Radio Frequency Mass Gauge Results from the Intuitive Machines Lunar Lander

A cryogenic propellant mass gauge known as the Radio Frequency Mass Gauge (RFMG) was integrated into the Intuitive Machines (IM) Nova-C lunar lander and provided an estimate of the liquid oxygen and liquid methane mass in the lander propellant tanks throughout the IM-1 mission. An RFMG controller was used to measure and record the spectrum of the RF signal reflected from an antenna sensor in each tank over the frequency range of 100–1300 MHz. The RF spectrum of each tank is unique and sensitive to the index of refraction of the propellant and the spatial distribution of liquid within the tank. Electromagnetic simulation software was used to simulate antenna response spectra for a given tank geometry, fluid properties, and liquid–vapor configurations within the tank. The simulated spectra served as a database against which measured tank spectra were compared. RFMG measurements were recorded during tank loading on the launch pad, during translunar coast, during lunar orbit insertion, during low lunar orbit, during powered descent to the lunar surface, and postlanding on the moon. This paper describes the RF and fluid simulations, the RFMG measurements and analysis of spectral data, the RFMG instrument, and the gauged results throughout all phases of the IM-1 mission.