Pascal Bauer

Ram accelerator operation in the superdetonative velocity regime

Preliminary experiments on superdetonative propulsive cycles at a ram accelerator facility using projectiles fabricated from aluminum and titanium alloys have demonstrated that acceleration is possible at velocities greater than the Chapman-Jouguet (CJ) detonation speed of a gaseous propellant mixture. Projectile materials were found to play a significant role in these experiments. Theoretical modeling was successful in predicting projectile drag in nonreactive gas mixtures at hypersonic velocities. When this drag was subtracted from the ideal thrust of a supersonic combustion ram accelerator, the net thrust closely matched that measured in the experiments. The dependence of the maximum operating Mach number on both the projectile diameter and propellant heat release was examined. The peak velocity capability of the experimental projectile geometry is predicted to be about 1.5 times the CJ speed of the propellant mixture. It was found that the drag resulting from an increase in projectile diameter was more than offset by the corresponding enhancement in thrust, and that velocities of nearly twice the CJ speed are possible. (Author)

Equations of state for 1-D modelling of ram accelerator thrust in thermally choked propulsion mode

A one-dimentional performance analysis code has been developed at eh Laboratoire de Combustion et de Detonique to predict the thrust in the thermally choked ram accelerator propulsion mode. This code includes the real gas effect in steady calculations and incorporates the following equations of state: ideal gas, Boltzman, Percus-Yevick (PY) and Becker-Kistiakowsky-Wilson (BKW). The code was validated against key experimental data from representative experiments at the University of Washngton 38-mm-bore facility. The predicted thrust and valicity agree well with experimental measurements. The present paper provides details about the algorithm of the calculation in the thermally choked propulsion mode.

Improved One-Dimensional Unsteady Modeling of Thermally Choked Ram Accelerator in Subdetonative Velocity Regime

The subdetonative propulsion mode using thermal choking has been studied with a one-dimensional (1D) real gas model that included projectile acceleration. Numerical results from a control volume analysis that accounted for unsteady flow effects established that the thrust coefficient versus Mach number profile was lower than that obtained with a quasi-steady model. This deviation correlates with experimental results obtained in a 38-mm-bore ram accelerator at 5.15 MPa fill pressure. Theoretical calculations were initially carried out with the assumption that the combustion process thermally choked the flow about one projectile length behind the projectile base. Thus the control volume length used in this 1D modeling was twice the projectile length, which is consistent with experimental observations at velocities corresponding to Mach number less than 3.5. Yet the choice of the length of the combustion zone and thus the control volume length remains a key issue in the unsteady modeling of the ram accelerator. The present paper provides a refinement of the unsteady one-dimensional model in which the effect of control volume length on the thrust coefficient and the projectile acceleration were investigated. For this purpose the control volume length determined from computational fluid dynamics (CFD) as a function of projectile Mach number was applied. The CFD modeling utilized the Reynolds-averaged Navier-Stokes (RANS) equations to numerically simulate the reacting flow in the ram accelerator. The shear-stress transport turbulence and the eddy dissipation combustion models were used along with a detailed chemical kinetic mechanism with six species and five-step reactions to account for the influence of turbulence and rate of heat release on the length of the combustion zone. These CFD computational results provided Mach number dependent estimates for the control volume length that were implemented in the 1D modeling. Results from the proposed improved 1D unsteady modeling were compared and validated with ram accelerator experimental data with significant improvements in terms of the predicted thrust dependence on Mach number.

Numerical Investigation of Thermally Choked Ram Accelerator in Sub-Detonative Regime

Numerical investigations of thermally choked ram accelerator in sub-detonative regime have been carried out. The simulation solves axis-symmetric compressible Navier-Stokes equations and includes modern turbulent combustion models. The experimental operating characteristics of a four-finned projectile in a 38-mm-diameter tube loaded with premixed propellant gas of methane/oxygen/nitrogen at a fill pressure and temperature of 5.15 MPa and 300 K, respectively, are considered. Simulations for projectile Mach numbers of 2.98, 3.05, 3.21, 3.41, 3.6, 4.36, and 5.01 were carried out. The shear-stress transport turbulence model and the eddy dissipation turbulent combustion model are used to simulate the turbulent reactive flow around the projectile. Complex chemical reaction mechanisms have been modeled with one-step, two-step and five-step simplified global reaction models. Not surprisingly, the non-dimensional pressure distributions from the simulation using the fivestep reaction model are in better agreement with the experimental measurements than those from the one-step and two-step models. The simulations reveal some key features of the shock system around the projectile, which are important in determining the characteristics of the thermally choked propulsive mode. These findings are useful in understanding the characteristics of supersonic turbulent combustion processes in the ram accelerator.

Thrust Prediction in Thermally Choked Ram Accelerator

Computational fluid dynamics solutions of the Reynolds-averaged Navier-Stokes equations have been used to numerically simulate the reacting flow in the ram accelerator mass-driver concept. The shear-stress transport turbulence model and the eddy dissipation combustion model are also used including a detailed chemical kinetic mechanism with six species and five-step reactions. Simulations for a series of incoming Mach numbers were carried out to investigate the details of the flow field in the thermally choked combustion regime. It was found that the predicted thrust-velocity characteristics agreed well with those obtained from the one-dimensional modeling computation and the experimental measurements. The present study provides some useful results such as the influence of Mach number on length of ram accelerator combustion zone at velocities approaching the Chapman-Jouguet detonation speed.

Detonation wave initiation of ram accelerator propellants

Abstract. The current ram accelerator operations have shown that data on the ability of the propellants to detonate are required. Previous studies examined the efficacy of initiation techniques based on piston impact. The purpose of the present work is to analyze the effects of detonation wave transmission from a detonating mixture into a low sensitivity mixture. One-dimensional modeling based on the analysis of pressure vs particle velocity for the mixtures is used to interpret experimental data. Furthermore, calculations based on chemical kinetics (CHEMKIN code) are provided. Experimental data together with the modeling of the detonation transmission provide some new insight into the limiting conditions necessary to establish a Chapman-Jouguet (CJ) wave in a detonable mixture.

The Ram Accelerator in Subdetonative Propulsion Mode: Analytical and Numerical Modeling and Simulation

The present chapter highlights the different steps which lead to a better understanding of the dynamics of the flow in order to predict the performance of a ram accelerator in the thermally choked propulsive mode which operates in the sub-detonative velocity regime; i.e., below the Chapman-Jouguet (C-J) detonation speed of the propellant. A two-dimensional numerical simulation is described and the CFD results are validated against test data from a representative experiment at the University of Washington 38-mm-bore test facility. The early form of the 1D modeling based on a quasi-steady assumption of the flow and involving compressibility effects as well as a real gas equation of state is then presented. The modeling is further improved by using a real gas equation of state for the reactants to account for the compressibility effects when high initial pressures are involved. In order to further account for high acceleration rates, an unsteady modeling of the flow is then assumed. The effectiveness of an improved 1D unsteady modelling applying the CFD predicted control volume length is further demonstrated. Moreover, based on data derived from different scale facilities; i.e., the University of Washington and the French-German Research Institute (ISL), several equations of state of the combustion products are investigated. A computer code, namely TARAM, has been developed for this purpose.

RAMAC in Subdetonative Propulsion Mode with Fin-Guided Projectile: Design, Modeling, Performance and Scale Effect

The aim of this chapter is to demonstrate the basic feasibility of the RAMAC and its higher ability to accelerate a projectile than any conventional gun. This capability comes from the constant high pressure thrust at the rear of the projectile, contrary to the pressure distribution in a conventional powder gun barrel. Yet, a better knowledge of the gas dynamics of the flow was expected in order to improve the ram accelerator operations. The present study refers to ram accelerator operations in the thermally choked propulsive mode which operates in the sub-detonative velocity regime; i.e., below the Chapman-Jouguet (C-J) detonation speed of the propellant. A numerical simulation of the flow characteristics around the projectile with and without fins is presented in order to show the influence of the geometry of the fins on the RAMAC performance. This series of calculations were conducted using the 3D numerical code TascFlow TM. Different types of fins, i.e., various shapes and number, were studied. The characteristics of the flow in terms of temperature, pressure and Mach number distributions were studied. The data show a drastic role of the fin geometry on the maximum temperature of the flow. The nature of the reactive mixture, which is a key parameter for the performance of the ram accelerator, was investigated as well, mainly in terms of detonability. For this purpose, an important basic research has been conducted in 90 mm tubes in order to yield the best composition of the reactants for a Sub-detonative propulsion mode. This studies included detonations experiments conducted in a 1.35 m long tube and a 3.15 m long tube. The other important aspect, which is addressed in this chapter, is the discussion on the scale effect, which provides some information on the feasibility of this novel technology for future space applications. All these data have become now part of the databank for the development of this technology throughout the world.

One-Dimensional Modeling of Thermally Choked Ram Accelerator Based on CFD Simulations

In order to improve one-dimensional unsteady modeling of ram accelerator thrust performance, computational fluid dynamics solutions of Reynoldsaveraged Navier-Stokes equations have been used to investigate the reacting flow field of a projectile accelerated in the sub-detonative velocity regime. Both shearstress transport turbulence and eddy dissipation combustion models were used, including a detailed chemical kinetic mechanism with six species and five-step reactions. Simulations for a series of incoming Mach numbers were performed to estimate the length within which the combustion reactions were completed. This in-depth calculation of the flow field allowed implementing the unsteady 1D modeling with an accurate Mach number dependent heat release zone length. A significantly better agreement of the predicted thrust-Mach number behavior with experimental data was observed.

Effects of Mach number on the combustion zone length for a RAMAC configuration at sub-detonative mode

Computational investigation of Mach number effects on the combustion zone length in supersonic flow over a ram accelerator has been carried out to provide valuable data for an on-going theoretical study of the same problem. It is found that the combustion zone length is inversely proportional to the Mach number with a significant reduction of about 33% at Mach 3.5 and about 44% at Mach 4, respectively, while comparing to the combustion zone length at Mach 2.5. This correlation provides some useful guidelines to define the control volume box in corresponding theoretical study.