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The paper focuses on numerical simulation of neutral entrainment in a Field Reversed Configuration thruster with a neon propellant. A fully kinetic approach based on a Particle-In-Cell method of modeling plasmas and the Direct Simulation Monte Carlo method of modeling neutrals is applied. An implicit PIC code Celeste3D extended to compute neutral transport and collision processes between neutral and charged particles are used. The current collision model includes all key elastic, collisional radiative, and reaction processes in neon. Temporal evolution of gas and plasma macroparameters and thruster performance properties is presented., The original document contains color images. Presented at the 44th AIAA Thermophysics Conference held in San Diego, CA in June 2013. Prepared in cooperation with ERC, Inc., Edwards AFB, CA.

Neutral entrainment in a field reversed configuration thruster is modeled numerically with an implicit PIC code extended to include thermal and chemical interactions between plasma and neutral particles. The contribution of charge exchange and electron impact ionization reactions is analyzed, and the sensitivity of the entrainment efficiency to the plasmoid translation velocity and neutral density is evaluated., The original document contains color images. Conference paper for the 28th International Symposium on Rarefied Gas Dynamics, Zaragoza, Spain, 9-13 July 2012.

Numerical analysis has been conducted to assess the feasibility of using radiometer arrays mounted on a near-space vehicle, for wind disturbance compensation. The results indicate the possibility of using radiometric force for that purpose for altitudes of 80 km and smaller, and head winds up to 30 m/s., The original document contains color images.

A propulsion system based on an array of radiometer vanes has been proposed for wind disturbance correction on near space vehicles. The radiometric force is produced by a vane due to a temperature gradient between a surface heated by insolation and a convectively cooled surface. Previous research has identified several key parameters that might affect the thrust produced by a radiometer array. These parameters include the heat transfer mechanisms on the individual vanes and between nearby vanes in an array, the proximity or spacing of radiometer vanes in an operational array, the effects of the vanes attached to a near space vehicle operating in the boundary layer near the surface of the vehicle, and the effects of attaching the vanes to a substrate on the vehicle itself. A combined numerical and experimental approach was used to investigate these effects. Numerical codes were used to assess the heat transfer mechanisms and the boundary layer flow. Experiments were designed and carried out to measure the force produced by a small array of radiometers. These results were combined to assess the performance of a notional array of radiometer vanes for the purpose of counteracting drag on a near space vehicle operating at an altitude of 60 km. The array performance was assessed based on the maximum force that can be produced as a function of array mass and area., The original document contains color images. Presented at the AIAA Thermophysics Conference, held New Orleans, LA, on 25-28 June 2012.

Chemical reactions between ions and electrons of a translating, high velocity field reversed plasmoid, including charge exchange and electron impact ionization, are examined in an adiabatic heat bath relaxation. Importance of different reaction mechanisms is analyzed for conditions typical for FRC thrusters, with gas and plasma densities on the order of 10(exp 18) molecule/cu m, plasma temperatures between 5 and 50 eV, and relative velocities up to 30 km/s. Then, an implicit Particle-In-Cell code, Celeste3D, extended to include plasma-neutral and neutral-neutral elastic and inelastic processes and Coulomb collisions, is used to model a planar flow between an FRC plasmoid initially at a Schmid-Burgk equilibrium, and neutral gas with varying relative velocities, plasma temperatures, and neutral gas densities. The contribution of charge exchange and electron impact ionization reactions is analyzed, and the sensitivity of the entrainment efficiency to the plasmoid translation velocity and neutral density is evaluated., 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 29 Jul-1 Aug 2012, Atlanta, GA. The original document contains color images.

The growing interest in the use of microspacecraft within government and industry is driving a critical need for new propulsion systems capable of fulfilling a wide range of mission requirements. The applicability of each propulsion system is highly mission dependent and subject to system constraints. The purpose of this research was to investigate the design, fabrication, and microspacecraft systems limitations associated with the scaling of micropropulsion devices., The original document contains color images.

A computational approach to homogeneous nucleation is proposed, based on Eulerian description of the gas phase coupled with a Lagrangian approach to the cluster phase. A continuum, Euler / Navier-Stokes solver VAC is used to model the gas transport, and a kinetic particle solver is developed in this work to simulate cluster nucleation and growth. Parameters in the new model were adjusted so as to match the known theoretical dimer formation equilibrium constants for two gases under consideration, argon and water. Reasonable agreement between computed and available experimental data was found in terminal cluster size distributions in nozzle water expansions in a wide range of stagnation pressures. The proposed approach was found to be orders of magnitude faster than a comparable approach based on the DSMC method., Published in the AIP Journal of Chemical Physics, v134 p104105-1-11, 2011. Prepared in cooperation with ERC Inc., Edward AFB, CA; and Air Force Research Laboratory (AFRL), Edwards AFB, CA. PDF updated to replace Preprint with Postprint on 29 Mar 2012.

This is a presentation to DSMC 11 on comparing reaction cross sections for two DSMC models and sensitivity in a nonequilibrium flow., For presentation at DSMC 11, Santa Fe, NM, 25-28 Sep 2011.

The Quantum Kinetic chemical reaction model proposed by Bird for the direct simulation Monte Carlo method is based on collision kinetics with no assumed Arrhenius-related parameters. It demonstrates an excellent agreement with the best estimates for thermal reaction rates coefficients and with two-temperature nonequilibrium rate coefficients for high-temperature air reactions. This paper investigates this model further, concentrating on the non-thermal reaction cross sections as a function of collision energy, and compares its predictions with those of the earlier total collision energy model, also by Bird, as well as with available quasi-classical trajectory cross section predictions (this paper also publishes for the first time the complete sets of these computed reaction cross sections). A rarefied hypersonic flow over a cylinder is used to examine the sensitivity of the number of exchange reactions to the differences in the two models under a strongly nonequilibrium velocity distribution., For submission to the journal Physics of Fluids. Prepared in collaboration with ERC Inc, Edwards AFB, CA and IMIP of CNR, Bari, Italy.

The solution of Kliegel's gas dynamic equations for seven moments of the velocity distribution functions that include mass, momentum, and directional temperatures, is examined for the one-dimensional heat transfer problem. The slip boundary conditions for the gas-surface interface are derived. The obtained solutions are compared to the DSMC predictions., International Symposium (27th) on Rarefied Gas Dynamics, Monterey, CA, 10-15 July 2010. Performed in collaboration with ERC, Inc., Edwards AFB, CA.

An overview of recent numerical and computational studies of radiometric phenomena is presented. Several important factors affecting radiometric forces have been analyzed, including the contribution of area, edge, and shear forces for different pressures and gases, chamber size effect, gas-surface accommodation, and multi-vane geometries., For presentation at the International Symposium on Rarefied Gas Dynamics (27th) to be held in Monterey, CA on 10-15 July 2010.

The impact of the radiometer vane thickness and edge geometrical shape on the total radiometric force is examined numerically solving the ES BGK model kinetic equation. The flow of argon over a single vane and a multi-vane configurations is considered in the range of Knudsen numbers from 0.02 to 1. The shear force is found to reduce the total radiometric force for most vane configurations. It is shown that the change in the vane shape may offset the losses due to the shear force in the multi-vane geometry., For presentation at the International Symposium on Rarefied Gas Dynamics (27th), to be held in Monterey, CA on 10-15 July 2010.

Molecular dynamics and DSMC models for energy transfer in argon dimer - argon monomer collisions are compared. The inelastic collision number for these collisions is evaluated, and found to decrease significantly when gas temperature increases., International Symposium (27th) on Rarefied Gas Dynamics, Monterey, CA, 10-15 July 2010. Performed in collaboration with ERC, Inc., Edwards AFB, CA; and Pennsylvania State University, University Park, PA.

A computational approach to homogeneous nucleation is proposed, based on Eulerian description of the gas phase coupled with a Lagrangian approach to the cluster phase. A continuum, Euler / Navier-Stokes solver VAC is used to model the gas transport, and a kinetic particle solver is developed in this work to simulate cluster nucleation and growth. The new model was found to reproduce well the known theoretical dimer formation equilibrium constants for two gases under consideration, argon and water. Reasonable agreement between computed and available experimental data was found in terminal cluster size distributions in nozzle water expansions in a wide range of stagnation pressures. The proposed approach was found to be orders of magnitude faster than a comparable approach based on the DSMC method., AIAA Thermophysics Conference, Chicago, IL, 24-28 June 2010. Performed in collaboration with ERC, Inc., Edwards AFB, CA; and the University of Southern California, Los Angeles, CA.

A near-space propulsion system using radiometric forces is notionally developed for the Lockheed HALE-D vehicle. The purpose of the system is to provide wind disturbance compensation for the vehicle operation at 20 km. Experimental results indicate that using a multi-vane configuration increases the force produced per unit mass of the radiometer. Previous numerical studies were implemented to determine the force production of the proposed concept design. The proposed design features in integrated system where multi-vane radiometers line the surface of the vehicle and operate on solar power. The HALE-D vehicle at 20 km requires 40.1 billion sub-vanes, at 26 micrometers in length, covering a total area of 101 square meters in order to produce the necessary 142.1 N of thrust to counter the wind drag. Results are also given for systems at altitudes above 20 km., Prepared in collaboration with the University of Colorado, Colorado Springs, CO; and ERC, Inc., Edwards AFB, CA. AIAA Joint Thermophysics and Heat Transfer Conference (10th), Chicago, IL, 28 June-1 July 2010.

A technique is proposed to assess gas-surface accommodation coefficients. The technique utilizes the fact that radiometric forces exerted on heated objects immersed in rarefied gases are governed by the interaction of gas molecules with the surface. In the present implementation, it connects measurements of radiometric forces on a heated vane in the transitional flow regime with the kinetic modeling of the flow, and derives the accommodation coefficients through the successive analysis of measured and computed results. A new combined ES-BGK / DSMC approach that allows accurate and time efficient analysis of radiometric forces on a vane in large vacuum chambers filled with rarefied gas is presented. Accommodation coefficients for the Maxwell model are estimated for argon, xenon, and helium on a machined aluminum surface, and found to be 0.81, 0.86, and 0.53, respectively., Published in Physics of Fluids, v21 n073101, 2009. Prepared in collaboration with the University of Southern California, Los Angeles, CA; and ERC, Inc, Edwards AFB, CA.

A kinetic approach is used to study the feasibility of increasing the efficiency of microactuators that use radiometric force through etching holes in a single radiometer vane. It has been shown that a radiometer that consists of small vanes is capable of producing at least an order of magnitude larger force than a single vane radiometer that takes up the same area. The optimum gap between the vanes is found to be slightly smaller than the vane size, with the optimum Knudsen number of about 0.05 based on the vane height. Several competing processes have been suggested for the actuation of micromechanical devices. Electrostatic micro-actuators have been suggested(1), (2) where a potential difference between two sets of combs produces a force generated by the associated electric field. Generally, electrostatic micro-actuators are linear drive devices and are not well suited for rotational operation. Piezoelectric devices(3) have been considered where an electric field applied to a material causes mechanical deformation in the material. The piezoelectric effect typically produces very small material deflections; however, they can have a very fast actuation frequency from the kHz up to the MHz range. Electrically conducting polymers(4) can be used to change the volume of a material to provide more actuation distance than a typical piezoelectric material with lower applied voltages.

An approach based on a discontinuous Galerkin discretization is proposed for the Bhatnagar-Gross-Krook model kinetic equation. This approach allows for a high order polynomial approximation of molecular velocity distribution function both in spatial and velocity variables. It is applied to model normal shock wave and heat transfer problems. Convergence of solutions with respect to the number of spatial cells and velocity bins is studied, with the degree of polynomial approximation ranging from zero to four in the physical space variable and from zero to eight in the velocity variable. This approach is found to conserve mass, momentum and energy when high degree polynomial approximations are used in the velocity space. For the shock wave problem, the solution is shown to exhibit accelerated convergence with respect to the velocity variable. Convergence with respect to the spatial variable is in agreement with the order of the polynomial approximation used. For the heat transfer problem, it was observed that convergence of solutions obtained by high degree polynomial approximations is only second order with respect to the resolution in the spatial variable. This is attributed to the temperature jump at the wall in the solutions. The shock wave and heat transfer solutions are in excellent agreement with the solutions obtained by a conservative finite volume scheme., Submitted for Publication in the American Institute of Aeronautics and Astronautics Journal in 2010.

Modeling of low speed gas flows at moderate Knudsen numbers, where conventional continuum CFD methods are inapplicable and kinetic methods are either inaccurate or prohibitively expensive, presents a significant challenge for computational modeling. In this work, an approach that combines a finite-volume solution of the ES-BGK model kinetic equation and the statistical DSMC method to accurately predict radiometric forces on a vane in large vacuum chamber filled with rarefied gas is presented. The approach in effect combines the accuracy of a statistical solution of the Boltzmann equation with the numerical efficiency of a deterministic solution of simplified model kinetic equations., For publication in Computer Methods in Applied Mechanics and Engineering.

Numerical study of a 5,900 lb solid propellant thruster plume at an altitude of 118 km is performed using a combined multi-step continuum/kinetic approach. The Navier-Stokes equations are solved for the flow inside the nozzle and the first several meters into the plume. The DSMC method is used to simulate the remaining plume and the plume-free stream interaction. A Monte Carlo based radiation code is applied in an overlay mode to calculate UV radiation in the near and far field of the plume. The computations take into account both alumina particles and soot. The effect of alumina particle emissivities and size distribution, as well as soot concentration, on UV radiation is clarified. Comparison of numerical results with available UV measurements is conducted.

Feasibility of using beamed microwave energy to increase thrust of solid rocket motors during launch is analyzed. Coupling of microwave radiation with internal energies of micron-sized alumina particles and the successive transfer of internal energy of particles to thermal and then kinetic energy of gas is expected to be the main mechanism of thrust increase. A two-phase two-way coupled capability has been developed. For the gas phase, the capability applies an Eulerian approach based on the solution of the Navier-Stokes equations, and uses VAC code modified to include particle source terms. For the particle phase, a Lagrangian approach is used based on a particle tracker that incorporates the impact of the gas and microwave radiation. The developed numerical capability is applied to compute the nozzle flow for a Castor 120 type motor with and without the microwave radiation. High gas density and thus fast collisional relaxation result in highly efficient transfer of microwave energy the kinetic energy of the flow. The total thrust was found to increase by 1.5% for a 100 MW/m2 beam, and 15% for a 1GW/m2 beam., For presentation at the AIAA Joint Propulsion Conference and Exhibit (45th) held in Denver, CO on 2-5 August 2009.

Numerical study of a 5,900 lb solid propellant thruster plume at an altitude of 118 km is performed using a combined multi-step continuum/kinetic approach. The Navier-Stokes equations are solved for the flow inside the nozzle and the first several meters into the plume. The DSMC method is used to simulate the remaining plume and the plume-free stream interaction. A Monte Carlo based radiation code is applied in an overlay mode to calculate UV radiation in the near and far field of the plume. The computations take into account both alumina particles and soot. The effect of alumina particle emissivities and size distribution, as well as soot concentration, on UV radiation is clarified. Comparison of numerical results with available UV measurements is conducted., Submitted for presentation at the AIAA Thermophysics Conference (40th), to be held in Seattle, WA, on 23-26 June 2008. Prepared in cooperation with University of Southern California (USC), Los Angeles, CA.

A multi-step continuum-kinetic approach is used to model a steady state plume flow from a Star 27 motor at an altitude of 188 km. Two-way coupled Navier-Stokes equations and the DSMC method are used to predict the interaction between plume and atmospheric gases and micron-sized alumina particles from the thruster. A Monte Carlo radiation code that accounts for photon scattering on particles is used to calculate UV radiation based on the obtained flowfield solutions. Comparison of computed spectral and integral radiant intensity with available flight data is performed. Photon scattering by submicron particles in the 200 nm to 400 nm range was found to be a dominant process in the far field UV emission., Submitted for publication in an AIAA Journal. Prepared in collaboration with ERC, Inc. and with the University of Southern California.

A two-phase plume flow from a small aluminized propellant side thruster interacting with rarefied atmosphere at 120 km has been examined numerically. A three step continuum-kinetic approach has been used, with the Navier-Stokes equations solved inside the nozzle, and a 2D/3D DSMC method employed to compute the plume nearfield and then the plume-atmosphere interaction region. At each of these steps, a two-way gas-particulate coupling has been used. The DSMC implementation uses molecular fluxes to calculate the number of gas-particulate collisions, and is based on a statistical approach to calculate deflection angles. A sensitivity study of various parameters of the approach is performed. The importance of two-way coupling, particle radiative cooling, and molecule accommodation on particle surface are analyzed.