Lesson Info:

• Lesson Number: 1037
• Lesson Date: 2001-08-22
• Submitting Organization: WSTF
• Submitted by: Jess Waller

Subject:

Description of Driving Event:

The Primary Reaction Control System (PRCS) pilot-operated valve (POV) controls the flow of hypergolic liquid propellants nitrogen tetroxide (oxidizer) and monomethylhydrazine (fuel) to the Space Shuttle Orbiter attitude control thrusters. The POV has experienced numerous in-flight and ground turnaround failures, including leakage and failure to open upon command. Corrosion in the oxidizer valves and seal extrusion in the fuel valves have been implicated in most failures. The desire to mitigate corrosion and seal extrusion led to an effort to develop a redesigned POV (RPOV) pilot seat assembly.

Machining of polytetrafluoroethylene (PTFE) pre-forms for the RPOV pilot seals was found to be a demanding art requiring special tooling, support equipment, and practice. For example, the need to machine PTFE to close tolerances can be thwarted by slight temperature changes. Given the small size of the machined parts (Figure 1) and the close tolerances required (Figure 2), it was essential to see and accurately measure the article being machined.

[D]

Figure 1. RPOV seat assembly piece-parts (Left: MP35N seat, Middle: PTFE preform, Right: MP35N retainer)

[D]

Figure 2. Final RPOV seat assembly design configuration

Also, proper cutting tool size, shape, material, cutting angle, nose radius, sharpening technique, cutting speed, and coolant were important considerations in obtaining desired tolerances and surface finish.

Lesson(s) Learned:

Machining of polymers such as polytetrafluoroethylene (PTFE) used in the fabrication of small piece-parts made to close tolerances presents special challenges.

Recommendation(s):

Appropriate experience and tooling must be sought, and ample time allocated for practice before viable seal preforms can be machined. For example, machining of PTFE to close tolerances using a conventional lathe required spraying isopropyl alcohol (IPA) directly on the PTFE during preform fabrication and final trimming to create the final seal profile. Implementation of established machining guidelines for PFTE was also found to be essential [1]. Accurate machining of small RPOV piece-parts was further assisted by the use of a video microscope and monitor system with "in-process" measurement and inspection capability.

Reference:

1. Product Literature. Teflon PTFE Properties Handbook, DuPont and Nemours and Company, Specialty Polymers, Wilmington, DE, November 1992.

Evidence of Recurrence Control Effectiveness:

Documents Related to Lesson:

Mission Directorate(s):

• Exploration Systems
• Aeronautics Research

Additional Key Phrase(s):

• Hardware
• Industrial Operations
• Parts Materials & Processes
• Test & Verification

Additional Info:

Approval Info:

• Approval Date: 2001-12-11
• Approval Name: Jan Jungewaelter
• Approval Organization: WSTF
• Approval Phone Number: 505-524-5161