraded splices, undetected maintenance damage, and operational damage due to other operating causes like delamination, exposure to chemicals and elemental oxygen.

Quantification based on assumptions and limited anomaly data resulted in 1.4E-4 failures per ISS docking mission. The failure is judged to be an improbable event that is much smaller than the failure probabilities of other hazards to the Orbiter. Common cause failures are dominant as they affect multiple wires simultaneously. The two most likely failure modes are arc fault and chafing. The results of this study aided in the management of ‘return to flight’ risk by helping prioritize proposed RCS wiring upgrades with respect to other ‘return to flight’ safety issues.

  • Summary/Abstract
  • 1. Introduction
  • 2. Hazard Consequence
  • 3. Model Based Likelihood Assessment
  • 4. Later Work
  • 5. Discussion
  • Acknowledgments
  • References

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