Monte Carlo simulation for backscatter of outgassing molecules from simple spacecraft surfaces

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A computer program has been developed for the Monte Carlo simulation of molecular flux impingement on simple spacecraft surfaces due to self- and ambient scattering of outgassing molecules. The outgassing and ambient molecules are assumed to be elastic spheres with a Maxwellian velocity distribution. Binary elastic collision processes have been adopted in a hard-sphere collision model. A test-particle Monte Carlo methodology is used to simulate sequentially the motions, collisions, and trajectories of the outgassing molecules inside a control volume by means of uniformly distributed random numbers. A unique Monte Carlo computational scheme has been developed that reduces the computer processing time by three to four orders of magnitude for a typical outgassing problem without introducing additional assumptions or compromising the solution accuracy. Monte Carlo results are obtained in the form of backscatter flux distribution and return flux ratios, for various sample sizes, controlvolume sizes, and outgassing and ambient flow conditions. The CPU time for a typical run is about 2 to 4 min on a VAX-8700 computer, and less than about 2 to 4 s on a Cray X-MP system.