Description of Driving Event:
Leaking Reaction Control System (RCS) Thruster Valves
Lesson(s) Learned:
The orbiter Reaction Control System (RCS) thruster valves continue to leak in flight. NASA has aggressively attacked this problem with some success. Procedural changes have improved thruster reliability, and the incidence of leakage has been reduced but not eliminated.
Recommendation(s):
Continued attention must be focused on the elimination of the root causes of RCS valve leakage/failures.
Evidence of Recurrence Control Effectiveness:
Several remedial actions have been and are being implemented as a result of a 1995 tiger team investigation into the causes of RCS valve leakage/failure. This has resulted in many procedural changes and several potential hardware improvement concepts. The procedural changes are reducing the number of in-flight thruster valve failures. Many of the hardware improvements are entering a development testing phase. Examples of procedure and hardware changes include: - Preventative maintenance flushing. Water flushing of the RCS pilot operated valves (POV) was developed at the White Sands Test Facility (WSTF) and has been verified to remove iron nitrate contamination from the POV.
- Manifold thruster preventative change-outs. It is now required to change all thrusters on a manifold when that manifold is drained for any reason. The removed thrusters are sent to WSTF for water flushing and then returned to spares.
- Hard-filled manifold processing. The POV has demonstrated substantially better sealing capability in a fully wetted, hard-filled condition. KSC procedures have been implemented that provide the ability to maintain a hard-filled manifold configuration when performing work on the RCS pods.
- Minimization of moisture intrusion. Several additional recommendations have been implemented that specifically address minimizing moisture intrusion into the propellant system.
- Redesigned POV (RPOV). Another result of the critical examination of the RCS POV failure history was the recommendation that the oxidizer POV be redesigned to improve the ability to withstand nitrate contamination. The resulting RPOV is proceeding through buildup and development testing. The RPOV design addresses the areas in the current valve that are known to be sensitive to nitrate contamination.
- Minimization of oxidizer moisture and iron content. The presence of iron and water in nitrogen tetroxide greatly increases the potential for precipitation of iron nitrate internal to the POV pilot cavity. Therefore, a molecular sieve is being designed and fabricated to reduce the levels of iron and water in nitrogen tetroxide.
Documents Related to Lesson:
N/A
Mission Directorate(s):
- Space Operations
- Exploration Systems
Additional Key Phrase(s):
- Aerospace Safety Advisory Panel
- Ground Operations
- Hazardous/Toxic Waste/Materials
- Logistics
- Research & Development
- Safety & Mission Assurance
- Spacecraft
- Test & Verification
Additional Info:
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