scope:

Description of Driving Event:

The RCS primary thruster throat ferry flight plug assembly is used to prevent moisture intrusion during the orbiter's ferry operations. This assembly is installed after the orbiter lands and is removed during deservicing operations. After deservicing, the primary engine cover is used during orbiter processing to control and/or prevent moisture. This cover has a butyl rubber o-ring which acts as a moisture seal, but is incompatible with N2O4. Vapor leakage could cause deterioration of the o-ring, severely limiting the effectiveness of the thruster ferry plug to seal out moisture, as well as contaminating the thruster nozzle.