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Microgravity Propellant Gauging Using Modal Analysis: Phase II

Completed Technology Project

Project Introduction

Microgravity Propellant Gauging Using Modal Analysis: Phase II
The Modal Propellant Gauging (MPG) experiment is designed to assess the mass gauging resolution of a novel implementation of experimental modal analysis (EMA). The central objectives of the MPG experiment are to (1) record the modal response of a model propellant tank at different fill-levels under unsettled, microgravity conditions, and (2) record the modal response of the propellant tank during simulated propellant transfer Phase II of the Modal Propellant Gauging (MPG) project will assess the gauging resolution of the MPG technique at low fill fractions for unsettled, sloshing propellant simulant. Phase II will utilize a full-fidelity propellant tank with flight heritage and will extend the gauging analysis to accommodate changes in tank pressure during low-gravity propellant transfer operations. Finally, Phase II will allow for the development and testing of slosh measurement and automated uncertainty assignments to gauging estimates The EMA technique is an inexpensive, non-invasive and computationally robust method of assessing propellant mass through its effect on structural resonance modes in the propellant tank. EMA is a computational technique that extracts the resonant modes of the structure from real-time vibrational data obtained from sensors affixed to the structure. The natural resonances of a structure depend on the stiffness and the effective mass of the structure, and so the EMA technique can be used to determine the effective mass of the propellant tank. The effective mass of the tank is, in turn, dependent on the mass loading of the structure by its liquid contents. EMA uses the shifts in resonant frequencies of the tank as liquid level changes to calculate the change in effective mass of the tank and thereby the change in liquid mass Propellant is the primary life-limiting resource on spacecraft. Accurate gauging of propellant volume in a microgravity environment has been identified by the NASA Exploration Systems Architecture Study (ESAS) final report as an area requiring significant further development. The microgravity environment renders direct volume measurement using traditional buoyancy- and level-based techniques ineffective. Instead, indirect methods are currently used to establish propellant volume. These methods incur considerable additional mass and expense to mission architectures. Additionally, traditional methods of propellant gauging suffer from decreasing accuracy as the tank empties, so that mass-gauging accuracy is lowest at the end of mission life when gauging accuracy is most important. More »

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