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The influence of chemical accommodation coefficients on the forebody heating of the ballistic MIRKA re-entry capsule was investigated numerically with the nonequilibrium Navier-Stokes code URANUS. Chemical accommodation coefficients have been modeled physically consistent for all possible catalytic recombination mechanisms at the surface, i.e. direct recombination, the Eley-Rideal reaction type as well as the Langmuir-Hinshelwood reaction type. The difference in surface temperature was below 6 K when comparing the thermal nonequilibrium simulations, taking into account chemical accommodation coefficients below unity, with the thermal equilibrium simulations. In a second step, typical plasma wind tunnel conditions with stagnation pressures of 800 Pa and a total specific enthalpy of 20 MJ kg used to determine recombination coefficients in the plasma wind tunnel at IRS have been addressed. Given the chemical accommodation coefficient, only little uncertainty in determining recombination coefficients has been found.

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.

The laser-induced fluorescence measurement setup has been installed at the plasma wind tunnel PWK3 of the Institut fur Raumfahrtsysteme (IRS). Number densities and translational temperatures of atomic oxygen have been measured in pure oxygen plasma. The examined data has been used to calculate local specific enthalpies using measured tem- peratures and examined dissociation degree from the number density measurements. For a well investigated plasma condition, the results are compared to previous measurements using probe technique and absorption spectroscopy. The results agree well with previous experiments using probe techniques.

In the inductive stage of the DC-HF hybrid thruster Attila, power characteristics are measured and interpreted. This is done with two different induction coils of 2.5 and 4.5 turnsunder variation of ambient pressure. Test gases used are Nitrogen and Oxygen. At the present operation frequencies, generator power is coupled into the plasma with higher efficiency for Nitrogen than for Oxygen. Efficiencies determined for the inductive plasma generator are 44% and 43% in total.

Miniaturized flight experiments being developed at IRS for the European capsule program EXPERT are presented. The following paragraphs provide a brief survey on the sensor systems: PYREX, a miniaturized pyrometric entry experiment, measures rear side temperatures and heat fluxes on ceramic TPS. It has already been flown twice (EXPRESS, MIRKA). PYREX-KAT38 is a fully qualified six-channel measurement system and was intended to be operated aboard the X-38 vehicle. A similar design is being further developed for temperature and heat flux measurements in the nose structure of the EXPERT capsule. PHLUX (Sensor system for catalytic experiments) is based on the experiences with PYREX. It is a pyrometric sensor for catalytic heat flux measurements developed for the EXPERT capsule. Here, a probe with two sensors opposite each other has already been developed for ground tests by means of plasma wind tunnel experiments to qualify the sensor functionally and to investigate the catalytic behavior of surfaces. The flight experiment aims to estimate the dissociation degree during the entry using measured heat fluxes on different catalytic surfaces. This information is important to understand the thermochemical interactions between plasma and TPS wall. RESPECT is a miniaturized spectrometer to be used to gain spectral data in the flow field around EXPERT during its entry. The main goal is to obtain more detailed information about the plasma state in the post shock regime of EXPERT by measuring the spectrally resolved radiation onto the surface. The measured data will provide a database consisting of radiation of multiple species. These data will be used for the comparison with results of numerical simulations using e.g. the IRS URANUS code for validation of aerothermodynamic models.

The paper describes the development of the inductively driven plasma wind tunnel PWK3 enabling the electrodeless generation of high enthalpy plasmas for the development, investigation and qualification of heat shield materials required for space vehicles performing entry manouevres in the atmospheres of Venus, Earth and Mars. The facility, equipped with modular inductive plasma generators enabling the operation with gases such as carbon dioxide, air, oxygen and nitrogen has been qualified for thermal plasma powers up to 60 kW. The measurement of these powers, coil currents, operational frequencies, electromagnetic fields, heat fluxes on probes, Pitot pressures and Mach numbers allows the extensive characterization of both the plasma generators’ operational behavior and the plasma flow conditions. Newly developed models and numerical simulations, that correspond pretty well with the experimental data, enable a maximum plasma power in combination with long operational periods using different operational gases and gas mixtures by optimizing the plasma container wall thickness, the number of coil turns and the number of capacitors in the resonant circuit.

Inductive Plasma Generator developed at the Institut fur Raumfahrtsysteme (IRS) was used to produce high enthalpy pure oxygen plumes. Translational temperature was obtained by laser absorption spectroscopy using an absorption line from meta -stable atomic oxygen at 777.19nm. As a result, emission and absorption signals were found to fluctuate at 300Hz. Maximum translational temperature w as 6,700K on the centerline and 4000K at the edge of the plume. Then, the spatially averaged static enthalpy and kinetic enthalpy estimated by measured temperature and corresponding frozen loss calculated under thermo -chemical equilibrium were 6.5MJ/kg 5.5 MJ/kg and 15.5MJ/kg, respectively . Then, the total enthalpy was estimated to be 27.5MJ/kg. This value agrees with input enthalpy of 30MJ/kg.

The objective of the presented work was the investigation of heat flux on a cooled copper surface in two different plasma flows, N2/O2 and N2/CO2. Both plasmas were generated by the thermal arcjet generator RB3. The mass fraction of CO2 was the same as the mass fraction of O2 in the air plasma. The formation of NO was investigated with laser-induced fluorescence measurements (LIF). The heat flux on copper is about two times higher in an N2/CO2–plasma than in an air plasma but NO recombination is much lower. The rotational temperature in front of the probe was estimated by comparison of excitation scans with simulated spectras.

The operating parameters and resulting plasma properties of a blind hollow cathode (BHC) discharge have been investigated. The hollow cathode was driven capacitively with a pulsed dc signal of 200 kHz in a power range between 50 and 100 W at an ambient pressure of about 10 Pa. The working gas was argon, which was introduced with a ceramic capillary at different positions of the longitudinal axis of the hollow cathode with flow rates of between 30 and 1000 sccm. The current–voltage characteristics were recorded.The pressure at the end of the BHC was measured with a miniaturized pressure transducer with varying volumetric flow rate and axial position of the capillary in the hollow cathode. To characterize the ignition behaviour of the system, the measured breakdown voltages were compared with phenomenological Paschen curves calculated from the pressure data.Optical emission spectroscopy was used to examine the origins of the light emission, comparing the glow mode and hollow cathode mode in particular.A high-speed camera recorded some plasma processes. A mounting with an indium tin oxide coated glass was used to observe the inner volume of the BHC along the longitudinal axis, while the plasma was operated with different parameters. The optical observations revealed an inhomogeneous plasma condition along the axis.

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.

An overview of the IRS payload development for ESA's EXPERT mission is given. The final design and performance parameters of the payloads PYREX, PHLUX and RESPECT are described. PYREX is a sensor system measuring the thermal protection system (TPS) rear side temperature. PHLUX is a catalysis based experiment to determine the dissociation degree of the boundary layer. RESPECT applies optical emission spectroscopy to measure spectrally resolved the radiation onto a TPS surface.

The research and development process of the ablative pulsed plasma thruster SIMP-LEX is summarized. By experiment, the magnetic field in between the electrodes was measured with an induction probe, and a high-speed camera was applied to estimate the propagation of the plasma plume. Further, the optimization process towards higher thrust efficiency by means of geometric and electrical modifications is described. By analysis, the calculation of magnetic field and change in inductance is summarized and the numerical models for the performance simulation of SIMP-LEX introduced. A peak value of 0.7 T was found for the magnetic field created during an 80 J discharge whereas a propagation velocity of 28 km/s was estimated for the first plasma plume. The optimization process towards a new engineering model yielded an increase of 40% in mean exhaust velocity to 26 km/s, and a maximum thrust efficiency of 32% was evaluated respectively. Calculation of magnetic field and change in inductance based on the Biot-Savart law yield input for the slug model, which can then predict the general performance parameters with a good accordance.

Health Monitoring in aerospace applications becomes an emerging technology leading to the development of systems capable of continuously monitoring structures for damage with minimal human intervention. A promising sensing method to be applied on hot structures and thermal protection systems is the electrical resistance measurement technique, which is barely investigated up to now. This method benefits from the advantageous characteristics of self-monitoring materials, such as carbon fiber-reinforced materials. By measuring the variation of the electrical resistance of these materials information on possibly present mechanical damage can be derived. In order to set up a database on electric properties of relevant materials under relevant conditions and to perform a proof-of-concept for this health monitoring method a facility has been laid out, which allows for the measurement of the electrical resistance of thermal protection system relevant materials at temperatures up to 2000°C. First preliminary measurements of the surface resistance of a graphite sample have been performed and are presented. It has been proven necessary to make some modifications to the setup. Therefore, the remaining measurements with graphite and C/C-SiC samples are subject of further investigation which will be performed in the future.

The inductively heated plasma source at the Institute of Space Systems was investigated with respect to the wall thickness of the plasma tube using an air plasma. For this, the wall thickness of the quartz tube was reduced in steps from 2.5 to 1.25 mm. The significance of reducing the wall thickness was analyzed with respect to both the maximum allowable tube cooling power and the coupling efficiency. While the former results from thermal stresses in the tube's wall, the latter results from a minimization of magnetic field losses near the coil turns of the inductively coupled plasma (ICP) source. Analysis of the thermal stress could be validated by experimental data, i.e. the measurement of the tube cooling power when the respective tube structure failed. The coupling efficiency could be assessed qualitatively by simplified models, and the experimental data recorded show that coupling was improved far more than predicted.

An overview is given on a numerical simulation program for applied field magnetoplasmadynamic (AF-MPD) thrusters, which is currently under development at the Institute of Space Systems (IRS). The program allows the simulation of argon plasma flows in thermal and chemical non-equilibrium. The code is based on an axisymmetric finite volume method on unstructured, adaptive meshes. An externally applied magnetic field can be taken into account employing the vector potential formulation. Azimuthal velocity and magnetic field are handled by a quasi three-dimensional approach with vanishing azimuthal derivatives. Additionally, a laboratory AF-MPD thruster for the power range of 10-20 kW has been built up at IRS. The goal of the experimental investigations is the characterization of the operation and performance parameters of the thruster.

In order to predict the thermal and mechanical loads during re-entry, the URANUS (Upwind Relaxation Algorithm for Nonequilibrium Flows of the University of Stuttgart) has been being developed at the Institute of Space Systems (IRS) of the Universitat Stuttgart. For the accurate determination of the thermochemical conditions, advanced thermochemical relaxation models for the gas-phase as well as sophisticated gas-surface interaction models have been developed. The Navier-Stokes equations for the 11-component air flow, which consists of N2, O2, NO, N, O, N2+, O2+, NO+, O+, N+ and e-, have been derived by the Chapman-Enskog method from the Boltzmann-Equation. The linearized system of equations is solved fully coupled and fully implicitly, employing Newton's method. In the paper, the modelling is described in some detail and selected simulation results are presented.

A facility for the determination of total and spectral emission coefficients is described. Values of measured total emission coefficients of the different high temperature ceramics such as HfO2, Al2O3, Yt2O3 and the metals/alloys Tungsten, TZM and PM1000 in the temperature regime of 750 - 1800 K are given. The drastic influence of the oxidation state of PM1000 on the emissivity is discussed. Additionally, results of an investigation of the influence of surface roughness and surface topology on emissivity are presented. Therefore, three SSiC samples with root-mean-squared roughness surface roughness from Rq = 0.05 to Rq = 0.66 have been prepared using common finishing operations. The tests showed that the emissivity increased about 10 % with an increase of the surface roughness even in the regime where Rq values are in the same magnitude or much smaller than the maximum emitting wavelength. Knowledge on emissivity is a basis for the analysis of thermo-chemical behavior and related properties (i.e. recombination coefficients) as the two properties influence each other. In a second step a high enthalpy inductively heated plasma wind tunnel (PWK3) is described and a methodology to derive recombination coefficients is explained. Recombination coefficients for the above mentioned materials have been determined in pure oxygen plasma. The methodology for the determination of recombination coefficients of ceramic and metallic Thermal Protection System (TPS) materials in single species gases used at IRS and its latest improvements are presented. Test results for the recombination coefficients in oxygen plasma are shown between 1469 and 2072 K.

More than 25 years of experience have been gained in the field of electric propulsion at IRS. Recent developments within the field of electric propulsion are summarized and foremost results are highlighted. The various types of electric propulsion systems are not considered as to be competitive. Here, system analysis shows that optimum parameter such as the required exhaust velocity or specific impulse result taking into account both the mission profile and system related sizes such as the power conditioner efficiency, the thrust efficiency and the specific mass of the corresponding power unit. Correspondingly, ion thrusters, Hall thrusters, thermal arcjets, or magnetoplasmadynamics (MPD) thrusters are preferable depending on the mission. Among the described electric propulsion systems are recent developments in the field of applied field MPD but also from high power hybrid thrusters. In addition, new concepts such as the hybrid systems TIHTUS and CETEP are analysed.

Inertial Electrostatic Confinement is under investigation regarding the applicability as an advanced space propulsion system at the Institute of Space Systems, University of Stuttgart. As part of an extensive characterization of operational conditions and the jet extraction a two-grid system has been tested for its breakdown and discharge conditions. One of the known operation modes, the jet mode, is the most promising operation mode for that application. The breakdown discharge conditions and operational discharge conditions for several pressure ranges are given and the corresponding discharge phenomena shown.

1 . Among experiments conducted by American and Japanese researchers, a contribution of Germany was made by the SLIT experiment. Here, a conventional spectrometer in a Cerny Turner configuration (f=300mm, grating 600 l/mm) was fed by fiber optics with light collected by a small mirror telescope. The telescope was able to collect light from a viewing angle of only 0.45 degrees. Manual tracking was performed using a tracking camera with a viewing angle of 20 degrees. A wavelength range from 324 nm to 456 nm was covered with a pixel resolution of 0.08 nm. Data was taken during 30 s around the point of maximum heating until the capsule left the observed region with a measurement frequency of 5 Hz. Due to the difficulties in tracking not every spectrum contained data. Nevertheless, the emission of CN as a major erosion product as well as N2 + and different atoms could be monitored successfully during that time. Due to the nature of the set up, no spatial resolution of the radiation data was achieved so all measured values represent an integration both over the visible part of the glowing heat shield and over the plasma in the post shock region. The measured spectra could be split up into continuum spectra which represent a superposition of the heat shield radiation and the continuum radiation of potential dust particles in the plasma and into line spectra from the plasma in the post shock layer. Planck temperatures as well as rotational and vibrational temperatures of CN and N2 +

*† ‡ § This paper describes the method for the determination of the recombination coefficients for ceramic materials in air plasma and pure oxygen plasma. Additional a new method for the in situ determination of the spectral emissivity of the high temperature ceramic materials is presented. Values of the recombination coefficients for some materials, resulting from the described methodology, are given and compared with literature. Values of the spectral emissivities of the investigated materials are presented 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 re-entry program EXPERT.

First results of the application of an improved sensor design for local mass–specific enthalpy estimation are presented. The sensor is based on the fact that the reduction of heat flux to a probe body by mass addition into the boundary layer depends on the free stream enthalpy. In reverse, measurement of this heat flux reduction boundary layer mass addition can be used for enthalpy determination. However, the classical, theoretical approach to solve the boundary layer equations is based on several assumptions, the most important of which is the self similarity of the boundary layer. The new design of the probe meets the requirements of these theoretical assumptions. The coolant is now fed through one bore hole in the stagnation point of the spherical probe head. The cooled area corresponds to the area used for the calorimetric heat flux measurement. The heat flux reduction for different coolant mass flow rates behaves similar to the former probe design. For a pure high enthalpy argon plasma flow the determined enthalpy value of h = 25.9 MJ/kg compares well to the semi–empiric approach of Marvin and Pope.

The appliedeld magnetoplasmadynamic thruster AF-MPD ZT1 was successfully put in operation at IRS. The thruster operated in steady-state mode with Argon as propellant at low electric arc power of 6 kW. Discharge voltage and power increased as expected almost linear with applied magnetic ux density. The variation of mass ow rate ratio between anode and cathode gas towards higher cathode gas fraction showed increasing thrust and thrust e ciency at applied magnetic ux density of 0:1 T and are comparable with DLR’s X13 thruster. Additionally a steady-state 100 kW AF-MPD thruster SX3 was developed, set in operation and preliminary characterized at IRS. The SX3 thruster was operated at relatively low arc powers up to 30 kW and applied magnetic ux density of 0:1 T generated by the modi ed ZT coil. Due to low arc current and magnetic ux level, the AF-MPD ZT1 thruster achieved thrust of 70 mN and exhaust velocity up to 10 km/s at 6 kW arc power and up to 6 % thrust e ciency. The SX3 thruster reached thrust of 362 mN and thrust e ciency more than 12 % in steady state-operation at 25 kW arc power and at applied magnetic ux density of 0:1 T. However both thrusters have been operated at limited magnetic uxes only. For SX3 a total operation time of more than 3600 s together with 30 ignitions could be accumulated. The electrodes, however, do not show signi cant erosion nor a respective degradation. The performances of thrusters provide an outlook for future investigations on AF-MPD thrusters at IRS and give a hint to improvement in thrust e ciency of presented devices with the new appliedeld coil, which will be manufactured in the future to produce magnetic ux densities up to 0:6 T allowing further increase in thrust e ciency up to 30 %.

The production of hi gh enthalpy flows in a stationary manner requires plasma wind tunnels using plasma ge nerators. Magneto plasmadynamic generators (MPG), thermal arcjet devices (TPG) and inductively hea ted plasma generators (IPG) have been de ve loped at the IRS for differ ent atmospheres, different atmos pheric entry regimes and/or different basic in vesti gations. The absence of electrodes enables the use of IPGs for basic TPS material tests (e.g. catalycity) and the simulation of re -active atmospheres such as the Martia n or Venerian atmosphere. With IPG3 an rf -source is provided where the coil is closer to the plasma than it was with previous de signs. Therefore, the electromagnetic field loss is re duced. The water cooling system surrounds both the induction coil and th e plasma container. This paper describes the design of IPG3 and the experimental results using O 2. Three test cases with pure O 2 plasma were investigated using the absorption spectroscopy system of Department of Advanced Energy , the University of Tokyo, in combination with a Pitot pressure probe and a material sample support system of IRS, Universitat Stuttgart. Number density and translational temperature distributions of atomic oxygen were obtained from the measured absorption line profiles including abso rption lines of meta -stable atomic oxygen at 777.19 and excited atomic oxygen at 844. 64 nm. Mach number was deduced by the ratio between pitot pressure and ambient pressure. Surface temperature of TPS materials was measured by thermocouplers . 1. Introduc tion A variety of stringent requirements upon technology are imposed by both space travel to a rather close Earth orbit and to more distant celestial bodies such as the planets of our solar system. Among those requirements there are the high temperature ef fects occurring during atmospheric entries. Here, spacecrafts encounter gases at velocities of more than ten km/s, thereby being subjected to great heat loads. This makes it necessary to use high temperature thermal protection systems (TPS) to prevent the destruction of the space vehicle. Both TPS and environment (plasma) during the entry have to be investigated by means of computational and ground facility based simulations. Such ground facilities are the five IRS plasma wind tunnels PWK1 -5 that reproduce the thermal, aerodynamic and chemical load on the surface of a space vehicle entering a celestial body’s atmosphere. MPGs enable gas flows on high enthalpy levels. They are mainly used to investigate the erosion of radiation cooled heat shield materials li ke C -C or C -SiC as well as the behavior of ablative ma terials under thermal and chemical loads. Two IRS plasma wind tunnels are operated with MPGs. Furthermore, a TPG producing moderate enthalpies and higher stagnation pressures to simulate the follow on flight path is in use [1, 2]. As an IPG is contactlessly heated by induction currents, exceptionally pure plasma flows of any gases can be produced. Therefore, the application of an IPG delivers two major advantages. First, researchers are aware that the catalytic behavior of TPS materials is one of the main topics to be investigated in future. A main part of the heat flux seen by a TPS material derives from the recombination of atomic plasma species. Using the IPG the related heat flux measurements are not falsified by impurities as with the electrode plasma generators. Furthermore, the catalytic mechanisms of single gases such as of O 2 and N 2 [3, 4] in the case of air of an atmosphere can be investigated. Fig. 1 shows a typical pure oxygen plasma plume of IPG3 at 500Pa ambient pressure, an oxygen mass flow rate 3g/s and at an anode voltage U A=6250V. The resonant circuit was adjusted to nominal 650kHz using a 5 -turn coil with 7 capacitors each of 6nF (see description below).

The development status of a Navier-Stokes code allowing for a variable thermo-chemical modelling of nonequilibrium flows is presented. The system of the conservation equations is implemented allowing for the simulation of air flows as well as other flows, e.g. CO2 and CO2 N2. A high enthalpy flow in thermal equilibrium around a cylinder and a sphere has been calculated using different chemical models in order to verify the implementation.