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Description
Background of the study:
With increasing interest in both manned and unmanned missions to Mars, human spaceflight to the red planet is becoming a critical focus. Unlike robotic missions, human missions require significantly shorter transit times between Earth and Mars to minimize astronaut exposure to space hazards. For a 120-day transit to Mars, entry velocities could reach as high as 15 km/s, compared to the current 6-10 km/s range for robotic missions.
Such high-speed entry results in significant kinetic energy that ionizes the shock layer in front of the spacecraft. This ionized layer leads to a complex and intense thermal environment, with a high heat load. Additionally, the high velocities necessitate large deceleration forces for safe landings.
Magnetohydrodynamic (MHD) aerobraking presents a potential solution to mitigate these issues by exploiting the interaction between the ionized shock layer and a magnetic field generated by the entry vehicle. This interaction induces azimuthal currents and produces a Lorentz force. The resulting force, called "MHD drag," opposes the flow, reducing velocity and heat. Another outcome is the increased shock standoff, which reduces velocity and temperature gradients within the shock layer, thereby decreasing convective heat flux.
For MHD studies, electron number density measurements are critical to understanding the ionizing flow interacting with the magnetic field. Since the numerical modeling of magneto-flow interactions is still under development, experimental diagnostics of the shock layer are important for validating simulations.
Methodology:
In this study, experiments were conducted using the X2 expansion tube to simulate high-speed Mars entry conditions at 13 km/s. Emission spectroscopy along the stagnation streamline of a quasi-2D shock layer on a cylindrical model was employed to diagnose the flow. Hydrogen-beta emission from the Balmer series, representing transitions between the 4th and 2nd energy levels, was used to estimate the electron number density within the shock layer.
Hydrogen contamination from the X2 facility contributed to the hydrogen-beta emissions, making it a suitable line for analysis. Various broadening effects impact the spectral line shape, with Stark broadening being the most significant for ionized flows. When the electron number density is relatively low and the shock is optically thin, the hydrogen-beta line exhibits a clear shape that can be analyzed using its full width at half maximum (FWHM), by deconvolving the measured FWHM from the FWHMs from other broadening effects. At high electron number density scenario, Stark broadening becomes so prominent that the H-beta line experiences physical separation into distinct peaks due to the splitting of energy levels in the presence of strong electric fields. The separation between these peaks, can be directly related to the electron number density, which enables electron number density to be estimated based on the separated hydrogen-beta lines.
Results:
Spectroscopic data from the UV to the IR were measured, and UV data have been calibrated and analyzed for electron number density calculation. UV spectra for both coarse and fine grating experiments showed good agreement on the calibrated intensity. Shock standoff was measured to be approximately 1.7 mm. Fine-resolution spectral profiles of the hydrogen-beta line at different positions along the stagnation streamline were processed and analyzed to estimate the electron number density from the Stark splitting.
The experimental electron density profile was compared with numerical simulations using the lmr CFD code, which utilized a two-temperature, 11-species CO2 model to simulate the viscous shock layer. The results showed good agreement between the experimental and simulated electron number densities, although the simulation overestimated the shock standoff distance.
Conclusions:
Electron number density for high-speed Mars entry conditions was successfully estimated by analyzing the Stark splitting of the hydrogen-beta line. The results showed a good quantitative match with numerical simulations. This study provides a reliable method for diagnosing ionizing flows, essential for future MHD research. Future work will involve applying this technique to analyze spectra with and without a magnetic field, enabling a deeper understanding of ionization in shock layers under the influence of MHD effects.
Summary
Electron number density for high-speed Mars entry conditions was successfully estimated by analyzing the Stark splitting of the hydrogen-beta line. The results showed a good quantitative match with numerical simulations. This study provides a reliable method for diagnosing ionizing flows, essential for future MHD research. Future work will involve applying this technique to analyze spectra with and without a magnetic field, enabling a deeper understanding of ionization in shock layers under the influence of MHD effects.
