in dsmcFoam when compared with analytical solutions for both inert and reacting conditions. A comparison is also made between the Q-K and total collision energy (TCE) chemistry approaches for a hypersonic flow benchmark case.

Results from 15 static test firings of lab scale hybrid rocket motors using 90% concentrated unstabilized hydrogen peroxide as an oxidizer with hydroxyl terminated polybutadiene fuel are presented. Thirteen of those tests used aluminum or aluminum/magnesium alloy additive in the fuel. The performance characteristics of the propellant combinations were determined. The experimental results indicated combustion efficiencies of 0.72-0.89 and regression rates of 0:5-1:3 mm=s for the metallized fuel combinations. A correlation of the regression rate data for the metallized propellants deviated from that which was derived for turbulent convective heat transfer dominated behavior. A numerical model of the hydrogen peroxide/nonmetallized hydroxyl terminated polybutadiene motor was built using a commercial computational fluid dynamics code. The model was combined with an in-house code to predict the regression rate of the propellant combination, and the flowfield characteristics at the initial operating conditions for two of the experimental tests. The results of the model indicated that the proposed numerical model is a promising tool for mapping the temporal and spatial variation of the regression rate in hybrid motors operating with homogeneous hydrocarbon fuels.

Re-entry vehicles designed for space exploration are usually equipped with thermal protection systems made of ablative material. In order to properly model and predict the aerothermal environment of the vehicle, it is imperative to account for the gases produced by ablation processes. In the case of charring ablators, where an inner resin is pyrolyzed at a relatively low temperature, the composition of the gas expelled into the boundary layer is complex and may lead to thermal chemical reactions that cannot be captured with simple flow chemistry models. In order to obtain better predictions, an appropriate gas flow chemistry model needs to be included in the CFD calculations. Although more arc-jet experimental data is becoming available for model comparison, very little flight data exists for comparison and validation. However, because of the observation mission campaign led by NASA, data is available for the re-entry of the Stardust sample return vehicle, which employed a heat shield constructed of phenolic impregnated carbon ablator (PICA). Using a recently developed chemistry model for ablating carbon-phenolic-in-air species, a CFD calculation of the Stardust re-entry at 71 km is presented. The result demonstrates the need to account for different species in the flow field than the ones composing the pyrolysis gas. It is also shown that the main heat flux reduction phenomenon is through mass diffusion, and not through translational-rotational conduction, as is the case at higher altitude. The flow field solutions are also used to generate nonequilibrium radiation spectra, which are compared to the experimental data obtained during Stardust re-entry by the Echelle instrument. The predicted emission from the CN lines compares quite well with the experimental results, demonstrating the validity of the current approach.

The degree of electron thermal nonequilibrium occurring in continuum, hypersonic, slender body, and blunt body flows is investigated. The effect of thermal nonequilibrium between the electron translational and vibrationalelectronic modes on the predicted electron density and electron temperature is quantified at flight conditions characteristic of a slender hypersonic vehicle, as well as at a higher energy, superorbital flight condition of a blunt reentry vehicle. The most significant effect of electron thermal nonequilibrium on the flowfield is through the influence of the electron temperature on the magnitude of the chemical reaction rates in the high density shock layer. A twofold reduction in peak plasma density is predicted in the flow around the slender body when the electron nonequilibrium model is used, and this results in better agreement between the simulation results and the experimental electron density measurements. A change in the shape of the electron density profile with the use of the electron nonequilibrium model is predicted along the stagnation streamline of the blunt body flow. In both cases, the changes in predicted electron density are much more pronounced at the higher density flight conditions examined. In all cases, the rise of the electron temperature precedes the rise of the vibrational-electronic temperature along the stagnation streamline. These results have potentially important implications for the prediction of the onset of radio frequency blackout during the flights of next generation hypersonic vehicles, a phenomenon that is governed by the properties of the electron gas.

Axisymmetric simulations of hypersonic flow about the FIRE II reentry vehicle using the direct simulation Monte Carlo method are presented. Flow field solutions for points along the FIRE II trajectory in the noncontinuum regime at 85 km altitude, and near continuum regime at 76 km altitude are computed using both an eleven species and a five species chemistry set. The ambipolar diffusion assumption is used to enforce charge neutrality in the flow. The sensitivity of the results for the noncontinuum flow condition are examined with respect to the molecular interaction model for all colliding pairs, and the vibrational relaxation model for collisions of electrons and nitrogen molecules. The results indicate that the degree of ionization of the flow field and the surface heat flux are insensitive to variations in both models. Experimentally and computationally determined total collision cross section data for the interaction of electrons and neutral particles are incorporated into the molecular interaction model. This modification again results in little change to the results for the noncontinuum flow condition. When possible, the simulation results are compared to previous results and to experimental data. The heat flux to the vehicle surface computed using the eleven species chemistry model agrees reasonably well with the data obtained during the FIRE II reentry at the noncontinuum flow condition. In the near continuum regime, the predicted heat flux shows reasonable agreement with the measured data provided the radiative component is included.