Ultrasonic three-dimensional reconstruction and measurement technology for complex propellant free surfaces

In microgravity, the free-surface morphology of propellants inside metallic tanks becomes highly complex, while conventional optical and capacitive measurement methods face significant limitations: optical techniques cannot penetrate opaque metal walls, and capacitive sensors are easily affected by wall conductivity. Ultrasonic techniques offer a promising alternative; however, most existing approaches are confined to two-dimensional planar detection and are inadequate for reconstructing intricate three-dimensional (3D) free-surface morphologies. To address this challenge, we propose a three-dimensional reconstruction method for complex propellant free surfaces based on the principle of multipath ultrasonic reflection. A multi-channel, array-based ultrasonic system was developed to enable synchronous tomographic acquisition. Building on ultrasonic reflection tomography, an elliptical reflection element method was formulated and implemented. To experimentally validate the method under terrestrial conditions, solid–liquid interfaces were employed as surrogates for gas–liquid interfaces. Experiments involving three representative surface models achieved a minimum relative error of 2.68% and a maximum below 6.17%. The results demonstrate that the proposed ultrasonic approach enables accurate three-dimensional reconstruction of free-surface morphology within closed metallic tanks.