Modeling and Parameter Estimation of Spacecraft Lateral Fuel Slosh
Author: Yadira Rodriguez Chatman Title: Modeling and Parameter Estimation of Spacecraft Lateral Fuel Slosh Institution: Embry-Riddle Aeronautical University Degree: Master of Science in Aerospace Engineering Year: 2008 Predicting the effect of fuel slosh on spacecraft and launch vehicle attitude control systems has been a very important and challenging task and has been the subject of considerable research over the past years. Analytic determination of the slosh analog parameters has been met with mixed success and is made more difficult by the introduction of propellant management devices such as elastomeric diaphragms. The experimental set-up in this research incorporates a diaphragm in a simulated spacecraft fuel tank subjected to lateral slosh behavior. This research focuses on the parameter estimation of a SimMechanics model of the simulated spacecraft propellant tank with diaphragms using lateral fuel slosh experiment data. An experimental investigation was conducted to determine and measure the slosh forces response of free surface slosh and diaphragms in an eight inch diameter spherical tank. The lateral slosh testing consisted of the tank assembly partially filled with different liquids, for other tests, diaphragms were incorporated into the tank. The experiment results from different testing conditions were compared for estimation of unknown parameter characteristics that include the pendulum model stiffness constants and damping coefficients.
Modeling and Parameter Estimation of Spacecraft Fuel Slosh with Diaphragms Using Pendulum Analogs
Prediction and control of liquid slosh in moving containers is an important consideration in the design of spacecraft and launch vehicle control systems. Even with modern computing systems, CFD type simulations are not fast enough to allow for large scale Monte Carlo analyses of spacecraft and launch vehicle dynamic behavior with slosh included. It is still desirable to use some type of simplified mechanical analog for the slosh to shorten computation time. Analytic determination of the slosh analog parameters has met with mixed success and is made even more difficult by the introduction of propellant management devices such as elastomeric diaphragms. By subjecting full-sized fuel tanks with actual flight fuel loads to motion similar to that experienced in flight and measuring the forces experienced by the tanks, these parameters can be determined experimentally. Currently, the identification of the model parameters is a laborious trial-and-error process in which the hand-derived equations of motion for the mechanical analog are evaluated and their results compared with the experimental results. This paper will describe efforts by the university component of a team comprised of NASA's Launch Services Program, Embry Riddle Aeronautical University, Southwest Research Institute and Hubert Astronautics to improve the accuracy and efficiency of modeling techniques used to predict these types of motions. Of particular interest is the effect of diaphragms and bladders on the slosh dynamics and how best to model these devices. The previous research was an effort to automate the process of slosh model parameter identification using a MATLAB/SimMechanics-based computer simulation. These results are the first step in applying the same computer estimation to a full-size tank and vehicle propulsion system. The introduction of diaphragms to this experimental set-up will aid in a better and more complete prediction of fuel slosh characteristics and behavior. Automating the parameter identification process will save time and thus allow earlier identification of potential vehicle performance problems.
Parameter Estimation of Spacecraft Fuel Slosh
The nutation (wobble) of a spinning spacecraft in the presence of energy dissipation is a well - known problem in dynamics and is of particular concern for space missions. The nutation of a spacecraft spinning about its minor axis typically grows exponentially, with the rate of growth being characterized by a Nutation Time Constant (NTC). For launch vehicles using spin -stabilized upper stages, fuel slosh in the spacecraft propellant tanks is usually the primary source of energy dissipation. For analytical prediction of the NTC this fuel slosh is often modeled using simple mechanical analogs such as pendulums or rigid rotors coupled to the spacecraft. Identifying model parameter values which adequately represen t the sloshing dynamics is the most important step in obtaining an accurate NTC estimate. Analytic determination of the slosh model parameters has met with mixed success and is made even more difficult by the introduction of propellant management devices and elastomeric diaphragms. By subjecting full -sized fuel tanks with actual flight fuel loads to motion similar to that experienced in flight and measuring the forces experienced by the tanks these parameters can be determined experimentally. Currently, the identification of the model parameters is a laborious trial -and -error process in which the equations of motion for the mechanical analog are hand -derived, evaluated, and their results are compared with the experimental results. The current research is an effort to automate the process of slosh model parameter identification using a MATLAB/SimMechanics -based computer simulation. Two different parameter estimation and optimization approaches are being evaluated and compared in order to arrive at a relia ble and effective parameter identification process. The first approach is conducted using Newton's method for nonlinear least squares. The second estimation method is a "black box" approach using MATLAB's Parameter Estimation Toolbox. To evaluate each pa rameter identification approach, a simple one -degree -of -freedom pendulum experiment is constructed and motion is induced using an electric motor. By applying an estimation approach to a simple system with known characteristics, its effectiveness and accur acy can be evaluated. The same experimental setup can then be used with fluid -filled tanks to further evaluate the effectiveness of the process. Ultimately, the proven process can be applied to the full -sized spinning experimental setup to quickly and a ccurately determine the slosh model parameters for a particular spacecraft mission. Automating the parameter identification process will save time and thus allow earlier identification of potential vehicle performance problems. This, in turn, can reduce the cost and schedule penalty associated with needed design changes.