Materials outgassing rate decay in vacuum at isothermal conditions
As a laboratory for scientific research, the International Space Station has been in Low Earth Orbit for over 17 years and is planned to be on-orbit for another 10 years. The ISS has been maintaining a relatively pristine contamination environment for science payloads. Materials outgassing induced contamination is currently the dominant source for sensitive surfaces on ISS and modelling the outgassing rate decay over a 20 to 30 year period is challenging. Using ASTM E 1559 rate data, materials outgassing is described herein as a diffusion-reaction process with the interface playing a key role. The observation of -1/2 (diffusion) or non-integers (reaction limited) as rate decay exponents for common ISS materials indicate classical reaction kinetics is unsatisfactory in modelling materials outgassing. Nonrandomness of reactant concentrations at the interface is the source of this deviation from classical reaction kinetics. A t-1/2 decay is adopted as the result of the correlation of the contaminant layer thicknesses and composition on returned ISS hardware, the existence of high outgassing silicone exhibiting near diffusion limited decay, the confirmation of nondepleted material after ten years in Low Earth Orbit, and a potential slowdown of long term materials outgassing kinetics due to silicone contaminants at the interface.
New Mathematical Functions for Vacuum System Analysis
A new bivariate function has been found that provides solutions of integrals having the form u (sup minus eta) e (sup u) du which arise when developing predictions for the behavior of pressure within a rigid volume under high vacuum conditions in the presence of venting as well as sources characterized by power law transient decay over the range [0,1] for eta and for u greater than or equal to 0. A few properties of the new function are explored in this work. For instance the eta equals 1/2 case reproduces the Dawson function. In addition, a slight variation of the solution technique reproduces the exponential integral for eta equals 1. The technique used to generate these functions leads to an approach for solving a more general class of nonlinear ordinary differential equations, with the potential for identifying other new functions that solve other integrals.
Characterization of Contamination Generation Characteristics of Satellite Materials
Abstract : The objective of this program was to develop a standard test method for measuring the kinetics of material outgassing and deposition of outgassing species. The program was divided into three phases. In Phase I the current state of outgassing and deposition measurement and modeling technology was reviewed and a test approach was selected for further development. The Phase I work is reported in AFWAL-TR-83-4126 Volumes I and II. In Phase II the selected method was developed and its measurement capability was critically evaluated. The Phase II work is reported in AFWAL-TR-85-4118. Following Phase II an apparatus capable of executing the test method was built under contractor funding. In the Phase II Extension the apparatus and test method were exercised and evaluated by generating a database for 20 typical spacecraft materials. This report reviews the work performed on Phases I and II, describes the test method and apparatus, and presents the data generated, under Phase II Extension. The materials tested included paints, film materials, adhesives, lubricants, and composites. Keywords: Spacecraft contamination; Contamination transport; Material outgassing; Spacecraft material screening; Material condensation; Contamination source kinetics; Reemission kinetics; Condensation kinetics; Outgassing tests; Reemission tests.