Acta Astronautica 65 (2009) 676–686www.elsevier.com/locate/actaastro

Surface characterization ofmetallic and ceramic TPS-materials forreusable space vehicles

M. Schüßler∗, M. Auweter-Kurtz, G. Herdrich, S. LeinInstitut für Raumfahrtsysteme (IRS), Pfaffenwaldring 31, 70569 Stuttgart, Germany

Received 12 December 2006; accepted 13 January 2009Available online 10 May 2009

Abstract

The newly qualified IRS facility for the determination of total and spectral emissivities and its recent numerical optimizationis described. Values of measured total emissivities of the ceramics HfO2, Al2O3, Yt2O3 and the metals/alloys tungsten, TZMand PM1000 in the temperature regime of 750–1800K are given. The drastic influence of the oxidation state of PM1000 on theemissivity is discussed. Additionally, results of an investigation of the influence of surface roughness and surface topology onemissivity are presented. Therefore, three SSiC samples with surface roughness from Rq =0.05 to 0.66 have been prepared usingcommon finishing operations. The tests showed that the emissivity increased about 10% with an increase of the surface roughnesseven in the regime where Rq values are in the same magnitude or much smaller than the maximum emitting wavelength.Recombination coefficients for the abovementioned 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 singlespecies gases used at IRS and its latest improvements is presented. Test results for the recombination coefficients in oxygenplasma are shown between 1469 and 2072K.© 2008 Published by Elsevier Ltd.

1. Introduction

Reusable launch vehicles (RLVs) encounter high gasvelocities and aero-thermal heat loads during ascent andre-entry. To withstand these loads, the vehicle requiresa thermal protection system (TPS). Within the ESAWinglet Launcher Studies (WLS), the Future EuropeanSpace Transportation Investigation Programme (FES-TIP) and the later Future Launcher Preparatory Program(FLPP) the development of a hybrid hot load-carryingTPS, partly of silicon based materials for hot regions

∗Corresponding author.E-mail addresses: schuessler@irs.uni-stuttgart.de

(M. Schüßler), auweter@irs.uni-stuttgart.de (M. Auweter-Kurtz),herdrich@irs.uni-stuttgart.de (G. Herdrich), lein@irs.uni-stuttgart.de(S. Lein).

0094-5765/$ - see front matter © 2008 Published by Elsevier Ltd.doi:10.1016/j.actaastro.2009.01.048

like the nose and metallic alloys for colder parts of theTPS like side panels has been advanced. Especially ox-ide dispersed strengthened (ODS) alloys like PM1000have proven promising because of their high temper-ature strength, creep behaviour and their oxidationresistance.In contrast to the amount of work done in the field

of mechanical properties characterization [1,2] andengineering development [3,4] less experimental workhas been conducted to characterize the catalytic andoptical properties of such materials. These propertiesare of great importance because they are determiningthe thermo-chemical behaviour of the TPS during agiven re-entry trajectory of a vehicle. Due to the highgas velocities in the re-entry phase, the oxygen andthe nitrogen molecules passing through the bow shockbecome at least partly dissociated. Depending on the

M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686 677

Nomenclature

A area (m2)cp specific heat capacity (J/(kgK))f correction function (–)h specific enthalpy (J/kg)kw recombination rate constant (1/s)Le Lewis number (–)m mass flow (kg/s)M molar mass (kg/mol)M specific radiation (W/m2)Ma Mach number (–)p pressure (Pa)q heat flux (W/m2)R radius (m)Reff effective probe radius (m)R universal gas constant (J/(molK))Sc Schmidt number (–)S signal of pyrometer (V)t time (s)T temperature (K)u velocity (m/s)x axial coordinate (m)y radial coordinate (m)� accommodation coefficient (–)� recombination coefficient (–)� emissivity (–)� angle of the point (rad)

� radiation heat flux (W)� wavelength (m)� isentropic exponent (–)� dynamic viscosity (kg/(ms)) dissociation degree (–) angle of the bow shock (rad)� density (kg/m3)� correction for catalytic effects (–)

Subscripts/superscripts

b backsideD dissociatione boundary layer edge (stagnation)fc fully catalyticfinitc finit catalyticMat materialN nitrogenL structural lossO oxygenPl plasmaPos positionS sampletot totalw wall∞ free stream

environmental conditions (e.g. pressure and tempera-ture of the TPS material) these atoms will recombine atdifferent rates following different mechanisms. In anycase, the released recombination energy of this exother-mal chemical reaction results in an additional heat flux(HF) on the TPS and the gas phase in the boundary layer.The increase in HF can be as much as 3 times for anair system, comparing a noncatalytic to a fully catalyticmaterial [5,6]. As the catalysis and also emissivity of aTPS material depend primarily on temperature, the HFonto a spacecraft itself becomes indirectly dependanton the TPS surface temperature. In case of commonsilicon based TPS materials, the raised temperaturesmay trigger another surface phenomena, which is con-nected to the catalysis properties: passive–active tran-sition (PAT) [7] along with a further rapid temperatureincrease and consequently a much higher erosion rate.In order to develop and weight-optimized TPS for thefuture, it is absolutely necessary to determine the mate-rial properties and closely investigate these surface phe-nomena and their interaction in ground tests and flightexperiments.

2. Determination of total emissivities

2.1. The emissivity measurement facility (EMF)

The IRS (EMF) has been qualified for total and spec-tral emissivities of ceramic and metallic TPS materials[8]. It is possible to test the same geometry (26.5mmdiameter European Standard Sample) as in the plasmawind tunnel facilities, thus it is possible to test blankmetal samples and samples that have been exposed tothe thermo-chemical loads during plasma wind tunneltesting. (Herein the samples from the double probe mea-surements see Section 3.5.)In its working principle the EMF has similarities to

a variable black body (BB) source. However, the ap-paratus is much more complex, as a moveable samplesupport system within the BB cavity is required. Thetest setup of the facility is shown in Fig. 1.

The BB cavity is realized with a graphite rod (�=0.9)with a high length to diameter ratio which is resistivelyheated up for the measurements. The realized configu-ration has an apparent emissivity �> 0.999, thus it can

678 M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686

Fig. 1. Setup of the EMF test facility.

Tem

pera

ture

/[K] P

ower

/ [W

]

TemperaturePower

3000

2500

2000

1500

1000

500100 150

Current [A]200 250 300 350 400

Fig. 2. Power–current characteristic of the EMF.

be regarded as a BB. The rod also has to maintain anisothermal profile to properly approximate a BB source.This was achieved by an optimization process, were theoutline and the wall thickness of the rod was varied overits length.Inside the rod the material sample is mounted on a

graphite sample holder which is itself fixed on a molyb-denum stick. On the back side of the vacuum vessel (seeright side of Fig. 1) the stick is guided through a spe-cial flange. The flange houses a sealing assembly thatallows the axial shift of the stick while maintaining thevacuum characteristic of the tank. For the measurementit is of importance that the time for the shift of the sam-ple is as short as possible to minimize the temperaturedrop in the sample. The utilized pneumatic piston drivetakes about 0.25s for a shift and has a special stop dy-namic to minimize the forces on the sample. The twostage vacuum system consists of a mechanical pump andan oil diffusion pump. The minimum pressure achievedis about 10−1 Pa. The power supply system is capableof a maximum current of 1000A. During operation thecurrent is controlled. Fig. 2 shows a typical power andcurrent profile for a desired temperature within the BB.

For temperatures higher than 1800K the increasingevaporation rate of graphite requires an inert gas at-mosphere inside the vessel. Therefore, an argon supplysystem is installed, providing an inert overpressureatmosphere during high temperature measurements.Moreover it has been found that the inert atmospherealso minimizes the oxidation process of the samplematerials, hence keeping test conditions more stableand reproducible. As a consequence, all tests were car-ried out in argon atmosphere. During the experimenta pyrometer is used for the temperature measurement.Depending on the purpose of the test, the detectorcan be changed to either a wavelength nonselectivedevice for total emissivity measurements or a spec-tral pyrometer for spectral emissivity measurements.As the total emissivity of a material is the decisiveparameter for the radiation characteristic of a TPS ac-cording to the well-known Stefan Bolzmann equationand necessary for the determination of the catalysisrelated properties this paper focuses on total emissivitymeasurements.This type of measurement postulates a pyrometer de-

vice which detects all wavelengths of Plank’s radiationdistribution. As this is technical infeasible, one has tomake the cut back to a most possible broadband de-tector type and accept a certain part of the intensitylost for detection. Respectively, a broadband thermopiletype pyrometer with a sensitive wavelength interval of1.1.22�m with a measurement rate of 5Hz was used.Hence at 750K the detected radiation fraction of Plank’sdistribution is 99.4%, at 1500K it is still 97.9% and at1800K it is 93.5%. A suitable optical window materialwas found to be KRS-5, that is used to provide opticalaccess though the front end of the vacuum vessel intothe BB cavity (see Fig. 3, left).As shown in Fig. 3, KRS-5 glass has a good trans-

mittance at wavelengths up to 35�m. As KRS-5 hasa very low melting point of about 690K [9], a hybridwater/air cooling system had to be installed to pro-tect the KRS-5 window from exceeding its workingtemperature.

2.2. Measurement principle

The measurement chain consists of the following ac-tions:

(A) The sample is placed in the sample holder at thebottom of the cavity (Pos. 1, Fig. 4). The design ofthe cavity defines an apparent emissivity close tounity at Pos. 1.

M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686 679

Fig. 3. Transmittance of KRS-5.

Fig. 4. Cut through the BB-Cavity and the moveable sample supportsystem.

(B) The vacuum vessel is evacuated and then refilledwith an inert argon atmosphere.

(C) The cavity is resistively heated up to the desiredtemperature at which emissivity has to be mea-sured. The electrical current can be adjusted henceemissivity can be measured at different tempera-tures. The temperature is constantly (5Hz) takenvia the pyrometer. Due to radiation exchange thesample comes in thermal equilibrium with thecavity.

(D) When the nominal temperature is stable, the sam-ple is rapidly shifted in Pos. 2 via the pneumaticpiston. As the shift is quick, it can be consideredisothermal.

(E) After another temperature is taken at Pos. 2, thesample is re-shifted to Pos. 1 for further measure-ments.

The total emissivity is evaluated by the obtained ap-parent temperature drop perceived by the pyrometer.According to the Stefan Bolzmann law:

SPos i ∼ Pos i ∼ MPos i ∼ � · �Pos i · T 4Pos i (1)

The ratio of the obtained signals (�Pos 1�1) yields in:

SPos 2SPos 1

= �Pos 2 · T 4Pos 2

T 4Pos 1

(2)

As the shift is isothermal (TPos 1 = TPos 2) the emis-sivity can be determined by the ratio of the two sig-nals or can be evaluated by the apparent temperaturedrop. Independently from Eqs. (1) and (2), neglectingthe offset values the used pyrometer, calibrated on a BB,allows for:

S ∼ T 4 consequently Sa ∼ T 4a (3)

Hence the emissivity can also be obtained through:

�Pos 2 = T 4a,Pos 2

T 4a,Pos 1

(4)

2.3. Results and discussion

For all tests, the surface of each sample has beenhoned to a surface roughness of about Rq = 0.5. Ad-ditionally, all samples were weighted and the thicknesswas determined with a micrometer screw before andafter the tests. The oxidation of metallic samples hasbeen conducted during the double probe measurements(see Section 3.5), where a blank sample was exposedto oxygen plasma for about 30min at high tempera-tures (see Fig. 16). An analysis of the accuracy has beenconducted, taking calibration uncertainties and nonlin-ear effects of the pyrometer into account. It predicts anuncertainty of the emissivity of ±5% from the nominalvalue. In Figs. 5–9 the results for the total emissivitiesof the ceramic and metallic candidate catalytic sam-ple materials are given. A quantitative comparison ofthe measurement result shows a reasonable agreementto the values found in literature for Al2O3, see Fig. 5.

680 M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686

No references could be found for HfO2 and Yt2O3.Interestingly, three different behaviours with risingtemperature could be observed: the emissivity of Yt2O3rises, whereas the emissivity of HfO2 stays nearlyconstant and the emissivity of Al2O3 decreases withincreasing temperatures.The investigation of PM1000 is of great importance

as it is foreseen as material for the side panels of the EX-PERT capsule [12]. The measurements (Fig. 6) showed

Y tO 3HfO2Al2O3Al2O3 [REF10]Al2O3 [REF11]

ε tot/[

-]

T [K]750 950 1150 1350 1550 1750

0.80.750.7

0.650.6

0.550.5

0.450.4

0.35

Fig. 5. Total emissivities of the ceramic materials.

Measurement (blank)

Measurement (pre-ox)

Calc.(blank) [REF13]

Exp.(blank) [REF13]

Calc.(pre-ox) [REF13]

Calc.(pre-ox) [REF14]

ε tot/[

-]

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2550 750 950 1150

T [K]1350 1550 1750

Fig. 6. Total emissivities of PM1000.

Fig. 7. Surface changes of the PM1000 samples.

that the emissivity for the blank/clean material deviatemore than a factor of two in some temperature ranges.The emissivity of the clean material (Fig. 7A) is below0.4, whereas it rises steep towards 0.72 at temperaturesabove 1100K, where oxidation processes start takingplace. In comparison, the pre-oxidized sample had anemissivity of 0.7 at low temperatures rising to about anear constant value of 0.9 at temperature levels above1100K.The pre-oxidized sample showed a brown-greenish

camouflaged surface (see Fig. 7B) after the plasmatronoxidation. The colour and pattern indicates that a layerof Cr2O3 has formed. This hypothesis is supported bya decrease of weight and an increase of thickness (seeTable 1) which could be due to the low density of aforming Cr2O3 layer. Additionally, one value of theemissivity of Cr2O3 of 0.6 could be found in rather rareliterature [15] which reasonable fits to the value of 0.7at the beginning of the measurement.During the emissivity measurement of this pre-

oxidized sample a further oxidation of the relativelysoft Cr2O3 layer has probably taken place. After themeasurement the colour had homogenously chancedto dark grey, together with a thickness decrease and amass increase. This might indicate a formation of anAl2O3 layer (density, colour) but then is in contrastto the emissivity measurement of Al2O3 (Fig. 5) andcould be closer investigated via secondary ion massspectroscopy (SIMS).The significant decrease of emissivity at temperatures

above 1500K may be explained with by melting of ma-terial at the surface.The measured emissivities of clean tungsten and

TZM show very similar values of about 0.3 at lowtemperatures constantly rising to values of about 0.43at 1800K. In contrast, the oxidized metals showeda clearly different behaviour: while the pre-oxidizedtungsten only changed slightly colour, the TZM nearlybecame black, indicating that the conditions during theplasma wind tunnel tests were severe enough to heavilyoxidize the TZM while the tungsten sample withstand

M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686 681

Table 1Weight and thickness changes of the PM1000 sample.

Mass m ± 0.00005 (g) �ma (%) Thickness s ± 0.001 (mm) �sa (%)

Blank (new material) 10.20482 0 2.039 0After pre-oxidation 10.20274 −0.20 2.065 +1.26After emissivity measurement 10.20397 +0.12 2.052 −0.19

aThe mass at the beginning of each test.

Tungsten (clean)Tungsten (pre-ox.)TZM (clean)TZM (pre-ox.)

ε tot/[

-]

0.950.850.750.650.550.450.350.25

700 900 1100T [K]1300 1500 1700 1900

Fig. 8. Total emissivities of tungsten and TZM.

visibly better. Investigating the measured emissivitiesthis hypothesis is fortified. While the emissivity of thepre-oxidized tungsten sample is only little higher: 0.35at 720K and 0.48 at 1800K the emissivity of the pre-oxidized TZM is about 2.5 times higher than that of theblank metal (Fig. 8).

Despite the research done in the general field ofemissivity and reflectivity dependence on roughness pa-rameters [16,17], little is known about the influence ofsurface roughness and surface topology on the emissiv-ity of typical TPS applications. Therefore three SSiCsamples have been prepared, each with a different de-gree of surface roughness, using common finishing op-erations: “as fired” (Rq = 0.67), grounded (Rq = 0.24)and polished (Rq = 0.05). The surface roughness wasdetermined after production. Using Wien’s Law, themaximum wavelength in the temperature range between750 and 1800K is between 1.61 and 3.75�m, thus be-ing in the same magnitude or much taller than the sur-face roughness. In optics a surface where Rq/�max>1can be considered smooth, thus approaching the proper-ties of a perfectly smooth surface. For the cases whereRq��max, classical diffraction models could be appliedto describe the radiative properties of the surface. Theypredict the independence of the hemispherical emis-sivity to the value of Rq . However, it has been found[16,18,19] that experimental results show the emissiv-ity to be very sensitive to the state of the surface in the

0.94

0.9

0.86

0.82

0.78

0.74

4.28 3.83 3.38 2.93 2.48 2.03 1.58λmax / [μm]

700 900 1100 1300 1500 1700 1900

Rq = 0.66Rq = 0.24Rq = 0.05

Rq

ε tot /

[-]

T [K]

Fig. 9. Surface roughness influence on emissivity of SSiC samples.

optical roughness range. Also the results of the SSiCmeasurements (see Fig. 9) show a clear tendency. Thelowest emissivities were obtained for the smoothest sur-face (polished), which is compliant to the results of [16].

3. Determination of recombination coefficients

3.1. Methodology and theory

As mentioned in the beginning, the determination ofthe recombination coefficient is of great importance forthe characterization of TPS materials. The methodologyto determine the recombination coefficients at IRS wasfirst reported by Pidan [20] and has been consequentlyimproved in the past [8]. Within this work, it is usedfor the determination of recombination coefficients ofthe TPS sample materials in pure oxygen plasmas. Thesupporting techniques, especially concerning the probemeasurements in the inductively driven PWK3-IPG3facility [21] at IRS will be discussed. A scheme of theinteraction of all steps involved in the determinationmethodology is shown in Fig. 10.

The determination of recombination coefficients iscarried out adapting Goulard’s Theory [22] and inverseHF calculation of material double probe temperaturemeasurements of the investigated materials in the

682 M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686

Fig. 10. Scheme for the determination of recombination coefficients.

stagnation point of oxygen (or nitrogen) plasmas. Ithas been found that structural heat losses due to con-ductivity of the samples and the probe have to be takeninto account for this inverse calculation. Further neces-sary plasma parameters (e.g. stagnation enthalpy, Pitotpressure and Mach number) have been derived usingvarious Probe techniques [6,23] developed at IRS. Nu-merical data such as transport coefficients, dynamicviscosity and effective probe radii were calculated withthe Upwind Relaxation Algorithm for Nonequilibriumflows of the University of Stuttgart (URANUS) [24].Most physical properties of the candidate materials andgases were supported by the supplier or can be foundin literature. The emissivities were measured using theIRS EMF, see Section 2.Based on Goulard’s Theory (Eqs. (5)–(7)), the re-

combination rate kw and hence the recombination co-efficient �Mat of a material can be determined from theratio of the full catalytic to the measured finite catalyticHF. The assumption of complete energy accommoda-tion to the TPS surface (� = 1) was made:

kwMat,SP = 2�Mat(Tw)

2 − �Mat(Tw)

√RTw2�MSP

(5)

�Mat =[1 + 0.665

ScMat�w,MatkwMat

(u∞�e�eReff

)0.5

×[�∞�e

(2 − �∞

�e

)]0.25]−1

(6)

Fig. 11. Calorimetric HF-pitot pressure double probe.

qfc = qfinitc

(1 − Le2/3MatSP(h

SPD /he)

1 + (Le2/3Mat − 1)SP(hSPD /he)

× (1 − �Mat)

)−1

(7)

The full catalytic HF was calculated measuring the HFon cooled oxidized copper and a correlation introducedby Pope [25]:

qNfc | htot=8.8MJ/kgptot=40 Pa

�1.25 · qNCuO (8)

In the case of oxygen, the HF measured was set equalto the fully catalytic HF as proposed in [6,20].

3.2. Calorimetric HF and pitot pressure

For the measurements the IRS calorimetric HF-Pitotpressure double probe (Fig. 11) was used. The probeemploys an insulated sample of oxidized copper whichis kept at a constant temperature of about 300K by aseparate high pressure water cooling circuit.The inlet and the outlet temperatures of the water

are measured by PT100 resistance thermometers. Aflow controller determines the flow rate and hence themass flow can be calculated. The HF is then derivedaccording to

qCuO|T=const = cpwm(Toutlet − Tinlet)/ASample (9)

The probe can be turned by 180◦ such that the other side(Fig. 11) can be used for the Pitot pressure measure-ments. The radial and axial mapping of the HF and thePitot pressure of the plasma plume is taken for the cal-culation of the local stagnation enthalpy at the boundary

M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686 683

layer edge [21]:

he(xPos, yPos)

htot≈ R2

Pl

2·

qfc(xPos, yPos)√ptot(xPos, yPos)∫ RPl

0qfc(xPos, yPos)√ptot(xPos, yPos)

y dy

(10)

Based on the measured radial profiles of HF and to-tal pressure a radius of the plasma plume of roughly100mm was determined [21]. The total enthalpy waspreviously measured with a cavity calorimeter describedin [21].

3.3. Mach number and plasma velocity

Free stream values of the Mach number and, re-spectively, the plasma speed were calculated using theRayleigh–Pitot equation and the Pitot pressure measure-ments

Ma2�∞ −(

2

� + 1

)(�+1)( ptotp∞

)(�−1)

Ma2∞

+(

2

� + 1

)(�+1)( ptotp∞

)(�−1)

= 0 (11)

or obtained via probe measurements with the cone probedescribed in [8].

3.4. Plasma parameters

The dissociation degree of the plasmawas determinedusing thermo-chemical data of the gas species (oxygenand nitrogen) of the NIST online databases [26]. Withthe measured stagnation enthalpy at the boundary layerand the corresponding total pressure, the temperature,the gas constant and the molar mass can be derivedunder the assumption of thermo-chemical equilibrium.Compared to [20], this procedure makes the determina-tion of recombination coefficients much more flexibleconcerning axial distances from the generator and thetemperature regime, especially towards colder temper-atures. The effective probe radius has been estimatedby iterative comparison of two URANUS calculations,one with the European Standard Probe geometry andone with a hemispherical blunt body of varying radius,both under the same free stream conditions. The effec-tive radius of the probe is determined by the consistenceof the two flow fields (e.g. Reff = 2.42Rprobe) at the fi-nal iteration. Lewis and Schmidt numbers as well as theviscosity have been determined according to [24].

Fig. 12. Pyrometric material double probe.

0

f (T)

/[-]

FEM Calc.Regression

0.5

0.4

0.3

0.2

0.1

1250 1500 1750

T/[K]

2000 2250 2500

fssic (T) = 2·10-7T2 - 1·10-3T+1.2942

Fig. 13. Structural heat loss for the pyrometric double probe (e.g.SSiC).

3.5. Pyrometric HF measurement

The HF and the wall temperatures of the TPS materi-als under investigation are determined through measure-ments with the IRS pyrometric double material probe.The probe (Fig. 12) is equipped with two PYREX [27]mini-pyrometers, measuring the rear side sample tem-peratures during the plasma wind tunnel tests.It has been shown in the past that despite high tem-

perature insulation of the material sample the structuralheat losses into the probe have to be taken into accountto calculate the HF [28,29]. The transient HF onto asample can then be expressed through

qs(T, t) = �(T )�Tw,b(t)4 + cp(T )�S

dTw,b(t)

dt+ qL (T )

(12)

During plasma wind tunnel testing the probe is hold atan axial position until thermal equilibrium between theenvironment and the sample is reached. This simplifiesEq. (12) to steady state. The HF loss term can further be

684 M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686

Table 2Test condition in PWK3-IPG3 facility.

Gas mass flow rate (g/s), O2 Capacitors (–) Coil-loops (–) Tank pressure (Pa) Ucoil (kV) Anode power (kW)

3 4 5.5 40 6.3 110

written as a proportion of the load term on the sample:

qL (T, t) = f (T ) · qs(T, t) (13)

The function f (t) then represents the loss factor of theheat load density on the sample. It has been estimated bya transient thermal analysis of the PYREX probe [29].Fig. 13 shows the result for f (T ) representatively onan SSiC sample. From the analysis data, the regressionfunction f (T ) was derived.It becomes clear that especially for recombination

coefficient determination at lower temperature regimeswhich are encountered on metallic TPS applications thestructural heat loss has to be taken into account. Fur-ther it can be seen from Eq. (12) that a calculated HF isdirectly proportional to the total emissivity. Therefore,the knowledge of the temperature dependant total emis-sivity of the sample material is essential to accuratelycalculate HFs.

3.6. Test conditions and results

The tests were conducted in the inductively driventest facility PWK3-IPG3, which operates electrode freeand thus generates plasmas without impurities due tocathode erosion. All metallic species were blank pol-ished to a surface finish of about Rq = 0.5 previous tothe tests. Table 2 shows the test conditions for the re-combination coefficient determination.From the temperature histories obtained with the dou-

ble material probe tests (Section 3.5) the HF was re-calculated at the stationary points and corrected forstructural losses as described. Thereby, the results of thetemperature dependant emissivities obtained in Section2 were used. Due to the improvements of the pyromet-ric HF measurements, the error could be reduced to 5%(mainly the error of the emissivity) compared to the ap-proximated 12% of [21]. The obtained axial HF profilesare shown in Figs. 14 and 15.The HFs allow for a qualitative comparison of the

catalytic behaviour of the investigated materials. Thusthe catalytic efficiency for the ceramics can be esti-mated as �O(Y2O3)> �O(HfO2)> �O(Al2O3), wherethe extremely high HF of Y2O3 thereby indicates high-est �O of all tested materials. The hierarchy withinthe metallic species could be tabulated as follows:

HF

[kW

/m2 ]

HfO2Al2O3Yt2O3

550

450

350

250

150100

Axial distance from generator [mm]350 400300250200150

Fig. 14. Axial HF profile of the ceramic materials.

150

HF

[kW

/m2 ]

PM1000TZMTungsten

450400350300250200150

100Axial distance from generator [mm]

200 250 300 350 400

Fig. 15. Axial HF profile of the metallic materials.

01400

Al2O3HfO2Yt2O3

γ/[-

]

0.5

0.4

0.3

0.2

0.1

1500 1600T/[K]

1700 1800 1900 2000 2100

Fig. 16. Oxygen recombination coefficients of the ceramic materials.

�O(PM1000)> �O(TZM)> �O (tungsten) at lower tem-peratures, whereas the catalytic efficiency of tungstenexceeds TZM at rising temperatures (Figs. 16 and 17).In the case of the ceramic samples, the previously

made qualitative comparison could be verified by thequalitative results. The uncertainty has been assessed to

M. Schüßler et al. / Acta Astronautica 65 (2009) 676–686 685

0.1 PM1000PM1000 [REF]TZMW

γ / [

-]

0.20.180.160.140.12

0.080.06

1400 1500 1600T / [K]1700 1800 1900 2000

Fig. 17. Oxygen recombination coefficients of the metallic materials.

be well within one order of magnitude of the calculatedvalues. It has indeed been found that Y2O3 has with0.3–0.4 the highest oxygen recombination coefficientof all tested samples. In this temperature regime Y2O3can be regarded close to fully catalytic. Observing thevalues of the metallic specimen it becomes clear thata qualitative comparison not necessarily gives the rightimpression when high variety in emissivities exists. Thisbecomes clear, comparing PM1000 to the other materi-als in Figs. 6 and 8. However, the results for PM1000show a good agreement with the results of [20], takinginto account that in [20] the HF was not corrected forstructural losses.With the overestimated HF on PM1000in [20] the slightly higher values for the recombinationcoefficients can be explained.

4. Conclusion

In this study, total emissivities of HfO2, Y2O3,Al2O3, PM1000, TZM and tungsten were determinedin the temperature range between 750 and 1800K. Inthe case of the metallic specimen tests with clean andpre-oxidized metals/alloys have been conducted. It hasbeen found out that the emissivities of PM1000 andTZM are highly affected through the different oxida-tion states within the investigated temperature regime.For TPS applications of these materials (e.g. PM1000foreseen as metallic TPS material of the European re-entry capsule EXPERT) a “full” pre-oxidation state asshown in Fig. 7C seems necessary for a predictable andoptimal radiative behaviour (�tot constant at �0.9) ofthe TPS. The investigation of surface roughness influ-ence on TPS emissivity values showed a clear trend ofrising emissivity with an increased surface roughnessfor SSiC. Since this behaviour is apparently difficultto predict with common theory, further experimentswith an enlarged roughness and topology ranges shouldbe conducted in the future. This work should include

C/C–SiC and metallic species due to the special interestfor TPS applications.Oxygen recombination coefficients of the investi-

gated materials have been derived using a combinednumerical and experimental methodology. The ob-tained results could be summarized as follows: Y2O3can be regarded close to fully catalytic in the investi-gated (short) temperature regime. The other ceramicsshow a rather moderate and very constant catalyticbehaviour. Especially together with a nearly constantemissivity of HfO2 within broad temperature levels,this makes the thermal performance of these materialsmore predictable. The metallic specimen showed tem-perature dependant individual behaviour from highlycatalytic to rather low catalytic behaviour. The resultsof PM1000 showed a good agreement with previ-ously found results, justifying the recent improvementswithin the pyrometric HF measurement. For the future,an enlargement of the temperature regime for the emis-sivity measurement is planned using a purpose-buildbroadband thermopile detector with an all-CaF2 opticalpath. This will allow for measurements from 1450 to2600K with a detected radiation fraction of Plank’sdistribution from 95% at 1450K to 99% at 2600K, thusfurther improving accuracy at medium temperaturesand extending the measurement range.

Acknowledgements

The author would like to thank the Ministry of Sci-ence, Research and the Arts of Baden-Wiirttemberg(Grant AZ: 23-729.86-1/1 to Markus Schuessler)and the Deutsche Luft- und Raumfahrtargentur DLR(Contract number 50JR0531) for the funding.

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Further reading

[10] K. Nakamoto, Infrared Spectra of Inorganic and CoordinationCompounds, Wiley, New York, 1963.

[11] Y.S. Touloukian, D.P. Dewitt, Thermophysical properties ofmatter, Thermal Radiative Properties, Non-metallic Solids, vol.8, Plenum Press, New York, 1972.

[13] D. Stewart, S. Bouslog, Surface characterization of candidatemetallic TPS for RLV, in: AIAA 99-3458, 33rd ThermophysicsConference, Norfolk, VA, USA, 1 July 1999.

[14] J. Buursink, On the development of a cooled metallicthermal protection system for spacecraft, Dissertation, TU-Delft, Netherlands, 2005.

[30] J.M. Muylaert, L. Walpot, H. Ottens, G. Tumino, W. Kordulla,G. Saccoccia, M. Caporicci, C. Stavrinidis, Preparing for re-entry with EXPERT: the EAS in flight ATD research program,IAC-03-V.5.09, in: 54th IAC, Bremen, Germany, September2003.