Georg Herdrich

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.

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.

Characteristics of plasma properties in an ablative pulsed plasma thruster

Pulsed plasma thrusters are electric space propulsion devices which create a highly transient plasma bulk in a short-time arc discharge that is expelled to create thrust. The transitional character and the dependency on the discharge properties are yet to be elucidated. In this study, optical emission spectroscopy and Mach-Zehnder interferometry are applied to investigate the plasma properties in variation of time, space, and discharge energy. Electron temperature, electron density, and Knudsen numbers are derived for the plasma bulk and discussed. Temperatures were found to be in the order of 1.7 to 3.1 eV, whereas electron densities showed maximum values of more than 1017 cm−3. Both values showed strong dependency on the discharge voltage and were typically higher closer to the electrodes. Capacitance and time showed less influence. Knudsen numbers were derived to be in the order of 10−3−10−2, thus, indicating a continuum flow behavior in the main plasma bulk.

IRS Ground-Testing Facilities: Thermal Protection System Development, Code Validation and Flight Experiment Development

Five plasma wind tunnels (PWK 1 - 5) are in operation for the investigation and qualification of TPS materials. Different plasma sources are developed for generating the high enthalpy plasma flows as they are expected for reentry flights. Exemplary results of measurements for trajectory points of X-38 and the planned EXPERT vehicle are presented. The PWKs and auxiliary facilities such as the IRS black body source and the emissivity measurement facility as well as their different application possibilities are described. Results are shown exemplary such as in-flight measurement technique calibration, emissivity measurements of SiC, investigation of passive-active transition (German research program ASTRA) and heat flux profile simulation for X-38 (TETRA). Within the PYREX-KAT38 development, a miniaturized pyrometric system which was planned to measure the temperatures in the nose structure of the X-38 during reentry, the stagnation point heat flux profile of X-38 was simulated by programming the computer controlled probe platform in the magnetoplasmadynamically driven PWK1. Efforts are being made at the inductively heated PWK 3,

OXIDATION BEHAVIOR OF SIC-BASED THERMAL PROTECTION SYSTEM MATERIALS USING NEWLY DEVELOPED 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. A recently developed reaction model has been implemented in the advanced non-equilibrium NavierStokes 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 120 K higher for active oxidation conditions as compared to passive oxidation conditions. In order to investigate transition behavior in detail various new probe measurement techniques have been developed. Important additional observations have been made in chemical non-equilibrium. 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.

Investigation of the Magnetic Field in a Pulsed Plasma Thruster

The magnetic field between the electrodes of an 80 J ablative pulsed plasma thruster was both measured and determined analytically. This was done to better understand the acceleration process and assess the accuracy of the analytical method. The measurements at different positions in both breech-fed and side-fed propellant configurations were performed using a shielded induction probe of 1 mm in diameter. To calibrate the probe, a Helmholtz coil was built. The magnetic field was calculated using the law of Biot-Savart and assuming a current sheet thickness of 3 mm. The measured magnetic field showed an overall peak at 0.7 T. It was possible to confirm the induced current loop formed between the electrodes at the time when the overall current passes through zero. The comparison between the magnetic field model and the measurements at the propellant surface of a breech-fed thruster showed reasonably good accordance.

Propellant Utilization Efficiency in a Pulsed Plasma Thruster

In a pulsed plasma thruster discharge, the propellant utilization efficiency is commonly seen as the weak point for its thrust performance, and for its low thrust efficiency respectively. However, the value depends strongly on operational settings and the design of the pulsed plasma thruster itself. To access this efficiency to understand further the influence of parameters, a method combining experimental and modeling efforts is proposed within this paper. By using various optical methods, including high-speed camera imaging, emission spectroscopy, and Mach–Zehnder interferometry, the plasma bulk velocity for different energy configurations of the SPPT-68 ADD SIMP-LEX is derived. Together with the total ablated mass and an improved slug model, the ratio of mass accelerated in the discharge process is computed and, thus, the propellant utilization efficiency is derived. Velocities in the range of 25 to 55  km/s were measured, leading to propellant utilization efficiencies of about 40 to 60% depending on t...