scope:

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.

To streamline RPOV seat assembly fabrication, prototype seat assemblies were built using a hotforming procedure.

The hot-forming procedure consisted of the following main steps:

• Fabricating a slightly oversized polytetrafluoroethylene (PTFE) seal preform

• Installating a loosely assembled preform, retainer, and seat into the hot-forming fixture

• Heating of the entire assembly to 120-150 °C (250-300 °F) under load

• Removing the heated assembly from the oven, followed by interference fitting (squeezing) of the PTFE seal while cooling Prototype RPOV pilot seat assemblies were fabricated using scrapped POV metal parts and generic PTFE bar stock. Dimensional tolerances were also relaxed on the seal preform. This approach was thought to minimize fabrication complexity, maximize conformance to the seal cavity shape, and maximize dimensional stability of the seal over the expected thruster operating temperature range, which was well below the seal processing temperature.

Success during valve-level testing of prototype RPOV seat assemblies gave early confidence in the RPOV design concept and assembly techniques. Based on this success, flight-configuration RPOV seat assemblies were then fabricated. Unfortunately, seal-related problems not revealed by prototyping hampered attempts to fabricate viable flight-configuration RPOV pilot seat assemblies.

There were three instances where reduced level of design rigor during prototyping contributed to the difficulties during fabrication of flight-configuration hardware:

1. It is suspected that the two-piece retainer and tiny screws used for securing the retainer to the seat inadvertently increased the mechanical compliance of the seal cavity. This minimized seal squeeze and mechanical deformation that ultimately produced micro-cracking and recession of the seal in flight-configuration hardware.

2. The convenient use of uncontrolled PTFE bar stock for the prototype seals, instead of using a carefully researched PTFE grade based on consultation with materials and processing experts, delayed optimum material selection.

3. A lack of precise strain rate control during prototype hot-forming trials, along with a design change to the prototype hot-forming fixture to accommodate the planned retainer weld, contributed to a lack of process repeatability.