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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 linked to most of these failures. The desire to mitigate corrosion and seal extrusion led to an effort to develop a redesigned POV (RPOV) pilot seat assembly.

Preform Size. Semitrapped PTFE seals, such as the PRCS POV pilot seal, will undergo coldflow during service [1]. This phenomenon can help to replenish lost material at a sealing interface or heal seal imperfections [2]. However, if the preform used during fabrication is too small, gaps may form in the seal cavity during fabrication that will cause the seal to recede into the seal cavity during qualification testing or service, causing loss of seal-to-poppet contact and ultimately leakage. Conversely, if the preform is too large, the compressive yield point of the PTFE can be exceeded during interference fitting of adjacent metal and plastic parts, resulting in irreversible plastic deformation and associated property loss. In general, the compressive yield strain of thermoplastic polymers such as PTFE is of the order of several percent. The interference fit used during initial attempts to fabricate a RPOV pilot seal (17 to 26 percent) exceeded the yield point of PTFE by a factor of about 5 to 7, resulting in the formation of translucent PTFE in regions of highest internal stress (Figure 1).

By comparison, the interference fit used in POV and final RPOV pilot seal fabrication (7 percent) exceeded the yield point of PTFE by a factor of about 2.

The use of an overly large seal preform can have another adverse consequence. Excessively oversized preforms give rise to higher levels internal stress within semitrapped parts. This frozen-instress will relieve over time or during thermal cycling, and could be manifested as unacceptable extrusion during service.

Preform Shape. If the preform is shaped incorrectly relative to the metal cavity dimensions (producing non-uniform squeeze within the seal), an internal stress gradient within the semitrapped PTFE seal can result. Depending on the magnitude and direction of the gradient, either recession or extrusion of the seal could result. The use of correctly shaped seal preforms that minimize the squeeze gradient within the seal is recommended. This could be accomplished in the RPOV by maintaining equivalent radial and axial seal squeezes during fabrication.

Exit Area/Volume Ratio. A lower seal exit area/entrapped seal volume (A/V) ratio was present in the initial and final RPOV designs (A/V = 0.22 mm-1) compared to original POV design (A/V = 0.35 mm-1). A differential expansion model for an annular, semitrapped seal like that in a POV configuration showed that twofold reduction in the A/V ratio will result in double the extrusion during heating [3]. Therefore, the highest practical A/V ratio should be used in semitrapped seal design.

Unoptimized Design Consequences. Evidence of undesirable structural and morphological changes was obtained during initial RPOV seal fabrication. Machined PTFE seals had a mottled, splotchy appearance in areas where the greatest amount of seal squeeze had occurred. This material was especially prone to fracturing and fibrillation during trimming. Subsequent sectioning of the seat assembly revealed PTFE material with a translucent appearance in the region of highest interference fit, while the remaining material in the seal retained a normal opaque appearance (Figure 1). One factor thought to be responsible for the formation of the translucent material was excessive shearing accompanied by irreversible property loss. Evidence of irreversible property loss was supported by decreasing Shore durometer hardness of PTFE test coupons with increasing squeeze (percent strain) (Figure 2). Hardness reductions in semicrystalline thermoplastics such as PTFE are generally regarded as indicative of irreversible deformation modes within crystalline lamellae involving chain slip and tilt at lower strains, followed by lamellar shear and crystal destruction at larger strains [4].