Development of a mN level Meso-Scale Thruster

Abstract

This dissertation focuses on the practical application of heat recirculating combustors as thrust chambers for micro-spacecraft systems, including: design, development, stability and operational characteristics of the thruster in both the steady state and pulsed configurations. Stable combustion was realized with partially premixed methanol/steam/oxygen, non-premixed methanol/steam/oxygen, and nonpremixed kerosene/steam/oxygen. The steam oxygen mixture is a surrogate for the decomposition products of hydrogen peroxide. The effect of channel geometry on the stability and thermal performance has also been conducted in addition to qualitative and quantitative comparisons of fuel/oxidizer injection configurations. In general it was found that non-premixed combustion is favorable in terms of both thermal performance and flame stability due to the predictable extinction characteristics at low flow rates and the absence of lean blow off at high flow rates. A quantitative extinction criterion was developed to predict extinction at the rich extinction limit. Additionally, nozzle discharge characteristics at low Reynolds number were studied and a correlation developed to predict the discharge coefficient from the Reynolds number for both cold and hot flow scenarios. It was found the discharge coefficient decays more rapidly for high temperature flows than low temperature flows due to the effects of temperature and viscosity on the boundary layer displacement thickness. Additionally, a milli-Newton level thrust stand was developed to indirectly measure the thrust level without allowing the thruster to translate, the thrust stand resolution was found to be <1mN. Using this device a study of the thrust characteristics was carried out in both the steady state and pulsed modes. Measurements of the specific impulse efficiency indicate that the conversion efficiency is high and any loss in thermal efficiency from the adiabatic scenario is due to wall heat losses and not incomplete combustion. Experiments conducted with hydrogen peroxide decomposed in the inlet channel of the combustor were used to validate the results taken with the steam/oxygen as the oxidizer and demonstrated that heat recirculation from the products to the exhaust is sufficient to promote efficient decomposition of the hydrogen peroxide.