Radiation measurements in a simulated Mars atmosphere

Eichmann, Troy Nicholas (2012). Radiation measurements in a simulated Mars atmosphere PhD Thesis, School of Mathematics & Physics, The University of Queensland.

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Author Eichmann, Troy Nicholas
Thesis Title Radiation measurements in a simulated Mars atmosphere
School, Centre or Institute School of Mathematics & Physics
Institution The University of Queensland
Publication date 2012
Thesis type PhD Thesis
Supervisor Tim McIntyre
Halina Rubinsztein-Dunlop
Total pages 282
Total colour pages 101
Total black and white pages 181
Language eng
Subjects 0205 Optical Physics
0901 Aerospace Engineering
Formatted abstract
A spacecraft entering the atmosphere of one of the terrestrial planets may be travelling at speeds of anywhere from 6 to 11 km/s, depending on parameters such as the vehicles trajectory and the composition of the atmosphere. As atmospheric gases interact with the spacecraft at such speeds, an immense amount of kinetic energy is converted to heat as a shock layer forms around the vehicle. At these conditions, the majority of the heat transfer to the vehicle will be radiative rather than convective in nature. Reliable estimates of the radiative heating of the vehicle are an important consideration in the design of a thermal protection system. Despite over 50 years of spaceflight, the radiative heating and chemical processes which occur at the extreme temperatures generated during a spacecraft's entry into a planet's atmosphere are not well understood. Present estimates arrived at by computational methods vary greatly and necessitate the inclusion of a significant safety margin in the design of any thermal protection system.

The goal of the research described herein was to advance our understanding of the physical processes involved through the development of quantitative measurement techniques and the accruement of experimental data, and provide a benchmark for comparison with predictions from numerical simulations as a test of their validity. A series of high-speed flow experiments was performed using the X2 facility at the University of Queensland. When operated in an expansion tube configuration, the facility can be used for subscale aerodynamic testing of vehicle geometries using a model. Three subscale models were used in the experiments - a cylinder providing data in a two-dimensional flow field, a sphere, and a 60 degree sphere-cone; the latter two both providing data in an axisymmetric flow field. The flow conditions used were intended to simulate what a spacecraft would experience at an altitude of approximately 60km during the period of peak heating as part of a Mars aerocapture trajectory. A mixture of carbon dioxide and nitrogen was used to approximate the atmosphere of Mars. At the temperatures produced within the shock layer, this combination of gases readily reacts to form the cyanogen radical (CN), a species known to be strong radiator in the ultraviolet region of the spectrum. The non-equilibrium chemistry occurring in the shock layer formed around the model during experiments was investigated by making calibrated measurements of the spectral power density and observing the distribution of CN and other species using emission spectroscopy. A detailed analysis of the relative heights of the vibrational and rotational band structure of the radiating CN was compared with the spectra predicted by two radiation codes, SPECAIR and LIFBase. Using a two temperature treatment, a trans-rotational and vibroelectronic temperature was estimated at a series of points along the stagnation streamline for the radiating flow passing over a cylindrical model.

The two-dimensional flow field data associated with the cylindrical model is conducive to the use of other line-of-sight measurement techniques such as holographic interferometry. This technique yields quantitative measurements of the phase shift due to density changes in the flow field through the application of phase unwrapping techniques. The phase shift can then be directly related to total density.

All these results were then compared to predictions made by other researchers using numerical simulations, prior to and concurrent with the experimental measurements. These comparisons have highlighted a number of areas where further improvements to the numerical simulations are necessary. Overestimates of the peak trans-rotational temperature behind the shock and the distance behind the shock needed to re-establish chemical equilibrium appear to have the knock on effect of an overestimate of the CN species number density and the spectral power density.
Keyword Mars
Nonequilibrium radiation
Emission spectroscopy
Holographic interferometry
Expansion tube

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Created: Tue, 25 Sep 2012, 15:19:43 EST by Mr Troy Eichmann on behalf of University of Queensland Graduate School