Computational results are presented for the flow through a helium microthruster. This device is to be used for fine adjustments in attitude control for a proposed space experiment. The mass-flow rates used by the thruster are very low giving Knudsen numbers at the nozzle throat between 0.01 and 1 based on the stagnation conditions and the nozzle throat diameter. These conditions indicate that low-density effects will dominate the fluid mechanics. Therefore, the flows are computed with a particle simulation scheme [the direct simulation Monte Carlo method (DSMC)]. This study presents an application of the DSMC technique to the complete expansion process of a real thruster: from the stagnation chamber of the thruster, to the far-field expansion of the plume. The numerical approach is evaluated by comparison with existing experimental data taken in the expansion plume. The computational results are employed to assess the effect of varying the mass-flow rate on the terminal state of the gas. In addition, the effect of including the background chamber pressure measured in the experimental vacuum facility is investigated and found to be significant. KM) P Nomenclature= Mach number at nozzle exit = mass-flow rate = background pressure in vacuum tank = Reynolds number at the nozzle throat = radial distance from the nozzle exit = flow velocity = axial distance from the nozzle throat -ratio of specific heats = flow angle = nozzle exit half-angle = maximum plume turning angle = Prandtl-Meyer expansion angle at Mach number M = density

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Gravity Probe-B and the Satellite Test of the Equivalence Principle are two proposed experiments in basic physics which will utilize drag-free satellites equipped with proportional helium thrusters. In order to calibrate the thrust systems for precise aeronomic measurements, it is necessary to model the effects of thruster plume impingement. Conventional plume models are invalid, owing to the high degree of rarefaction in the nozzles (Kn=0.01–1.1). An experiment was devised to measure the plume angular mass flux distribution using a helium mass spectrometer. The results suggest that the plume shapes are unchanged with mass flows around the nominal, and are generally wider than for conventional spacecraft control jets. A narrowing effect was observed at very low mass flows, in qualitative agreement with Monte Carlo results from the literature. The continuum model for the plume was found to be surprisingly accurate at nominal mass flows, and the inverse-square law was demonstrated to be a valid description of the far-field radial density profile.

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