Monika Auweter-Kurtz

PLASMA GENERATORS FOR RE-ENTRY SIMULATION

The qualification of thermal protection systems (TPS) and numerical design tools for re-entry vehicles and space probes requires the ability to understand and duplicate the prevailing complex physico-chemical phenomena, including thermal and chemical nonequilibrium near the surface of a body that enters the atmosphere of the Earth or another celestial body. At the Institut fur Raumfahrtsysteme of the University of Stuttgart, four plasma wind tunnels (PWK1-4) are in operation to simulate the thermal, aerodynamic, and chemical loads on the surface of a space vehicle. Three different plasma sources have been developed for this purpose: 1) a magnetoplasmadynamic generator for the simulation of the highenthalpy and low-pressure environment during the first phase of re-entry, 2) a thermal arcjet device for the follow-on flight path at moderate specific enthalpies and higher stagnation pressures, and 3) an inductively heated generator for basic materials experiments over a wide range of specific enthalpies and pressures. Special efforts were made to avoid electrode erosion to preclude impairing the erosion and catalytic behavior of TPS materials. A detailed description of these plasma generators and an overview of the simulation regions and operation areas of the plasma wind tunnels are presented.

Operational Behavior of Inductively Heated Plasma Source IPG3 for Entry Simulations

Thedesignoftheplasmawindtunnel (PWK3),itsinductivelyheatedplasmagenerator (IPG3),andexperimental results using O 2 and CO2 as operational gases are described. Various operational conditions (mass e ow rate, frequency) were applied. The thermal plasma powers measured with a calorimeter in the chamber of PWK3 are presented, leading to thepossibility to determineefe ciencies. When thepowerwas varied, two discrete operational transitions were observed. The simultaneous differences in the discharge behavior of IPG3 are represented by sudden changes of parameters such as calorimetric powers and local heat e uxes measured with a stationary heat e ux probeinthechamberofthefacility. Additionally,thecoolingpoweroftheIPG3plasmatubeispresented.Here, a plasma stabilization effect appearing with the second operational transition was observed when the tube cooling power decreased suddenly despite the simultaneous increase of the plasma power in the chamber. An imaging spectrometer measuring the radial intensity of the plasma through an axial optical window of IPG3 was used. Both the operational behavior, in particular the stabilization effect, and the heat e ux measurements show that PWK3 enables high-enthalpy tests for both basic thermal protection material tests and atmospheric entry simulation of spacecrafts.

High-Power Hydrogen Arcjet Thrusters

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.

Cathode Erosion Studies on MPD Thrusters

The fundamental processes occurring at the cathode interphase of magnetoplasma dynamic (MPD) arcs and their consequences on material erosion are by no means fully understood, despite the great many publications in this field on various different arc types of similar conditions. This paper presents the results of a cathode erosion study on an MPD arc thruster under pulsed and steady-state operating modes and, moreover, gives an explanation of the cathode attachment mechanisms for both operating modes.

Enthalpy Measurement in Inductively Heated Plasma Generator Flow by Laser Absorption Spectroscopy

Laser absorption spectroscopy was applied for diagnostics of inductively heated plasma generator flows. Temporal variation of translational temperature was deduced from measured absorption line broadening because the flow properties fluctuated at 300 Hz in synchronization with the induction current. The specific total enthalpy and mole fraction of oxygen were estimated from the deduced temperature assuming thermochemical equilibrium. Consequently, the averaged degree of dissociation of oxygen is 0.92. The specific total enthalpy was estimated at 33.7 ± 2.9 MJ/kg; 39% of it was in the form of chemical potential. The results show good agreement with intrusive measurements.

Recombination Coefficients and Spectral Emissivity of Silicon Carbide-Based Thermal Protection Materials

The method for determining the recombination coefficients for ceramic materials in air plasma and pure oxygen plasma is described. Additionally, a new method lor the in situ determination of the spectral emissivity of high-temperature ceramic materials is presented. Values of the recombination coefficients for considered materials, resulting from the described methodology, are given at temperatures between 1483 and 1851 K and pressures between 356 and 950 Pa and compared with literature. Values of the spectral emissivities of the investigated materials are presented in a temperature range from about 1200 to 1820 K and compared with values obtained by other authors. Obtained results will be used for the development of catalytic sensors in the frame of the European reentry program EXPERT.

New Inductively Heated Plasma Source for Reentry Simulations

Apart from magnetoplasmadynamic plasma generators (MPG) and thermal arcjet devices (TPG) for re-entry simulation, inductively heated plasma generators (IPG ) have been developed for basic TPS material tests at the IRS. These inductively heated plasma generators consist mainly of a water-cooled coil surrounding a tube in which the plasma is located Due to this setup no electrode erosion appears; impurities in the plasma are minimized. Hence, both the behaviour of the gas components can be examined singularly and basic material tests such as the determination of the catalytic behaviour can be performed. With IPG3 an rf-source is provided where the coil is closer to the plasma than it was with previous designs. Therefore, the electromagnetic field loss is reduced. The water cooling system surrounds both the induction coil and the plasma container. IPG3 was qualified up to an anode power of 180 kW (argon), which is an essential improvement regarding the intended re-entry simulations in combination with the IRS MPG-simulations in the plasma wind tunnels 1 and 2 (PWK). The structure of the rf-plasma source IPG3 and the facility PWK3 are presented in this paper. This paper describes the design of IPG3 and the first experimental results for the plasma source. Power characteristics of IPG3, which were measured under variation of gas, are shown. Within this variation of gas frequency measurements were made, which will be a help for later numerical simulations of IPG3. A CID camera was used to measure the radial intensity of the plasma through an axial optical window of IPG3. The results led to a rough determination of the skin depth for the operation with air.

Oxidation Behavior of Siliconcarbide-Based Materials by Using New Probe Techniques

Hysteresis of passive to active and active to passive transition of SiC oxidation behavior has been investigated theoretically, numerically, and experimentally. Theoretical and experimental investigations show a strong interaction between transition and catalysis. Dependence on plasma composition is shown.Arecently developed reaction model has been implemented in the advanced nonequilibrium Navier–Stokes code URANUS. Results are presented for the highly dissociated flow around the MIRKA capsule. In this case, radiation adiabatic surface temperatures have been found to be 120Khigher for active oxidation conditions as compared to passive oxidation conditions. To investigate transition behavior in detail, various new probe measurement techniques have been developed. Important additional observations have been made in chemical nonequilibrium.Within plasma wind-tunnel testing, a sudden temperature increase of up to 400 K was found with the transition from passive to active oxidation. Theoretical and numerical predictions show good qualitative and quantitative agreement with experimental results.

Numerical Modeling of the Flow Discharge in MPD Thrusters

Two theoretical models for calculating the current and flow distributions in self-field MPD thrusters have been developed and are applied to evaluate the effects of geometry, propellant type, scaling, and other parameters on the thruster performance. For continuous thrusters, a stationary code has been developed. The extended Ohms law is used to calculate the current contour lines, and a one-dimensional, two-component expansion flow model is applied to obtain the velocity, temperature, and pressure distributions for calculating the gas properties, which are again used in Ohms law. An integration over the volume and thermal forces equals the thrust. The differential equation is solved by means of a finite-difference method for the geometry of the nozzle-type plasma thruster DT2-IRS, which has been investigated experimentally in a steady-state as well as in a quasi-steady-state mode. The calculated current density distribution and the computed thrust are compared with these experimental results. For the starting phase of the steady-state MPD thrusters as well as for pulsed thrusters, a time-dependent, fully two-dimensional code has been developed. It uses a modified McCormack FD method in cylindrical coordinates to calculate the time-dependent flow, temperature, and pressure fields.

Arcjet Thruster Development

For several years an intensive program has been in progress at the University of Stuttgart to investigate and develop thermal arcjets for propellants including ammonia, nitrogen-hydrogen mixtures simulating hydrazine, and hydrogen. Since hydrogen yields the highest specific impulse /sp and best efficiencies TJ, special emphasis was placed on this propellant. Arcjet power levels between 0.7-150 kW have been studied, including water- and radiation-cooled laboratory models and flight hardware. Results yielded a maximal attainable 7sp as a function of the design and power level and showed that increasing power increased /sp. Radiation-cooled arcjets show better 17 and 7sp than water-cooled devices, but raise technical problems because of the high temperatures of the thrusters, which require the use of special refractory materials. Proper arcjet optimization was done with a thorough thermal analysis, including the propellant flow. A further improvement of these thrusters was reached by regenerative cooling and by optimizing the constrictor contour. The constrictor flow is modeled by a three-channel model, the results of which are compared with experimental data. A new two-dimensional computational fluid dynamics (CFD) approach for hydrogen arcjet thrusters is presented. In 1996 a 0.7-kW ammonia arcjet is scheduled for a flight on the P3-D AMSAT satellite.