Lessons Learned During Redesign of Shuttle Reaction Control Thruster Pilot Seat Assembly

摘要

Current generation Space Shuttle Orbiter primary reaction control subsystem thrusters have been plagued by problems associated with oxidizer leakage, internal corrosion, and fuel valve pilot seal extrusion. In an effort to fix these problems, and thus improve thruster reliability and reduce life-cycle costs, the pilot-operated valve on the thruster was redesigned. Efforts to improve self-flushing characteristics and minimize oxidizer-induced corrosion within the pilot-operated valve were largely successful; however, a variety of problems stemming from the redesign of the pilot seat assembly within the valve were encountered. The lessons learned from the design, materials selection, and fabrication of the pilot seat assembly are addressed. For example, maximizing the likeness of prototype to final parts and exercising rigorous process control during seal fabrication were found to facilitate the transition from concept to finished hardware. Selection of a suitable polytetrafluoroethylene resin was found to be a predetermining factor in fabrication of viable pilot seals with acceptable mechanical properties. Last, use of a correctly sized and shaped pilot seal preform was found to be essential in preventing excessive and unbalanced stress within the seal during fabrication and in minimizing seal extrusion or recession after fabrication.