The MPG project is a collaboration between Carthage Space Sciences and Kennedy Space Center Cryogenics Laboratory to develop a high-resolution low-gravity fuel gauging technology by exploiting the effect of fluid loading on the structural properties of liquid-filled propellant tanks.
Conventional methods of measuring propellant volume are ineffective in microgravity conditions. Instead of using direct buoyancy based techniques, spacecraft must rely on indirect techniques such as equation of state methods and burn-time integration. Indirect methods often require additional hardware to pressurize tanks and are inaccurate at lower fuel levels. Microgravity causes fluid sloshing within the tanks, further complicating the estimate of propellant volume. We present a non-invasive, indirect fuel gauging technique based on real- time measurements of the tank’s resonant acoustic modes.
Experimental Modal Analysis (EMA) involves recording the vibration spectrum of a solid object and using the spectral characteristics to infer the structural properties of the object. The EMA technique requires that we stimulate all acoustic resonances in the tank structure by applying a broadband white noise signal. Tank response is measured at discrete locations on the surface of the tank and the Fourier Transforms of the response and input functions are computed in real- time. Finally, the Frequency Response Function (FRF) for each structural response is computed by essentially taking the ratio of the response FFT to the input FFT. The resulting FRF is a complex-valued function in which the real part is a function of the effective mass of the vibrating object and the imaginary part is related to the rate of energy dissipation through structural damping. Adding fluid to a tank lowers the real part of the frequency by increasing the effective mass. In addition, fluid in a closed tank changes the imaginary part of the frequency by damping the vibrations.
Carthage College’s Microgravity Team has been developing this method since a 2011 reduced gravity flight campaign under the NASA Systems Engineering Education Discovery (SEED) program. In 2012, the SEED team demonstrated that EMA is a promising fuel gauging method with a resolution of 10% of the total volume of the tank in the case of unsettled, sloshing liquid. Carthage’s 2013 RockSat-C team further tested the precision of this technique with an experiment on a sounding rocket launched from Wallops Flight Facility: the maximum resolution achieved was 1.5% in volume for fill fractions from 30% to 70% and 7.4% in volume between 0% and 20% fill fractions. A 2014 USIP/FOP flight campaign demonstrated a liquid volume resolution of 2.0% for fill fractions between 30% and 70% of total tank volume (unsettled, zero- g measurements). The 2015 NASA/FOP flight campaign achieved resolutions of 1.5% for sloshing propellant simulants at fill fractions between 20% and 60%.