scope:

Description of Driving Event:

In order to mitigate seal extrusion-related failures in the Primary Reaction Control System (PRCS) pilot-operated valve (POV), an effort was undertaken to develop a redesigned POV (RPOV) pilot seat assembly. The POV controls the flow of hypergolic liquid propellants to the Space Shuttle Orbiter attitude control thrusters. Specifications and selection assistance from suppliers were found to be inadequate for choosing an appropriate grade of PTFE for seal material without further testing.

The two PTFE resins under consideration had similar properties per current material procurement specifications. For example, the dimensional stability, tensile strength, and percent elongation requirements were virtually the same. Meaningful properties such as dimensional stability under load (creep), retention of properties at higher operational temperatures, and chemical resistance to propellants were not covered. Resin selection design assistance from commercial resin suppliers for specific applications such as a thruster valve seal was found to be lacking. Resin candidates were also hard to distinguish by most laboratory tests, but each candidate was later found to exhibit unique physical and mechanical properties that made the difference between fabricating a viable versus nonviable seal.

Initial RPOV seal fabrication efforts focused on using a pre-sintered, free-flowing extrusion molding PTFE grade per AMS 3658 [1], sold under the trademark of Algoflon(r)-E2 [2]. Seals made from Algoflon-E2 exhibited significant splotchiness, microcracking, and fibrillation after the hot-forming assembly process and seal recession after valve testing. To resolve these issues, an as-sintered, nonfree- flowing compression molding PTFE per AMS 3660 [3], sold under the trademark of Teflon(r)- 7A [4], was used.

Given the limited utility of material procurement specifications, a squeeze test which allowed the compressive strain to be varied from 15 to 90 percent was devised to better differentiate between resins. Specimens made from the pre-sintered, free-flowing Algoflon-E2 resin produced a lower strength PTFE that was more susceptible to fracturing (Figure 2, bottom). Since Algoflon-E2 consists of coarse, agglomerated particles, fracturing was attributed to adhesive failure along resin particle boundaries. By comparison, a specimen made from the as-sintered, non-free-flowing resin gradually transitioned from fully opaque to translucent material without any evidence of fracturing or splotchiness.