A new method for accelerating projectiles from velocities of ~0.7 km/s up to -12 km/s using chemical energy is presented in this paper. The concept, called the "ram accelerator," is based on gasdynamic principles similar to those of an airbreathing ramjet but operates in a different manner. The projectile, which resembles the center body of a ramjet, travels through a tube filled with a premixed gaseous fuel and oxidizer mixture. The tube becomes the outer cowling of the ramjet, and the energy release process travels with the projectile. By tailoring the propellant mixture along the tube, a nearly constant acceleration can be achieved. In principle, the ram accelerator can be scaled for projectile masses ranging from grams to hundreds of kilograms and is capable of ballistic efficiencies as high as 30%. A straightforwar d, quasisteady, one-dimensional approach is used to model the acceleration process. The experimental facility developed to investigate the concept is described, and the results of recent experiments are presented. The velocity range of 690-1500 m/s has been explored in a 4.88-m long, 38-mm bore accelerator tube. Using methane, oxygen, and various diluents, accelerations of up to 16,000 g have been achieved with 75 gm projectiles and gas fill pressures of 20 atm. Proof of concept has been demonstrated, and agreement between theory and experiment has been found to be very good.

Ram accelerator - A new chemical method for accelerating projectilesto ultrahigh velocities

Analytical modeling of thermal and mechanical response is a fundamental step in the design process for ultra-high-temperature ceramic components, such as nose tips and wing leading edges for hypersonic applications. The purpose of the analyses is to understand the response of test articles to high-enthalpy flows in ground tests and to predict component performance in particular flight environments. Performing these analyses and evaluating the results require comprehensive and accurate physical, thermal, and mechanical properties. In this paper, we explain the nature of the analyses, highlight the essential material properties that are required and why they are important, and describe the impact of property accuracy and uncertainty on the design process.

Material property requirements for analysis and design of UHTC components in hypersonic applications

Anew air-radiationmodel is presented for the calculation of the radiative flux from lunar-return shock layers. For modeling atomic lines, the data from a variety of theoretical and experimental sources are compiled and reviewed. A line model is chosen that consists of oscillator strengths from the National Institute of Standards and Technology database and theOpacity Project (formany lines not listedby theNational Institute of Standards andTechnology), as well as Stark broadening widths obtained from the average of available values. Uncertainties for the oscillator strengths and Stark broadening widths are conservatively chosen from the reviewed data, and for the oscillator strengths, the chosen uncertainties are found to be larger than those listed in the National Institute of Standards and Technology database. This new atomic line model is compared with previous models for equilibrium constantproperty layers chosen to approximately represent a lunar-return shock layer. It is found that the new model increases the emission resulting from the 1–6-eV spectral range by up to 50%.This increase is due to both the increase in oscillator strengths for some important lines and to the addition of lines from the Opacity Project, which are not commonly treated in shock-layer radiation predictions. Detailed theoretical atomic bound–free cross sections obtained from the Opacity Project’s TOPbase are applied for nitrogen and oxygen. An efficient method of treating these detailed cross sections is presented.The emission fromnegative ions is considered and shown to contribute up to 10% to the total radiative flux. The modeling of the molecular-band systems using the smeared-rotational-band approach is reviewed. The validity of the smeared-rotational-band approach for both emitting and absorbing-band systems is shown through comparisons with the computationally intensive line-by-line approach. The absorbingband systems are shown to reduce the radiative flux by up to 10%, whereas the emitting-band systems are shown to contribute less than a 5% increase in the flux. The combined models chosen for the atomic line, atomic bound–free, negative-ion, and molecular-band components result in a computationally efficient model that is ideal for coupled solutions with a Navier–Stokes flowfield. It is recommended that the notable increases shown, relative to previous models, for the atomic line and negative-ion continuum should be included in future radiation predictions for lunarreturn vehicles.

Spectrum Modeling for Air Shock-Layer Radiation at Lunar-Return Conditions

The possibility of controlling scramjet inlets in off-design conditions by operating a near-surface magnetohydrodynamic (MHD) system upstream of the inlet is examined. The required electrical conductivity in air is supposed to be created by electron beams injected into the air from the vehicle along magnetic field lines. A simple model of a beam-generated ionization profile is developed and coupled with plasma kinetics, MHD equations, and two-dimensional inviscid flow equations. Calculations show that an MHD system with reasonable parameters could bring shocks back to the cowl lip when flying at Mach numbers higher than those for which the inlet was optimized. The MHD effect is not reduced to heating only because the work by j X B forces is a substantial part of the overall effect. Power requirements for ionizing electron beams could be lower than the electrical power extracted with MHD, so that a net power would be generated onboard. Problems associated with high Hall fields are discussed

Magnetohydrodynamic Control of Hypersonic Flows and Scramjet Inlets Using Electron Beam Ionization

DOI: 10.2514/1.39034 We study the behavior of the excited electronic states of atoms in the relaxation zone of one-dimensional airflows obtained in shock-tube facilities. A collisional-radiative model is developed, accounting for thermal nonequilibrium between the translational energy mode of the gas and the vibrational energy mode of individual molecules. The electronic states of atoms are treated as separate species, allowing for non-Boltzmann distributions of their populations. Relaxation of the free-electron energy is also accounted for by using a separate conservation equation. We apply the model to three trajectory points of the Fire II flight experiment. In the rapidly ionizing regime behind strong shock waves, the electronic energy level populations depart from Boltzmann distributions because the highlying bound electronic states are depleted. To quantify the extent of this nonequilibrium effect, we compare the results obtained by means of the collisional-radiative model with those based on Boltzmann distributions. For the earliesttrajectorypoint,weshowthatthequasi-steady-stateassumptionisonlyvalidforthehigh-lyingexcitedstates and cannot be extended to the metastable states.

Fire II Flight Experiment Analysis by Means of a Collisional-Radiative Model

Shock-Heated Air Radiation Measurements at Lunar Return Conditions

NASA's In-Space Propulsion Technology program is supporting the development of shock radiation transport models for aerocapture missions to Mars and Venus. Phenomenological models of nonequilibrium shock radiation will be incorporated into high-fidelity flowfield computations used to predict the aerothermal environments for a Mars or Venus aerocapture entry vehicle. These models are validated with shock radiance measurements obtained at flight-relevant conditions. A comprehensive test series in the NASA Ames Electric Arc Shock Tube facility at a representative freestream condition was recently completed. The facility's optical instrumentation enabled spectral measurements of shocked gas radiation from the vacuum ultraviolet to the near infrared. The instrumentation captured the nonequilibrium postshock excitation and relaxation dynamics of dispersed spectral features. A description of the shock tube facility, optical instrumentation, and examples of the test data are presented.

Shock Radiation Measurements for Mars Aerocapture Radiative Heating Analysis

This paper analyzes the shock layer radiative heating environment for a large entry vehicle on a lunar return trajectory. Modeling results show that much of the shock layer plasma is in local thermodynamic equilibrium (LTE) and is not optically thin. The ionization level is generally high (15%) and the air is almost fully dissociated. A significant amount of vacuum ultraviolet (VUV) radiation is produced due to bound-bound and bound-free transitions of N and O atoms. The sensitivity of total radiation to Stark broadening, which dominates over other line broadening mechanisms, is quantified. The latter part of this paper reports the status of ongoing validation of the current radiation models with measurements in the Electric-Arc Shock Tube (EAST) facility at NASA Ames Research Center. Model predictions are compared with the calibrated radiation spectra measured in the equilibrium portion of the shock layer at 0.3 Torr. The reasons for discrepancy between model and measurements are also discussed with possible hypotheses presented for further investigation.

Analysis and Model Validation of Shock Layer Radiation in Air

Assessment of nonequilibrium thermochemical models for shock-layer radiation in N 2 /CH 4 mixtures is presented via comparisons with spectrally and temporally resolved intensity measurements from a set of shock tube experiments. The experiments were carried out at the Electric Arc Shock Tube facility at NASA Ames Research Center in a rarified environment [13.3-133.3 Pa (0.1 and 1 torr)] representative of the peak heating conditions of a Titan aerocapture trajectory (5-9 km/s). The baseline model that assumes a Boltzmann population of the CN excited states consistently overpredicts the shock-layer radiation intensity at lower pressure [13.3 Pa (0.1 torr)]. A nonlocal collisional radiative model that solves a simplified master equation and includes radiative transport and nonlocal absorption in the shock tube is presented. The proposed model improves the prediction of the nonequilibrium radiation overshoot peak, but still underpredicts the intensity decay rate in the low-pressure case. Further analysis suggests possible reasons for the remaining disagreement, the most likely being a slow CN consumption in the current chemical kinetics model in the intensity fall-off region.

Modeling and experimental assessment of CN radiation behind a strong shock wave

The coefficient of reaction of nitrogen atoms with solid carbon to form gaseous cyanogen CN is measured in a shock tube. A stream of highly dissociated nitrogen is produced in a shock tube and passed over a grid of metal wire coated with carbon. The radiation emitted by the wake of the wire grid is observed with a monochromator set at 386 nm, where CN is known to radiate strongly. From the intensity of the CN radiation, the concentration of CN in the wake is determined. The concentration of nitrogen atoms in the stream are calculated by integrating conservation equations. From these concentration values, the fraction of the collisions of nitrogen atoms producing CN, that is, the reaction coefficient, is deduced. The results show that the reaction coefficient is about 0.3 at both 300 and 1100 K. The uncertainties in the results are described, and improvements are proposed.

David W. Bogdanoff

Papers

Shock-Heated Air Radiation Measurements at Lunar Return Conditions

Shock Radiation Measurements for Mars Aerocapture Radiative Heating Analysis

Analysis and Model Validation of Shock Layer Radiation in Air

Modeling and experimental assessment of CN radiation behind a strong shock wave

Shock-Tube Measurement of Nitridation Coefficient of Solid Carbon