scope:

Description of Driving Event:

In order to mitigate pilot seal extrusion-related failures of the Primary Reaction Control System (PRCS) pilot-operated valve (POV), effort were made 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. During the redesign, specific factors were found to be essential in fabricating a dimensionally stable pilot seal.

Seal Extrusion. Seal extrusion (Figure 1) is caused by thermal expansion mismatch between adjacent PTFE and metal parts in the POV or RPOV, and is potentially aggravated by:

• excessive internal stress or internal stress gradients in as-fabricated pilot seals

• low seal exit area relative to entrapped seal volume

• thermal cycling during seat assembly (e.g., retainer welding)

• vacuum bake-outs during periodic thruster maintenance

• heat soak-back after thruster firing, especially multiple sequential burns or long burns

• any presence of oxidizer vapor close to a fuel pilot seal

Data on semi-trapped PTFE specimens simulating a POV seal configuration show that extrusion is:

• incremental and irreversible

• increases with the size of the thermal excursion

• decreases with successive thermal cycling (stress relief)

• is accompanied by gap formation in the seal cavity

The above factors must be considered when developing a viable pilot seal.