The expansion of argon from microtubes into hard vacuum has been extensively investigated using a three dimensional unstructured DSMC code. The simulation results for cases with an aspect ratio of 1.5 have been shown to compare well with theoretical formulations of free jet expansion. The discrepancies between the theoretical formulation and the DSMC results have been found to increase with increasing Knudsen number. DSMC simulations have been used to investigate the effects of Knudsen number, aspect ratio, Reynolds number and microtube scale on plume structure. The plume shape has been found to narrow with increasing Knudsen number, increasing aspect ratio and decreasing Reynolds number. The relative number density drop along the flow axis has been found to decrease with increasing Knudsen number and Reynolds number.

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