Mid-IR Laser Absorption Diagnosis on Flow Characteristics for Mars Entry

The Mars exploration mission is important to the national deep space exploration project. The successful landing of "Tianwen" on Mars in 2021 is a big step forward in China's space exploration and research on Mars. The Martian atmosphere is mainly dominated by carbon dioxide, and the surface pressure is much lower than the Earth's. Due to the differences in atmospheric composition and entry orbit, the thermal environment of Mars entry vehicles is very different from that of Earth space vehicles. The entry process involves a non-air high-speed flow, leading to serious aerothermal heating problems. The arc-heated wind tunnels are used to simulate the aerothermal heating environment for Mars entry, which is a key step in validating the thermal protection system design for Mars exploration. The -high-temperature flow simulated by the arc-heated wind tunnel includes a dominated CO2 dissociation reaction, and dissociation species such as CO interact with the thermal materials of the Mars entry vehicles, which would cause a catalytic effect and significantly affect the aerothermal environment. In this paper, the mid-infrared quantum cascade laser absorption spectroscopy is carried out for in-situ and quantitative measurements of the flow characteristics of ground-based simulation for Mars entry in the arc-heated facility. The in-situ diagnosis in the arc-heated facility utilizes the spectral line of CO with a central wavelength of 2 212. 625 cm(-1) (nu '' = 0, R(19)) near 4. 5 mu m, to achieve a high signal-to-noise ratio measurement of the ground-based flow field. A single-line direct absorption spectroscopy obtains the static temperature and CO mole fraction of free stream in the arc-heated wind tunnel. Under a typical condition for Mar entry, the static temperature and CO mole fraction of freestream remain stable throughout the operation, showing good stability in the arc-heated flow field. Six repeated experiments show that the static temperature and CO mole fraction are in the range of (1 757 +/- 1 69) K and (0. 189 +/- 0. 027) respectively, with fluctuation of less than or equal to 3. 9% in flow temperature and fluctuation of less than or equal to 14. 3% in CO concentration, demonstrating great repeatability for the ground-based simulated flow field. The developed mid-infrared laser absorption spectroscopy diagnostic technology in the arc-heated wind tunnel can provide refined measurement capabilities for research of the aerothermal environment and ablation-catalytic properties of the thermal protection materials of the Mars entry vehicles.