Engineering topics
Publications and source records attributed to Rawls, J. Richard.
The Space Transportation System (STS) orbiters are known to be significant sources of outgassing in low earth orbit (LEO). Infrared and mass spectra of residues and outgassing from orbiter thermal protection tile and an external blanket are presented. Several sources of methyl and phenyl methyl silicones are identified. About fifty pounds of silicones are estimated to be outgassed during an STS mission.
Inadequacy of mass loss cleanliness criteria for selection of materials for contamination sensitive uses, and processing of flight hardware for contamination sensitive instruments is discussed. Materials selection for flight hardware is usually based on mass loss (ASTM E-595). However, flight hardware cleanliness (MIL 1246A) is a surface cleanliness assessment. It is possible for materials (e.g. Sil-Pad 2000) to pass ASTM E-595 and fail MIL 1246A class A by orders of magnitude. Conversely, it is possible for small amounts of nonconforming material (Huma-Seal conformal coating) to not present significant cleanliness problems to an optical flight instrument. Effective cleaning (precleaning, precision cleaning, and ultra cleaning) and cleanliness verification are essential for contamination sensitive flight instruments. Polish cleaning of hardware, e.g. vacuum baking for vacuum applications, and storage of clean hardware, e.g. laser optics, is discussed. Silicone materials present special concerns for use in space because of the rapid conversion of the outgassed residues to glass by solar ultraviolet radiation and/or atomic oxygen. Non ozone depleting solvent cleaning and institutional support for cleaning and certification are also discussed.