UVM Theses and Dissertations
Format:
Print
Author:
Kujawa, Jeffrey P.
Dept./Program:
Mechanical Engineering
Year:
2004
Degree:
M.S.
Abstract:
NASA and the Department of Defense agencies have expressed interest in using "nanosats," or satellites featuring a mass <20 kg for the next generation of next space science missions. The nanosats will be capable of operating in distributed networks not currently achievable with traditional architectures. As such, they will require unique propulsion systems to correct for small disturbances to ensure precise maneuverability. In fact, thrust on the order of mN and impulse bits of approximately 1-100 uN x s are expected. Micro-electromechanical systems (MEMS) techniques have been proposed to successfully develop a micro-thruster to accommodate the design thrust levels. Owing to the extremely small geometry, the flow is subjected to complex micro-scale effects. A prototype has been developed by the NASA Goddard Space Flight Center (GSFC), for which this thesis will discuss a numerical simulation of the nozzle region.
The research represents an independent and necessary extension to the work completed by NASA/GSFC. Specifically, the focus will be to (1) complete a numerical model of steady-state micro-nozzle operation, (2) determine the effects of heat transfer on the system, and (3) simulate transient micro-thruster shutdown. Two-dimensional continuum flow models are used in conjunction with the FLUENT6.1 computational fluid dynamics software to perform simulations of the flow and performance in a micro-nozzle based upon an existing MEMS thruster prototype developed at NASA/GSFC. Owing to the small length scales considered, viscous effects and external heat loss may significantly reduce thruster efficiency and performance. For a highly conductive silicon substrate, effects of heat loss can be represented by imposing isothermal walls. Our simulations indicate performance degradations as much as 3%.
Typical micro-thruster operation involves thrust throttling via a micro-valve, and so is inherently transient in nature. A two-dimensional numerical model has been developed to simulate micro-thruster shutoff by applying various decaying mass flow profiles at the inlet. Residual thrust during the. micro-valve operation could result in high attitude control uncertainties. The actuation time scale is consistent with micro-valve design concepts specified by NASA/GSFC. The transient boundary layer thickness is discussed and it is shown that the calculated impulse bit more than doubles some of the design specifications of the microthruster.
The research represents an independent and necessary extension to the work completed by NASA/GSFC. Specifically, the focus will be to (1) complete a numerical model of steady-state micro-nozzle operation, (2) determine the effects of heat transfer on the system, and (3) simulate transient micro-thruster shutdown. Two-dimensional continuum flow models are used in conjunction with the FLUENT6.1 computational fluid dynamics software to perform simulations of the flow and performance in a micro-nozzle based upon an existing MEMS thruster prototype developed at NASA/GSFC. Owing to the small length scales considered, viscous effects and external heat loss may significantly reduce thruster efficiency and performance. For a highly conductive silicon substrate, effects of heat loss can be represented by imposing isothermal walls. Our simulations indicate performance degradations as much as 3%.
Typical micro-thruster operation involves thrust throttling via a micro-valve, and so is inherently transient in nature. A two-dimensional numerical model has been developed to simulate micro-thruster shutoff by applying various decaying mass flow profiles at the inlet. Residual thrust during the. micro-valve operation could result in high attitude control uncertainties. The actuation time scale is consistent with micro-valve design concepts specified by NASA/GSFC. The transient boundary layer thickness is discussed and it is shown that the calculated impulse bit more than doubles some of the design specifications of the microthruster.