Modal Propellant Gauging in Low Gravity.

We present results from parabolic flight testing of a low-gravity propellant gauging technology that relies on detection of resonant acoustic mode shifts due to changes in the tank's effective mass as liquid propellant is removed from the tank. The modal propellant gauging (MPG) method is shown to be (1) relatively robust against sloshing, (2) of comparable effective resolution to existing gauging methods for settled propellant, and (3) particularly robust at low fill-fractions where conventional gauging methods are known to be inaccurate. Lab (1-g) measurements of gauging resolution show no more than a 1.0% error for fill-fractions between 10-50% of total tank volume. Data for sloshing liquids in zero-gravity suggest an error of no greater than 1.5% over the same range of fill fractions. An algorithm for the real-time identification of modal response behaviors in zero-g is presented and demonstrated to accurately associate zero-g frequency response functions with the equivalent 1-g FRF, allowing for the automated assignment of fill levels. Gauging applications for small satellite missions and for propellant transfer operations in orbital fuel depot concepts are considered.