Propellant Quantity Gauging in Microgravity Using Radio Frequency Tank Modes

The Radio Frequency Mass Gauge (RFMG) is a novel propellant quantity gauging technique developed at NASA for the purpose of gauging cryogenic propellant tanks in low-gravity. The RFMG operates by sensing several resonant electromagnetic modes of a tank and comparing the measured tank mode frequencies to a lookup table of results from several thousand numerical simulations. The numerical simulations are performed in advance in order to predict the electromagnetic eigenmode frequencies at different propellant fill levels, liquid configurations, and temperatures. A best match between measured and simulated eigenmode frequencies is used to gauge the fluid mass inside the tank. An RFMG instrument flew on the International Space Station (ISS) and was used to gauge the mass of liquid methane in a 50 L tank as part of the Robotic Refueling Mission 3 (RRM3) payload operations. Shifts to the measured tank RF spectra during the mission indicated significant changes to the fluid configuration as a result of various ISS maneuvers and payload tests. During the four months of RRM3 cryogenic payload operations the RFMG produced a mean gauged mass of 19.0 kg, in agreement with the expected value, with a one-sigma error distribution of ±2% of the full-scale mass.