Experimental and numerical determination of micropropulsion device efficiencies at low Reynolds numbers

The need for low-thrust propulsion systems for maneuvers on micro- and nanospacecraft is growing. Low-thrust characteristics generally lead to low-Reynolds-number flows from propulsive devices that utilize nozzle expansions. Low-Reynolds-number flows of helium and nitrogen through a small conical nozzle and a thin-walled orifice have been investigated both numerically, using the direct simulation Monte Carlo technique, and experimentally, using a nano-Newton thrust stand. For throat Reynolds number less than 100, the nozzle-to-orifice thrust ratio is less than unity; however, the corresponding ratio of specific impulse remains greater than one for the Reynolds number range from 0.02 to 200. Once the direct simulation Monte Carlo model results were verified using experimental thrust and mass flow data, the model was used to investigate the effects of geometrical variations on the conical nozzle's performance. At low Reynolds numbers, improvements to the specific impulse on the order of 4-8% were achieved through a combination of decreasing the nozzle length and increasing the nozzle expansion angle relative to the nominal experimental geometry.