Evolution of Liquid-Oxygen Interface Shape Based on Drop-Tower Experiments

To support complex aerospace missions including manned spaceflight, Mars exploration and space station construction, it is critical to conduct deep-space exploration research for solar-system planets and even extrasolar space. High-specific-impulse cryogenic fluids such as liquid hydrogen-liquid oxygen (LH₂-LOX) and liquid oxygen-liquid methane (LOX-LCH₄) are primary propellant for deep-space missions. During flight, cryogenic fluids in propellant tanks undergo gravity changes, leading to interface relocation. Owing to low surface tension and viscosity, their interfaces easily deform and break up, resulting in complicated flow behaviors. Meanwhile, increased interfacial and contact areas enhance heat transfer, triggering intense phase change due to low boiling points and latent heats, making thermal states unpredictable. This study aims to reveal the evolution of interface dynamics and thermal behavior during relocation, which is essential for the design of on-orbit propellant management devices.

In this study, a drop-tower experimental platform for LOX reorientation was constructed. The setup, housed in a stainless-steel vacuum chamber with sapphire windows for optical access, uses multilayer insulation and combined LED lighting to enable high-speed visualization. A liquid nitrogen cooling circuit connected via copper braids provides stable precooling and suppresses boiling. The test cell, made of sapphire with high pressure resistance, is instrumented with multiple temperature sensors. LOX is condensed and stabilized at a height of 18.5 mm until thermal drift is below 0.0001 K/s. The system is then installed in the drop tower tube, adjusted for center-of-mass, and released from 83 m to generate nearly 3.5 s of microgravity at approximately 0.0044 g₀. High-speed images are recorded and processed using a MATLAB edge-detection algorithm to analyze interface evolution during the relocation process. Meanwhile, the vapor-phase temperature and pressure were measured. During the drop-tower process, the overload environment transitioned from normal gravity to microgravity.

The LOX propagated along the inner wall and formed a liquid layer. The motion of this liquid layer was decoupled from that of the bulk liquid, and the interface center oscillated continuously. Owing to the ascend of the contact line, the gas-liquid interface area increased, and LOX evaporated continuously at the contact line. The emergence of the liquid layer resulted in a pressurization rate of 3227 Pa/s at the first oscillation of the contact line, which is approximately 1.8 times higher than the final stabilized pressurization rate. During the entire 2.5 s reorientation process, the pressure in the gas-phase region increased by 4217 Pa. The temperature variation at 15.2 mm from the interface was affected by not only heat transfer from the solid wall but also disturbances from the low-temperature gas flow induced by interface oscillations. Additionally, the temperature at this measurement point increased by only 0.351 K during the entire reorientation process.

This study concludes that the evolution of the LOX interface, temperature and pressure in the ullage in the interface reorientation process. This experiment provides the cryogenic fluid data for simulation validation and guidance for the configuration and design of cryogenic-propellant management devices.