Facility Effects on Performance Measurements of Micropropulsion Systems that Utilize Gas Expansion

Abstract

Facilities can affect the measurement of spacecraft propulsion system performance in a variety of ways. In this study an underexpanded freejet is used to simulate experimentally a micropropulsion system such as those thatoperate on the expansion of propellant gases through nozzle geometries. Facility effects on the thrust measured by a torsion thrust stand installed in two facilities with distinctly different characteristics have been quantified over a steady-state thrust range from 10 to 480 μN. The two facilities varied in dimension and pumping capacity with background pressures ranging from 10 - 6 to 10 - 4 torr. In both facilities the measured thrust decreased with increasing facility background pressure. At a thrust level of 10 μN, the thrust decreased by approximately 20% at a facility background pressure of 2 x 10 - 4 torr relative to the thrust measured at 2 x 10 - 6 torr; however, a similar background pressure only resulted in a 4% reduction in thrust at 120 μN. The larger percent decrease in measured thrust for a given background pressure in the low thrust range has implications for the design of micropropulsion test facilities. The facility background pressure effect contributes to less than 2% error for the thrust range from 10 to 480 μN at background pressures less than 10 - 5 torr, suggesting that low background pressures are necessary for accurate thrust measurements below 500 μN. An empirical formulation based on background-plume penetration theory provides estimates of the thrust degradation as a function of facility background pressure. The empirical model fits the experimental data reasonably well for jet centerline values of the background gas radius of penetration into the freejet plume.