Acoustic detection of the liquid–vapor interface in settled propellant tanks

This study presents a vibration-based method for detecting the liquid–vapor interface in settled propellant tanks. A theoretical framework is introduced to predict the vibrational response of patch sensors mounted on the tank wall and actuated by a broad-band acoustic excitation. The theory predicts the form of response transitions between wetted and dry conditions as the interface moves past the sensor location. Relationships between the instantaneous location of the liquid–vapor interface and the measured root-mean-square (RMS) response signal are established in a scale-free manner that does not depend on physical properties of the tank or fluid. A computational model, implementing a modified Euler–Bernoulli beam approach, is developed to simulate sensor response, with the interface treated as a boundary between two distinct mass densities on the beam. Experimental results, obtained from a model propellant tank, show that the liquid–vapor interface is detected with an average error in the full tank volume estimate of 0.7% across the measurement range from 4% to 90% fill. These findings demonstrate a promising non-invasive technique for liquid–vapor interface detection in settled conditions, with potential applications in spacecraft propellant management and in-space refueling systems following appropriate engineering qualification for the space environment.