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 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.

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.