Search

Future space missions, such as return from Mars, will require entry speeds of the order of 15 km/s. These harsh conditions are dominated by radiative heat flux and have large modeling uncertainties related to radiative coupling, such as precursor radiation and ablation radiation coupling. These conditions have also never been tested in a flight scenario and have limited ground testing data available. This study investigates the feasibility of generating these conditions in the X2 expansion tube at the University of Queensland so that these flows can be tested over scaled test models for the study of shock relaxation, heat flux, shock standoff, or ablation. This paper presents a theoretical performance map of the X2 facility for the generation of these conditions before it presents some of the unique issues that may be encountered when generating and investigating them. Some preliminary experimental data are then presented, showing that these 15 km/s equivalent flight speed test conditions can be generated in X2 within the current performance limits of the facility. [ABSTRACT FROM AUTHOR]

The thermal protection system of a reentry vehicle must endure extreme heating loads, leading to high surface temperatures and associated phenomena such as thermochemical ablation. Accurate modeling and prediction of these processes are critical for determining heat-shield sizing requirements; however, large uncertainties still remain. Due to the high costs associated with flight tests, ground testing remains an economical option for addressing these uncertainties. In this study, experiments with electrically preheated graphite samples are conducted in the University of Queensland's X-2 expansion tunnel. Calibrated spectral measurements of the shock-layer emissions are taken in the wavelength region of 353 to 391 nm, targeting the carbon-nitrogen violet band for surface temperatures from 1770 to 2410 K. It is observed that carbon-nitrogen concentrations clearly increased within the boundary layer at higher surface temperatures. Numerical simulations fail to reproduce this behavior and instead indicate that production should be relatively constant. Increased levels of carbon-nitrogen radiation with surface temperature are also found throughout the entire shock layer and even the freestream, another observation that is not predicted by the simulations. It is proposed that this effect is due to spallation; however, further experiments are required to verify this. [ABSTRACT FROM AUTHOR]

This study presents calibrated vacuum ultraviolet spectroscopic measurements and filtered cyanogen violet band images for steel, cold graphite, and preheated graphite models at three reentry-relevant conditions in the X2 Expansion Tube. An analysis of these experiments concluded that there were no unique radiating species in the boundary layer of an ablating model within the spectral range of 120-180 nm. Spectral measurements showed an increase in atomic carbon emission in the boundary layer with preheated graphite models. Filtered images revealed significant cyanogen emission in the boundary layer of the preheated graphite models. A comparison with spectral measurements concluded that the production of cyanogen in the boundary layer did not reduce the nitrogen line emission relative to the shock layer. The increase in the cyanogen boundary-layer emission relative to the shock-layer emission was found to vary with freestream conditions. [ABSTRACT FROM AUTHOR]

Vehicles entering planetary atmospheres at high speed require an ablative heat shield to withstand the high thermal energy flux to the body. The interaction between the ablative products and the flowfield is not well characterized. In this study, numerical simulations were conducted to investigate the influence of carbon ablation on shock-layer radiation. Data collected from experiments performed in the X-2 expansion tunnel at the University of Queensland were used to compare to the simulations. The model was a short half-cylinder made of isomolded graphite and was tested in 8.6 km/s8.6km/s Earth entry flow. The graphite model was heated within a temperature range of 1770-2410 K over the course of the experimental campaign. The radiation emitted from the CN violet bands was measured by ultraviolet spectrometry in a spectral range from 353 to 391 nm. The simulations used the Park as well as Zhluktov and Abe finite-rate surface kinetic models for determining the chemical state of the ablating boundary layer. The Park model consistently overpredicted the radiative heat fluxes, but better comparison was achieved when a modified nitridation rate was used. The Zhluktov and Abe predictions displayed good agreement with the measured radiative heat fluxes at 1770 K but remained well below the measured values at the higher wall temperatures. The lack of a nitridation mechanism in the Zhluktov and Abe model is suggested as the reason for these underpredictions. [ABSTRACT FROM AUTHOR]

In 2010, the U.S. National Research Council proposed planetary entry probe missions to Uranus and Saturn. This paper details an investigation exploring the operating limits of the X2 superorbital expansion tube at the University of Queensland for the simulation of atmospheric entry test conditions related to these missions. Theoretical calculations indicate that X2 can recreate the stagnation enthalpy of the proposed 22.3km/s Uranus entry, but not of the proposed 26.9km/s Saturn entry. Experiments were generally able to confirm the theoretical predictions; however, internal loss mechanisms caused experimentally measured shock speeds to be up to 10% slower than predicted. One test condition was shown to achieve the required 22.3km/s of the proposed Uranus entry, and it was proposed that, with a slightly more powerful driver condition, Saturn entry and scaled Uranus entry test conditions could be simulated in X2. The use of a faster response, optically based shock detection system was also proposed to lower test condition uncertainties for such high-speed conditions. [ABSTRACT FROM AUTHOR]

Flight into the gas giant planets involves atmospheric entry velocities between 20 and 50 km/s, which are mostly beyond the capabilities of current ground testing facilities that make use of test models. Using an expansion tube, this paper investigates how increasing the percentage of helium in the H2/He test gas, or substituting the helium for the heavier noble gas neon, can be used to generate representative shock-layer conditions associated with Uranus and Saturn entry. Theoretically, it was found that either test gas substitution should make it possible to simulate Uranus or Saturn entry shock layers, which was confirmed experimentally for the helium substitution. Lower percentages of the substituted gas were required for neon; however, the helium substitution was found to be able to maintain the flow stagnation enthalpy in the driven sections of the facility with changing diluent fraction. Noting the current interest in sending atmospheric entry probes to both of these planets, this demonstrates that the required experimental capabilities are available for performing simulated experiments using test models. [ABSTRACT FROM AUTHOR]