How scientists simulated the decompression process of a Mars rover in the taking-off stage?

In the launch phase of Mars exploration mission, the pressure-bearing capacity of the cover on the rover is limited and is one of the factors influencing the success of the space mission. Moreover, the ambient pressure in the rocket fairing drops dramatically in the take-off stage and the pressure differential between the inside and outside of the rover might exceed the bearing capacity of the cover. Therefore, holes are made in the low heat flow area on the surface of the rover to make sure that the pressure differential between the inside and outside in the launch phase falls within the range of the bearing capacity of the cover. In a research paper recently published in Space: Science & Technology, Rui Zhao from Beijing Institute of Technology proposed the numerical simulation method to analyze the decompression process of the Mars rover in the taking-off stage. The effects of environmental pressure setting, time step and mesh density on simulation results are studied to improve the accuracy of calculation. The laws of variation in the pressure differential between the inside and outside of the module resulting from changes in the ambient pressure in the rocket fairing is studied.

The subject of research in this paper is a non-sealed module. The structure of the module is simplified, with only the bodies of the large/small modules, small holes between modules, cables, openings on the large module and baffles remained. It has a maximum volume of less than 12m³, a maximum outer diameter of 3401mm and a height of 2608mm. The cover is on the top of the module and the opening is located in the low heat flow area on the leeward surface of the module, with a diameter of 130mm. The diameter of the opening between the large module and the small module is 35mm. The cable that passes through the small opening is 20mm.

Afterwards, the author focused on the mesh generation techniques and used the software POINTWISE to obtain full-structure meshes. To ensure the accuracy of the results, a radiant outer domain along the direction of openings is added on the outside of the opening of the large module and the boundary of the outer domain serves as the pressure boundary condition. Thus, the computational domain consists of three parts: large module body, small module body and radiant outer domain, which involves 1.45 million meshes. Based on the full-structure meshes, the fluid simulation module FLUENT is used to perform numerical simulation on the decompression process of the module. In this decompression process, the velocities in most areas are close to zero. Therefore, a pressure-based solver is used to solve problems. The detailed settings in the process of numerical discretization are demonstrated as follows. The advection terms are discretized with a second-order upwind scheme and least-squares construction based on cells is utilized for diffusion terms; pressure-velocity coupling is performed using SIMPLE algorithm; a second-order implicit scheme is used in time discretization, and the ideal gas model and Realizable k-s turbulence model are applied.

Finally, the influences of three main factors on the accuracy of calculations are discussed, including ambient pressure boundary settings, time scheme and computational meshes. Two simulation scenarios are conducted to validate the efficiency of the proposed method. (1) The decompression in the separate large module is simulated to analyze the variation characteristics of pressure in the large module when the ambient pressure is the upper and lower limits of internal pressure in condition 1. (2) Considering the impact of the large/small modules, the actual deflation process of the large/small modules is simulated to analyze the variation characteristics of pressure inside the large/small modules when the ambient pressure is the upper and lower limits of internal pressure in conditions 1 and 2. The main conclusions are two. Due to the small volume of the small module, the results for the separate large module and the large/small modules are basically consistent. Moreover, the pressure differential between the inside and outside of the rover is mainly affected by changes in ambient pressure. As for the subsequent researches, on the one hand, the area of the opening should be increased and the following performance of pressure inside the module be sped up. On the other hand, the distortion of ambient pressure inside the fairing should be cut down to further reduce the pressure differential between the inside and outside of the rover.

Reference

Author: Weizhang Wang,¹ Wei Rao,² Qi Li,² Hao Yan,¹ and Rui Zhao¹

Title of original paper: Numerical Simulation of Decompression Process of a Mars Rover in the Launch Phase

Article link: https://spj.sciencemag.org/journals/space/2022/9827483/

Journal: Space: Science & Technology

DOI: https://doi.org/10.34133/2022/9827483

Affiliations: 

¹School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China

²Beijing Institute of Spacecraft System Engineering (ISSE), Beijing 100094, China

About the author

Rui Zhao, PhD., Assistant Professor at School of Aerospace Engineering, Beijing Institute of Technology. Prof. Zhao is also a member of Chinese Society of Theoretical and Applied Mechanics. His research interests focus on numerical simulation method for supersonic complex aerodynamic problems, Large computational fluid dynamics (CFD) software development, and the development of supersonic thermal protection system based on hyper-surface.