Showing papers on "Arcjet rocket published in 1998"

Spectroscopic Emission Measurements Within the Blunt-Body Shock Layer in an Arcjet Flow

Abstract: In the present study, radiation emanating from the freestream and shock-layer e ow over a 15.24-cmdiam, e at-faced cylinder model was measured in the NASA Ames Research Center’ s 20-MW Arcjet Facility. The test gas was a mixture of argon and air. Spatially resolved emission spectra were obtained over a 200- to 890-nm wavelength range using a charged-coupled device camera (1024 3 256 array) attached to a spectrograph. The optical system was calibrated using tungsten and deuterium radiation sources. Analytical tools were used to determine the following line-of-sight-averaged thermodynamicproperties from the calibrated spectra: 1 ) rotational temperature of the freestream and 2 ) rotational, vibrational, electronic temperatures, and species number densities within the shock layer. An analysis was performed to estimate the uncertainty bounds of the determined properties.

High-Power Hydrogen Arcjet Thrusters

Abstract: A radiation-cooled thermal arcjet thruster named HIPARC-R has been developed and investigated. It has been designed for the 100-kW power level and is operated with hydrogen as its propellant. A specie c impulse of 1970 s was obtained with a mass e ow rate of 150 mg/s at the 100-kW power level and at a thruster efe ciency of about 28%. This equals a specie c input power value of 670 MJ/kg. Parallel to the experiments a numerical code system was developed to further optimize the next generation of hydrogen arcjet thrusters. This code system consists of a e nite volume e ow code coupled with program modules for the calculation of thermal, chemical, and electronical properties. In addition, a program module for the calculation of the heat e ow inside the thruster, including heat exchange, has been applied to model the heat transfer processes during thruster operation. The thruster has been operated over a wide power range and has been intensively investigated for the qualie cation of the numerical code system. Within this paper the experimental setup and the code system are described, the performance data are presented, and experimental and numerical results are compared.

Effect of Propellant Temperature on Efficiency in the Pulsed Plasma Thruster

Abstract: : A pulsed plasma thruster (PPT) benefits from the inherent engineering simplicity-and reduced tankage fraction gained by storing the propellant as a solid. The solid is converted to the gaseous state and accelerated by an electric discharge across the propellant face. Previous research has concluded that as little as 10% of the consumed propellant is converted to plasma and efficiently accelerated. The remaining propellant is consumed in the form of late-time vaporization and particulate emission, creating minimal thrust. Critical to improving the PPT performance is improving the propellant utilization. The present work demonstrates one possible method of increasing the PPT propellant efficiency. By measuring the PPT thrust, propellant consumption, and propellant temperature while varying the power level, duration of the experimental run, and total propellant mass, a correlation is established between decreased propropellant temperature and increased propellant efficiency. The method is demonstrated by performance measurements at 60 W and S W, which show a 25% increase in thrust efficiency, while the propellant temperature decreases from 135 to 42 deg C. Larger increases in the efficiency may be realized on-orbit where operating temperatures are commonly subzero. The dependence of propellant consumption on temperature also creates systematic errors in laboratory measurements with short experimental runs, and orbit analyses where the PPT performance measured at one power level is linearly scaled to the power available on the spacecraft.

Application of Planck-Rosseland-Gray Model for High-Enthalpy Arc Heaters

Abstract: An upgraded ARCFLO computer code using a radiation model named Planck-Rosseland-Gray(PRG) model is developed. The developed method is capable of analyzing both an air and a carbonaceous gas flow in the constrictor of an arcjet wind tunnel. In the first part of this paper, the method is tested against the experimental data for air. The testing is carried out by modifying the turbulence parameters employed in the ARCFLO code. Radiation calculation is made by accounting for electron thermal nonequilibrium. Agreements between experimental data and calculation are obtained by choosing a set of the turbulence parameter appropriately. In the second part, a carbonaceous gas flow is preliminary calculated with the turbulence parameter so chosen. The depth of carbon condensation is evaluated approximately. The characteristic data such as mass averaged enthalpy, etc., are presented for typical operation conditions.

Arcjet flow characterization using laser-induced fluorescence of atomic species

Abstract: Flow properties are determined from recent experimental measurements in the Aerodynamic Heat- ing Facility (AHF) Arcjet at NASA Ames Research Center. In the experiments, two-photon Laser-Induced Fluorescence (LIF) is used to measure flow velocity, translational temperature, and atomic nitrogen con- centrations over a range of facility operating conditions for air/argon gas mixtures. Sensitivities of the mea- sured flow properties to changes in primary arcjet con- trol parameters, pressure and current, are examined. With additional facility data and certain assumptions the flow property measurements are used to determine the centerline enthalpy and its apportionment into ki- netic, thermal, and chemical contributions. Compar- isons of the relative modal contributions to the total enthalpy and results from previous LIF measurements in the AHF are used to assess the validity of the as- sumptions. Based on the analysis of the experimental measurements, conclusions are drawn about the ther- mochemical state of the nonequilibrium free stream flow.

Arcjet flow properties determined from laser-induced fluorescence of atomic nitrogen

Abstract: Flow property measurements that were recently acquired in the Ames Research Center Aerodynamic Heating Facility arcjet using two-photon laser-induced fluorescence (LIF) of atomic nitrogen (N) are reported. The flow properties, which include velocity, translational temperature, and N concentration, were measured simultaneously over a range of facility operating conditions for N2–argon test gas flows in the 30-cm-diameter nozzle. A recent measurement of the two-photon excitation cross section for the 3p4D° ← 2p4S° transition of atomic nitrogen is used to convert the relative nitrogen concentration measurements to absolute values, and a nitrogen flow reactor is used to provide a room-temperature, reference-wavelength calibration of the translational temperature and velocity measurements. When combined with information from facility measurements, an analysis of the flow properties obtained using two-photon LIF of N yields the total free-stream flow enthalpy.

Electrode Configuration Effect on the Performance of a Two-Dimensional Magnetoplasmadynamic Arcjet

Abstract: Thrust performance and internal plasma flowfield of a 1-MW class self-field magnetoplasmadynamic (MPD) arcjet were measured to evaluate their dependence on the cross-sectional geometry of the electrodes. A multichannel two-dimensional MPD arcjet in quasisteady operation was used to visualize the two-dimensional flowfield and reveal the correlation between the internal flowfield and the thrust performance. The experimental results for six different electrode configurations show that the thrust performance strongly depends on the thruster chamber cross-sectional geometries for the 7sp range of interest, 1000-3000 s. The cathode length determined the engine performance, regardless of the anode geometry. In particular, the convergent-divergent anode with a short cathode showed the best performance. The superior acceleration mechanism of the short cathode was explained on the basis of two-dimension al plasma distributions such as discharge current contours and plasma density obtained by Mach-Zehnder interferometry. A dense plasma region near the tip of the short cathode was observed and subsequent expansion guided by the diverging nozzle can enhance aerodynamic acceleration, which contributes to large thrust generation.

Ablation and thermal response program for spacecraft heatshield analysis

Abstract: An implicit ablation and thermal response program is presented for simulation of one-dimensional transient thermal energy transport in a multilayer stack of isotropic materials and structure which can ablate from a front surface and decompose in-depth. The governing equations and numerical procedures for solution are summarized. Solutions are compared with those of an existing code, the Aerotherm Charring Material Thermal Response and Ablation Program, and also with arcjet data Numerical experiments show that the new code is numerically more stable and solves a much wider range of problems compared with the older code. To demonstrate its capability, applications for thermal analysis and sizing of aeroshell heatshields for planetary missions, such as Stardust, Mars Microprobe (Deep Space n), Saturn Entry Probe, and Mars 2001, using advanced light-weight ceramic ablators developed at NASA Ames Research Center, are presented and discussed.

Influence of constrictor size on thrust performance of a very low power arcjet

Abstract: An experimental study was performed to evaluate the feasibility of arcjet operation at very low power levels ranging 5 W 35 W. The very low power arcjet was run using nozzles with different material and geometry. Nitrogen gas was used as the propellant. Both propulsive performances and thermal characteristics at the constrictor exit were investigated for conventional nozzles which consist of an assemble of tungsten nozzle parts, and modified nozzles, consisting of an assemble, an insulator and a tungsten anode. In the modified nozzles, a ceramic material or an insulator was used as a part of a constrictor to allow an arc column penetrate further downstream of the constrictor or to maintain the high-voltage mode discharges and to reduce the electrode losses. Stable operations with the specific impulse levels of ~ 270 sec at very low power levels ranging about 5 W 35 W with the constrictor diameter of 0.3 mm or 0.5 mm were confirmed at efficiencies between 30 and 40 percent, except a singular case, glow discharge, in which little effect in propulsive performance was observed with the expense of electrical power. At higher specific powers the specific impulse was relatively independent of mass flow rate. At lower specific powers, the specific impulse for the lower mass flow rate was slightly above that for the higher mass flow rate. The constrictor diameter was found to have significant effect on the thermal characteristics (heavy particle temperature and thermal efficiency) of the internal gas flow and the performance of the device. With partially insulated nozzles the specific impulse and thrust efficiency were significantly increased compared to conventional nozzles.

Ultrahigh vacuum arcjet nitrogen source for selected energy epitaxy of group iii nitrides by molecular beam epitaxy

Abstract: The key technical challenge in the molecular beam epitaxial (MBE) growth of group III nitrides is the lack of a suitable source of incorporatable nitrogen. In contrast with the growth of the other III–V compound semiconductors by MBE, direct reaction of N2 with excess group III metal is not feasible, because of the high bond strength of dinitrogen. An incorporatable MBE nitrogen source must excite N2 forming a beam of atomic nitrogen, active nitrogen (N2*), or nitrogen ions. rf and electron cyclotron resonance sources use electron impact excitation to obtain atomic nitrogen and in the process generate a wide variety of excited ions and neutrals. Experiments have shown that ionic species in the beam degrade the morphology of the epitaxial layer and generate electrically active defects. Recent theoretical studies have predicted that ground state atomic nitrogen will successfully incorporate into the growing GaN surface, while atomic nitrogen in either of the excited doublet states will lead to etching. In this article, we report on the development of an ultrahigh vacuum-compatible arcjet source which uses an electric arc to thermally dissociate N2. The thermal excitation mechanism offers selective excitation of nitrogen and control of kinetic energy of the active species. This source has been fabricated from refractory materials and uses two stages of differential pumping to minimize the pressure in the growth chamber. The arcjet has been reliably operated at power levels of 10–300 W, with no visible degradation of the thoriated tungsten cathode after 300 h. No metal contaminant lines can be found in the optical emission spectrum. Using an Ar-seeded beam for calibration of the optical spectrum, we find that the arcjet plasma is far from local thermodynamic equilibrium, and show that the fraction of atomic nitrogen in the beam ranges from 0.3% to 9%. This corresponds to a flux of 0.1–4 monolayers per second at the MBE sample location. With an articulated Langmuir probe sampling the beam at the MBE growth position, we find a positive ion flux of less than 4× 10−9 A/cm2, a maximum ion kinetic energy of 3.5 eV, a median electron energy of 1 eV, and a maximum electron energy of less than 4 eV. With increasing arcjet power, the ion and electron fluxes increase and the ion energy distribution shifts to lower energies. No change in the electron spectrum is observed. Quadrupole mass spectra of the ion flux measured on the arcjet axis show that the N+/N2+ ratio has a maximum at an arcjet power of about 35 W.

Computational Simulation of Emission Spectra from Shock-Layer Flows in an Arcjet Facility

Abstract: This paper reports computational comparisons with experimental studies of a nonequilibrium bluntbody shock-layer e ow in a high-enthalpy arcjet wind tunnel at NASA Ames Research Center. The experimental data include spatially resolved emission spectra of radiation emanating from a shock layer formed in front of a e at-faced cylinder model. Multitemperature nonequilibrium codes are used to compute the conical nozzle e ow, supersonic jet, and shock-layer e ow. A line-of-sight (LOS) radiation code is employed to predict the spectra from the computed e owe eld. Computed LOS emission intensities are directly compared with the experimental data at several axial locations along the stagnation streamline. Various LOS-averaged e ow properties, such as vibrational and rotational temperatures, and species number densities, deduced from the experimental spectra, are compared with computed results. Comparisons provide an assessment of thermochemical equilibration processes in an arcjet shock layer.

Techniques for Estimating Rotational and Vibrational Temperature in Nitrogen Arcjet Flow

Abstract: Molecular nitrogen and molecular nitrogen ion spectra are used to infer vibrational and rotational temperatures in an arcjet sbock layer under the assumption of Boltzmann populations. Various techniques correlate spectral features with temperatures, among which are determining intensity integrals and forming ratios that are then correlated with rotational and vibrational temperatures. Sensitivity factors, determined by correlating these ratios with temperature, are indicators of the potential accuracy of different spectrum regions for determining temperature. In another technique, least-squares fits of measured spectra are made to fit temperature-dependent computed spectra, including global fits to computed spectra as a function of temperature or fits to vibrational-level basis spectra. Technique accuracy is described and precision improved by combining the results of several techniques. When results from the various techniques are combined, overall temperature determination accuracy at a single point in an arcjet shock layer is about ±4% for vibrational temperature and ±10% for rotational temperature.

Advanced designs for high pressure, high performance solid propellant rocket motors

Abstract: A solid rocket propellant formulation with a burn rate slope of less than about 0.15 ips/psi (2.54 cms/69.102Pa) over a substantial portion of a pressure range of about 1,000 psi (69.102Pa) to about 7,000 psi (69.102Pa) and a temperature sensitivity of less than about 0.15 %/°F (0.15 %/0.56 °C) is provided. A high performance solid propellant rocket motor including the solid rocket propellant formulation is also provided. The rocket motor is encased in a high strength low weight motor casing which is further equipped with a nozzle throat constructed of material that has an erosion rate not more than about 2 to about 3 mils (2.54.10-3cm) per second during motor operation. The solid rocket propellant formulation can be cast in a grain pattern such that an all-boost thrust profile is achieved.

Overview of major U.S. industrial programs in electric propulsion

Abstract: Commercial activity in electric propulsion remained robust over the past year, with 146 spacecraft now on orbit using electric propulsion, an increase of 13 in the past year. For the past 12 months, the most notable first was the launch of the Hughes 702 spacecraft (Galaxy XI) using ion thrusters for orbit raising and stationkeeping. Outside the geostationary market, there was a significant slowdown induced by the failure and/or delay of the Low Earth Orbit constellation programs. Additionally, considerable progress has been made developing arcjet, Hall thruster, ion thruster, pulsed plasma thruster, and microthruster system technologies, including power processor and feedsystem elements. Introduction Electric propulsion continues to gain acceptance in the commercial spacecraft community due to its clear economic advantages over chemical propulsion. During the past year, Loral has initiated integration of Hall thruster systems on the Telstar 8 spacecraft (to be launched in 2001), Hughes launched 6 spacecraft using ion thruster systems, and Lockheed Martin launched 7 A2100 spacecraft using arcjet systems. All of these spacecraft are geostationary communications satellites for which electric propulsion provides either increased life, increased payload, or decreased launch mass. Figure 1 shows all operational satellites using electric propulsion. In addition to the conventional applications, micropropulsion has emerged as a growing area of technology development. Micropropulsion technology will allow microsatellites and nanosatellites to be organized as local clusters and constellations to provide unique mission capabilities. With the exception of rapid inclination change, kilogram-class nanosatellites require 1-to-1000 jlN thrusters for on-orbit maneuvering. Several options for these technologies exist, including colloid thrusters, pulsed plasma thrusters, and an electric/chemical hybrid technology using electrolysis. For each of the key techology areas, this paper summarizes the launch status of spacecraft on which it is applied and provides an overview of new technologies under development by U.S. industry. Technology and Application Programs Arciet and Resistoiet Systems Lockheed Martin has been utilizing electric propulsion thrusters on its satellites since the incorporation of the Primex Aerospace Company hydrazine electrothermal hydrazine thrusters (EHTs) providing 292 sec. Isp with 0.43 kw input power on the Lockheed Martin Astrospace Series 3000 satellite bus. Modest improvements in performance were achieved on the Series 5000 bus with higher power level Improved EHTs (IMPEHTS), Primex EHTs are now also used by Orbital Sciences on their geosynchronous satellites. The Lockheed Martin Series 7000 satellites bus allowed the introduction of the next generation electrothermal thruster, the MR509 hydrazine arcjet (1.8 kW power level, 502 sec. Isp), also developed by Primex. The arcjet continued to evolve with the latest commercial satellite bus currently being delivered by Lockheed Martin Space Systems Company the A2100 (TM). A2100 (TM) satellites utilize 2 kw, 585 sec. nominal Isp thrusters for station acquisition, North-South stationkeeping, East-West station keeping, and orbit relocation. The A2100 arcjets continue to evolve with the implementation and flight demonstration of ruggedized arcjets with gas arrestors and higher performance arcjets are currently under development at Primex for future programs with an expected nominal Isp of 620 seconds. At the Aerospace Corporation, the first laser-induced fluorescence and mass spectrometric studies for a flight-type arcjet thruster were completed on a Primex MR-512 arcjet. This arcjet has similar performance to the MR-509 but utilizes a low-voltage input PPU. An extensive series of Direct Simulation Monte Carlo (DSMC) computations were performed in-house to compute plume impingement torque and heating effects, and in collaboration with lain Boyd of University of Michigan to compute detailed properties in the near-field. The synergy between theory and experiment has helped produce an improved understanding of species-specific and general properties in both near-field and far-field.

Arcjet nozzle flow-field characterization by laser-induced fluorescence

Abstract: Laser-induced fluorescence of the Balmer–alpha (Hα) transition of atomic hydrogen was performed within the nozzle of a 1-kW class radiatively cooled arcjet thruster operating on hydrogen and synthesized-hydrazine propellants. Axial velocities were determined from the Doppler shift of the Hα line center relative to a stationary reference, whereas translational temperatures and electron number densities were determined from a line-shape analysis of the Hα transition. The results are compared with a numerical model and indicate excellent agreement with the velocities, as well as temperatures near the nozzle exit. There are discrepancies, however, in the temperatures far upstream of the exit and in the electron densities, suggesting needed improvements in the modeling of the recombination chemistry.

Cathode Sheath Voltage Models for Hydrazine Arcjets

Abstract: We present two different approaches to predict the cathode sheath potential necessary to account for the total voltage in a 1-kW hydrazine arcjet operating at 10 A, 50 mg/s. The first approach is a jump condition cathode sheath model, which treats the cathode surface conditions and properties at the sheath edge. The input of the sheath model includes attachment area and the temperature at the cathode, and the near-cathode bulk plasma properties, which are obtained from a nonequilibrium arcjet model. The sheath potential from the sheath model is approximately -30 V for a pure tungsten cathode with 2 x 10 -8 m 2 attachment area, and 3680 K cathode tip temperature. The second approach is a detailed nonneutral plasma cathode sheath model, which employs Poisson's equation to calculate the charge distribution in the sheath and Saha's equilibrium equation to calculate species densities in the ionization region. The sheath voltage, electron temperature, and electron number density predictions from the two cathode models agree well. However, the cathode surface electric field differs by a factor of 10 or more

Propellant Management and Conditioning within the X-34 Main Propulsion System

Abstract: The X-34 hypersonic flight vehicle is currently under development by Orbital Sciences Corporation (Orbital). The Main Propulsion ystem as been designed around the liquid propellant Fastrac rocket engine currently under development at NASA Marshall Space Flight Center. This paper presents analyses of the MPS subsystems used to manage the liquid propellants. These subsystems include the propellant tanks, the tank vent/relief subsystem, and the dump/fill/drain subsystem. Analyses include LOX tank chill and fill time estimates, LOX boil-off estimates, propellant conditioning simulations, and transient propellant dump simulations.

Cathode sheath voltage models for hydrazine arcjets

Abstract: We present two different approaches to predict the cathode sheath potential necessary to account for the total voltage in a 1-kW hydrazine arcjet operating at 10 A, 50 mg/s. The first approach is a jump condition cathode sheath model, which treats the cathode surface conditions and properties at the sheath edge. The input of the sheath model includes attachment area and the temperature at the cathode, and the near-cathode bulk plasma properties, which are obtained from a nonequilibrium arcjet model. The sheath potential from the sheath model is approximately -30 V for a pure tungsten cathode with 2 x 10 -8 m 2 attachment area, and 3680 K cathode tip temperature. The second approach is a detailed nonneutral plasma cathode sheath model, which employs Poisson's equation to calculate the charge distribution in the sheath and Saha's equilibrium equation to calculate species densities in the ionization region. The sheath voltage, electron temperature, and electron number density predictions from the two cathode models agree well. However, the cathode surface electric field differs by a factor of 10 or more

Spectroscopic emission measurements within the blunt-body shock layer in an arcjet flow

Abstract: In the present study, radiation emanating from the freestream and shock-layer flow over a 15.24-cm-diam, flat-faced cylinder model was measured in the NASA Ames Research Center's 20-MW Arcjet Facility. The test gas was a mixture of argon and air. Spatially resolved emission spectra were obtained over a 200- to 890-nm wavelength range using a charged-coupled device camera (1024 x 256 array) attached to a spectrograph. The optical system was calibrated using tungsten and deuterium radiation sources. Analytical tools were used to determine the following line-of-sight-averaged thermodynamic properties from the calibrated spectra: 1) rotational temperature of the freestream and 2) rotational, vibrational, electronic temperatures, and species number densities within the shock layer. An analysis was performed to estimate the uncertainty bounds of the determined properties

Low Bus Voltage Hydrazine Arcjet System for Geostationary Satellites

Abstract: The capabilities of arcjet propulsion systems were recently extended to accommodate operation on the NASD A Data Relay Test Satellite (DRTS) providing a power bus voltage between 31 and 51.5 VDC. This paper summarizes the newly attained qualification status of the MR 509-A/B arcjet system demonstrating the flexibility of the current design. A redesign of the Power Processing Unit (PPU) became necessary as well as a delta-qualification of the thruster to validate spacecraft integration, and to provide compliance with the DRTS satellite environmental requirements. Two types of thrusters with different thrust levels were made available to meet mission requirements. The delta-qualification included a pyro-shock test, vibration tests to a higher level than previously tested, and performance mapping beyond the original range. Included in the paper is an assessment of the PPU performance characteristics as well as the discussion of the system operation and system telemetry.

The structure of an expanding hydrogen arcjet plasma

Abstract: The structure of an expanding low-power hydrogen arcjet plasma is investigated through probe-based measurements of impact pressure and mass flux. Comparisons are made to direct simulation Monte Carlo models (DSMC) of the nonignited flow (cold flow) and continuum magnetohydrodynamic (MHD) models of the arc-heated flow. While general agreement with previous spectroscopic data and DSMC calculations are obtained, the cold flow impact pressures are shown to exhibit features that may be due to probe–flow interactions and rarefied gas effects. The ability to identify shocks in the plume, predicted by the DSMC model and previously observed by Raman spectroscopy, shows that the cold flow probes were capable of resolving major flow features. The impact pressure measurements of arc-heated flow also agree quite well with the results calculated from the MHD model, and it is demonstrated that integrated thrust densities derived from impact pressure measurements are in agreement with direct measurements of thrust using ...

Arcjet Semi-Elliptic Nozzle Simulations and Validation in Support of TPS Testing

Abstract: Much of the ground based testing of advanced thermal protection system (TPS) components for the X33 program is done in arc-heated wind tunnels such as those located in the Arc-Jet Complex at NASA Ames Research Center. These facilities are capable of simulating the high temperature, chemically reacting flow environment experienced by the vehicle during flight. This allows one to test critical design issues such as maximum reuse temperatures, seals, gaps, and increases in heating due to interfaces between different materials. Computational fluid dynamics (CFD) has evolved to the point where it now can be used in the vehicle design process for accurate and timely prediction of trajectory based aerothermal heating environments for re-entry vehicles. It can also be used for simulation of the flow environments in ground based facilities such as arcjets. By utilization of the same CFD code and solution methodology, the important differences between ground test and flight may be quantified. The goal of this paper is to utilize CFD to provide validated simulations of the flow environment in the NASA-Ames semi elliptic nozzle arcjet facilities. The validation of the ground simulations will come From comparison to existing calibration data. Specific tests in support of the X33 TPS test program will ilso be simulated. In this manner, the differences between the ground test simulation and the flight environment can be identified for a measure of ground test to flight traceability.

Arc Jet Flow Properties Determined from Laser-Induced Fluorescence of Atomic Nitrogen

Abstract: An laser-spectroscopic investigation of the thermocheMical state of arcjet flows is currently being conducted in the Aerodynamic Heating Facility (AHF) Circlet at NASA Ames Research Center. Downstream of the nozzle exit, but upstream of the test article, Laser-Induced Fluorescence (LIF) of atomic nitrogen is used to assess the nonequilibriuM distribution of flow enthalpy in the free stream. The two-photon LIF technique provides simultaneous measurements of free stream velocity, translational temperature, and nitrogen number density on the flow centerline. Along with information from facility instrumentation, these measurements allow a determination of the free stream total enthalpy, and its apportionment in to thermal, kinetic, and chemical mode contributions. Experimental results are presented and discussed for two different niti-ogen/argon test gas flow runs during which the current is varied while the pressure remains constant .

Arcjet Anode Plasma Measurements Using Electrostatic Probes

Abstract: A 1-kW hydrazine arcjet thruster has been modie ed for internal probing of the anode sheath boundary layer with an array of 14 electrostatic microprobes e ush mounted into the anode body. Axial and azimuthal distributions of the plasma properties e oating potential, anode sheath potential, wall current density, electron number density, and electron temperature have been obtained for arc currents between 7.8 and 10.6 A and propellant e ow rates of 40 ‐60 mg/s. The specie c power ranged from 18.8 to 27.4 MJ/ kg. Azimuthal symmetry has been verie ed for all arcjet operating conditions. The electron temperature data show that the near-anode plasma is highly nonequilibrium. Most of the current density and anode heating is located within 2 ‐4 mm of the constrictor exit, with the location affected more by mass e ow rate than by arc current. The axial anode heating distribution is closely coupled to current density and accounts for ;18‐24% of the total input power. Reasonable agreement between a numerical model and experimental results is found for a constant value of the electron inelastic energy-loss factor.

A theoretical study on alkali seeding of arcjet thrusters

Abstract: A model has been developed to assess the feasibility of reducing the frozen losses in a hydrogen arcjet by adding very small amounts of easily ionizable cesium vapor. It is found that within reasonable constraints on the constrictor geometry, and without allowing the electron temperature to exceed about 7000K, both the ionization and the hydrogen dissociation losses can essentially be eliminated, and a speci c impulse of about 850 seconds can be obtained. A small perturbation analysis was performed on the nonequilibrium governing equations with the intention of probing the physics of ionization instabilities that occur in both the cesium and hydrogen ionization ranges. The results from this analysis show that the seed ionization instability occurs at electron temperatures where Coulomb interactions are important. Its occurence in the system was found to result in driving the electron temperature above its threshold value for near-full ionization of the seed. The hydrogen ionization instability was found to occur when the ratio of electrons from hydrogen to the total present is about 2%. Thesis Supervisor: Manuel Martinez-Sanchez Title: Professor

Modeling the plume contamination and emissions of an ammonia arcjet

Abstract: The direct simulation Monte Carlo method is employed to compute cold flows of ammonia for a large arcjet that is to be tested in space in the upcoming ESEX flight experiment. The nozzle flow computation indicates that the flow is almost in thermal equilibrium at the nozzle exit. A very large and expensive computation of the back flow region of the actual spacecraft geometry is performed to provide predictions of mass fluxes that will be measured in flight by quartz crystal micro-balances. It is indicated that contamination of the spacecraft occurs even in regions lying behind a plume shield. A further computation is performed to simulate the interaction of the arcjet plume with the ambient atmosphere. The high impact energy is offset by the very low atmospheric density at the spacecraft operational altitude of 833 km. Nevertheless, it is indicated that ammonia chemistry occurs and the primary products are NH, NH2) and OH. These species radiate strongly in the ultra-violet. The estimated emission intensities of the molecules are similar to those measured previously in situ by a lower velocity reentry experiment. An estimate is also made of the intensity of emission from ammonia. In all cases, it is concluded for the cold flow that none of the emissions will be detectable by the ground based observation facility that is part of the space experiment. Introduction Arcjets represent a mature electric propulsion technology that is replacing chemical propulsion engines for orbit maintenance procedures on many satellites. The U. S. Air Force has developed the Electric Propulsion Space Experiment (ESEX) to study several of the effects of using a very high power ammonia arcjet on a spacecraft. The ESEX experiment is part of the ARGOS flight that will orbit the Earth at an altitude of about 833 km. The flight experiment, due for launch in the winter of 1998, includes diagnostic measurements to analyze exhaust plume contamination and radiative heating effects as well as arcjet performance. Remote 1 Associate Professor. Mechanical and Aerospace Engineering. Senior Member AIAA. 2 Graduate Research Assistant. Mechanical and Aerospace Engineering. 3 Post-doctoral Research Associate. Mechanical and Aerospace Engineering. 4 Research Professor. Chemistry. Member AIAA 5 Research Scientist. Deceased. optical measurements are also planned from a ground station at Maui. Several arcjet operational modes will be investigated including the cold flow case where the arc is not ignited. The goal of this paper is to conduct numerical simulations of the cold flow of the arcjet. The primary motivation for these computations is to provide estimates of the contamination on the spacecraft by ammonia that will be measured by quartz crystal micro-balances. Hence, the focus of these computations will be the back flow region behind the thruster exit plane. A secondary focus of the study is to investigate the interaction of the ambient atmosphere with the ammonia jet exhausting from the thruster. In particular, the goal is to estimate the radiative emission generated under these conditions. The layout of the paper is as follows. First, the arcjet thruster and diagnostics of ESEX are described. Next, the computational approach is discussed. Due to the relatively low densities involved, the direct simulation Monte Carlo method (DSMC)2 is employed. Discussion of results is divided into separate sections concerning the back flow contamination studies and the radiative emission calculations.