Propellant Quantity Gauging Using the Radio Frequency Mass Gauge

The Radio Frequency Mass Gauge (RFMG) is a novel propellant quantity gauging technology being developed at NASA for gauging cryogenic propellant tanks in low-gravity. The RFMG operates by sensing the natural electromagnetic modes of a tank, comparing several mode frequencies with a database of calculated mode frequencies, and finding a best match at a particular fill ratio which is then reported as the gauged liquid quantity. Here, we summarize recent test results in liquid oxygen and liquid hydrogen, and present a sample of test data acquired during a low-gravity aircraft campaign at lunar and low-gravity flight levels. Liquid oxygen and liquid hydrogen testing was conducted in a normal, 1g settled liquid configuration to assess the gauging accuracy of the RFMG as compared to a reference weighing system. These cryogenic tests indicate an RFMG gauging uncertainty of approximately ±1% of full-scale. Testing of the RFMG on a low-g aircraft using an inert fluid shows very good performance at lunar-g flight levels, and approximately ±6% of fullscale gauging uncertainty at low-g flight levels. Changes in the tank RF spectra during the low-g parabolic flights indicate significant fluid motion inside the tank. Despite the higher uncertainty at low-g flight levels, averaging of the RFMG output data during the low-g portion of the aircraft flight shows very good agreement between the RFMG gauged fill level and the actual fill level.