AD-A016 786

AFRPL GRAPHITE PERFORMANCE PREDICTION PPOGRAM. VOLUME II.ARC PLASIA GENERATOR EVALUATION OF GRAPHITE REACTIONKINETICS IN ROCKET PROPELLANT ENVIRONMENTS

H. Tong, et al

Acurex Corporation

Prepared for:

Air Force Rocket Propulsion Laboratory

October 1975

DISTRIBUTED BY:

National Technical Information ServiceU. S. DEPAR'MENT OF COMMERCE

3151 3"r

AFRPL-TR-75-59 AEROTHERM PROJECT 7113AEROTHERM REPORT NO. 75-143

AFRPL GRAPHITE PERFORMANCE PREDICTION PROGRAM

INTERIM REPORT

VOLUME II, ARC PLASMA GENERATOR EVALUATION OF GRAPHITE REACTIONKINETICS IN ROCKET PROPELLANT ENVIRONMENTS

AEROTHERM DIVISION/ACUREX CORPORATION485 CLYDE AVENUEMOUNTAIN VIEW, CALIFORNIA 94042

AUTHORS: H. TONG

d.C. HARTMANE. K. CHU

OCTOBER 1 9 7 5

APPROVED FOR PUBLIC RELEASE:DISTRIBUTION UNLIMITED

AIR FORCE ROCKET PROPULSION LABORATORYDIRECTOR OF SCIENCE AND TECHNOLOGYAIR FORCE SYSTEMS COMMANDEDWARDS, CA 93523

IiNC-;ASSITFTE• ECURITY CLASSIFICATION OF THIS PAGE (When lot. Ent'rdJ)

REPORT DOCUMENTATION PAGE READ INSTRUCTIONSBEFORE COMPLETNG FORM

SRLPORT NUMBER 2. GOVT ACCESSION NO. 3. PFCIPILNT'S CATALOG NUMBERAFRPL- TR-75-59 I

4. TITLE (adSubUfl.) S. TYPE OF R1EPORT A PERIOD CC VERED

AFRPL GRAPHITE PERFORMANCE PREDICTIO; Interim ReportI May 1974 to May 1975

Vol IT Arc Plasma Gene'rator Evaluation of 6. PERPIRMING ORG. RP.ORT NUMBER

Graphite Reaction Kinetics in Rkt Propellant f7. AUTHORP(, Envirorments 1. CONTRACT Oil GRANT NUMBER)

H. Tong F04611-74-C-0023J.C. HartmanE. K. Chu

9. PERFORMING ORGANIZATION NAME AND AORESS 10. POGRAM ELEMENT, PROJFCT. TASK

Aerothem/Acurex Corporation Project No. 6230F

485 Clyde Avenue ' BPSN: 305909HUMountain View, CA 94042 _

It. CONTROLLING OFFICE NAME AND A-DF.ZSS 1Z. REPORT DATE

Air Force Rocket Propulsion Laboratory/MK October 1975Edwards, CA 93523 13. NLPASER OF PAGES

14 MONITORING AGENCY NAME & AODRE - -sf .ffer fron C..ntn±0n0Oifice) IS. SFCUPT f CLASS. (Q.thlo report)

,L . UNCLASSIFIED

?5P. OECLASSIFICATION/DOWNGRADINGSCHEDULE

16. OiSTRI9JTION STATEMENT (of this Rep,rt)

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

I?. DISrRIBiJTION STATEMENT it the a.,R c e."rtr.d In flock !1, if d frf W11 iroa, RFPnPt

18. SUPPLEMENTARY NOTCS

?9. KEY *OROS rCiritue on reverzi sidej/ neeosary Kieti s h.' Layer An

Rocket Nozzle Reactio Kinetics Boundary Layer AnalysisCarbon Ablation Analysis ARC plasma generatorGraphite Surface Ther;,6cha-is..rPyrolytic Graphite Peat Transfer

20. ABSTRACT fCwlfnue on rever,, sid* if P -7,x~7 And idenflfy by block nrm.e.)

Test gases and conditions were selected for use in the arc plasma generationsimulation of advanced propellant environments. Calibration and checkoutresults are reported. The experimental program to determine kinetic reactionrates of carbon and graphite materials will now proceed and be completed inearly 1976.

DO M 1473 EDITION OF I NO5 5IS OBSOLETE UNCLASSIFIED

SECUR:7'- CLASSIFICATION OF THIS PAGE (*Won DWS Entered)

TABLE O CONTENTS

Section Pde

1 INTROlXUCTION ........ .......................... 1

2 EXPERIMENTAL APPARATUS ........ ..................... 4

2. i Arc Plasma Generator ........ ................... 42.2 Test Nozzle Configurations ..... .. ................ 72.3 Fume Collection System ....... .................. 72.4 Instrumentatita ......... ...................... 13

3 TEST GASES AND TEST CONDITIONS ...... ................. 15

3.1 Test Gas Selection Criteria ................... .... 15

3.1.1 Rocket Motor Environments ... ................ .... 153.1.2 Important Surface Reactions ..... ............... 173.1.3 Required Information for Predictions ......... 193.1.4 Sensitivity of Kinetic Response ...... ............ 203.1.5 Arc Plasma Generator Limitations ..... ............ 293.1.6 Potential Test Gases ..... .. .................. 30

3.2 Test Gas Selection ........ .................... 33

3.2.1 Test Gas Evaluation. ....... ................... 333.2.2 Recommended Test Gases for Full Characterization

Studies ...... ...................... . . 34

4 CALIBRATION TEST RESULTS ........ .................... 36

4.1 Test Conditions .... .................... . . 36

4.2 Performance Maps ..... ... ..................... 454.3 Test Nozzle Response .... ................... .... 45

5 SUMMARY AND CONCLUSIONS ..... .................... .... 53

i

LIST OF ILLUSTRATIU.-7

figure Pg

1 Aerotherr l-Wf.' constrictor ,r,- heater ....... .............. 5

2 Axisymmetric nozzle assembly ........ .................. 8

3 Nominal test sectiorn insert configuration ...... ............ 9

4 Calorimeter nbzzle ass-mbly ..... ................... ..... 10

5 Axisymwtric t, st section, cal-,riircter installed ... ......... 11

6 Fume collection. syter . ....... .. .................... 12

7 Typical surface a~s compositien at throat for C-Plane PG .... 18

8 Effect of bo'lnd;2ry lay!r .-._ or ca.rbon consuption rates . . .. 24

9 D .1ineativ)n of kinetic and transition regi.mes for A-5'elane PG ........ ............................ .... 25

10 Delineaticr: uf kinezic an tr,r'sitic;, v ies for bluegraphite ........ ............................ .... 26

II Equilibrium,, graphite consur.;tiun rates ... ............. .... 27

12 Effect of mass transfer cc:fficipnt on qraphite consumptionrates ....... .. .............................. ... 28

13 Performance map, test .s ...... ................... .... 46

14 Performance maP, test gas 2 ........ ................... 47

15 Performance map, test qas I ..... ................... ... 48

16 Performance . test . ... . .. . ........ 49

17 Performance map, test rcs s.. . ... ................... .... 50

,i i

LIST OF TABLES

Table

1 Representative composition and flame temperature of advanced

MX propellants ........ ..........................

2 Propellant gas composition (A1203 removed) ..... ............ 1

3 MX baseline - PEG/FEFO propellant hroat thprmodynamicsconditions at boundary, layer edge .. ..............

4 Potential APG material characterization test qases ... ........ 31

5 Surface reactions ..... ... ........................ 32

6 Recommended material characterization test gases ........... 3

7 Test gas 1 calibration resu;s ...... .................. 38

8 Test gas 2 calibration results ....... .................. 39

9 Test gas 3 calibration results ....... . ................ 41

10 Test gas 4 calibration results ...... .................. 4V

11 Test gas 5 calibration results ....... .................. 43

12 Preliminary test results, surface temperature comparison, hi.ahesttest condition ....... .......................... .. 51

13 Check out test series, test nozzle response ..... ........... 52

iv

SECTION 1

INrTRODUCTION

Because it is not possible to write an analytiral expression which exactly

models the real events associated with carbon consumption in rocket nozzles, one

must resort to developing engineerinq approximaticns which represent the overall

observed phenomena. Expressions of .,is type with a set of empirical coefficients

were developed in Reference 1. The empirical coefficients relate the local thermo-

chemical state to the surface temperature and carbon coV-sumption rates. Thus, given

an accurate set of data with these variables known, it is possible to write a kine-

tic equjation for analytic prediction of ablatior, ratcs.

There are two viable experiments that can be conducted to obtain ablation

data. First and certainly the most relevant is to fire actual rocket motors.

Difficulties associated with this approach are:

1. Surface temperature is not a measured variable and can be experimentally

adjusted only over a small range.

2. It is not possible to isolate even simple reactions to simplify the

data analysis procedure.

3. Chamber conditions may vary widely during the ablation run.

4. The surface is subjected to an unknown quantity of ambient air oxidation

during coo~down.

5. Heat transfer rate is not an e,:sily controllable parameter nor can it

be independently mea:ured.

6. Rocket nozzles, in crder to be structurally sound, are complex and may

result in a variety uF fln-disturbing features such as steps and gaps.

7. Motor firings are high-cost experiments.

The obvious advantage of motor data is that there is an exact simulation of the

environment and material response.

The second approach is to conduct laboratory scale ablation measurements in

prescribed simulation environments. Here, one has the option of selecting from a

large number of potential experiments which range from heated filaments to miniature

nozzles heated in an arc plasma generator (APG) The latter was selected because

it provides a good compromise between "academic' quality data and rocket motor

data. The disadvantages of this ani n'o t laboratorY tests are:

1. Surface temperatures and systemi presure, are less than those experienced

in rocket motors.

2. The experiment must be viewed carefully to determine if test is kinetically

or diffusively limited or soen'..wrre in between.

3. Full-size components cannot be te.ted.

4. Some analytical technique must be availaule to sort out the effect of

various reactants and to scale for prediLtin, full-size perfornance.

The obvious advantage of a ldbiratory ei,,lt is thdi+ 1i- can obtain detailed

measurements in a controlled environmern. Thfis data provides a separation of the

effect of each constituent and yields temnperatur,, dependent reaction rates. In

addition a large number of ldooratnry tests. can be coriduttr-d at the cost of one

motor firing.

Laboratory data and motor firing data drC both important in the development

of an accurate analysis procedure and should therefore be treated as being compli-

nentary rather than competitive. In the current study, the complimpntary aspects

are being exploited wherever sufficient miotor (dtd dre 3,%Ildble. Laboratory ddtd

are being obtained in an arc plsmra generator (AFG) and are beinq analyzed simul-

taneously with motor recession rates which are analyized and manipulated to look

like APG data. Since the final analysis treats motor ddta and APG data as a single

set of data, it is not possible to unequivocaly define th,- funrtion of each. How-

ever, in a loose jeneral sene, an APG provides a large data bdse which allows d

separation of the effect of various constituents or reactants whereas motor data

provides high temperature, high pressurc data point, which rtake the analysis simul-

taneously relevant and accurate. The analysis procdurp has been described else-

where and will not be considera here.

The Aerothenr 1-megawatt arc plasma generator ,s o ver-,atile test facility

for determining the response of r-aterials to hyp,-ther,al environments. This facil-

ity has been used extensively for the- eviluation of ;p] e shuttle and reentry

vehicle thermal protection systen s and rocket nozzle linsr mcterials. Air and

inert gases such as argon and helium are common gas es to pass ,rough the arc heater

and are usually sufficient for simulating earth entry condilons. However, in a

rocket nozzle, a substantial constituent is hydrogen and pt, r 4imulation arid

separation of kinetic events requires that hydrogen be ,_. -' .'e arc plasma gen-

erator. The Aerotherm arc plasma facility is unique in this reqard since it is

one of the few arc plasma facilities in the country that uses hydrogen gas routinely.

2

This report will describe the facility and itS application to the determination ofkinetic reaction rates for graphitic rocket nozzle materials.

SECTION 2

EXPER IHEITAL APPAP ATOS

The experimental apparatus L nsists of the "rc plasma generator used to pro..

duce tne high temperature reactive environments, the test nozzles which are exposed

to these environments, the fune collection, cooling and scrubbing system used to

remove the test gases from the facility, ond thoe instruptintation used to character-

ize the test conditions and model respon- e. The art. plasma ?enerator and support

equipment are discussed in Section 2.1. The test nozzles drel described in Section

2.2. The fume collection syitcm i, described in Section 2.3, and the instrumenta-

tion is presented in Section 2.4.

2.1 ARC PLASMA GENERATOR

The Aerotheri 1-megawatt constricted arc plasi:a (ienerator (APG) is shown

schematically in Figure la and physically in figure lb. This APG is a constant

mass flow rate device with a flow rate controlled by throttling at the gas injec-

tion ports. The APG uses d seqnented constrictor ar' with a tunqsten cathode and

a w&ter cooled copper anode to transfer (energy fu thH prii,,ary test gas. This testnas is injected tangentially between the (dthodp aid the tirt constrictor segment

to p-ovide a stable, high vol taqe operation. Additional , to simulate propel-

lant gases are injected downstream of the anode and inixed with the primary arc-

heated gas in a plenum chamber. Thermocheical equflibriu:, is achieved in this

plenum and the resulting simulation gases are expanoea throijh a choked converging-

diverging nozzle. The test section is the throat M 6r, ,vf this nozzle.

The arc unit is water cooled with ambient temperaturp, nigh pressure deion-

ized water. The APG input poner is supplied by d 666 kw continuous rated, saturable

core reactor, dc rectifier power suoply. A mraxiitu, overload power level of 1.2 MW

is achievable for 5 mainutes. Th power supply has 1,000, 2,000, or 4,000 volts

open circuit voltage rodes to match APG operating characteristics for various test

gases, flow rates, and pressures. Arc starting is accomplished by imposing power

supply open circuit voltages across the APG electrodes while an argon flow is main-

tained. Then a momentary RF discharge in the APG column provides an initial ioniza-

tion path for the arc. Once the arc is started, test gases are immediately intro-

duced as necessary to provide the required test gas composition.

4

CO w

00

0. 0

EEu 0)

0)C 1

>, x

4-)1

CUl0

50

4-)V)

00

C-,

0. U

E0 a

0 -=

-0'0

*~0 a0

00

01-D o

The arc unit exhibits very low contamination levels. Based on the results

of Reference 2, total gas stream contamination should not exceed 200 parts per

million (0.02 percent). The source of this contamination is the tungsten cathode

and copper anode. A third potential source of contamination is the boron nitride

insulators of the constrictor section; however,, their contribution to the above

figure is felt to be very small.

2.2 TEST NOZZLE CONFIGURATIONS

The nominal test configuration is an axisymmetric nozzle as shown in Figure 2.

The test section inserts form the throat region of the nozzle. The PG washer ime-

diately upstrea'n of the test section insert insures a smooth transition into the

insert and holds Li boundary layer trip. This trip, a thin Grafoil disk, is em-

ployed to promote turbulent flow, and therefore high trinsfer coefficients, in the

throat. The test section insert is retained by a crushable high temperature insu-

lator. The test section could therefore expand thermally without imposing excessive

compressive stresses. The test section insert configuration is shown in Figure 3.

This is the nominal-dimension configuration; the details of the actual test insert

depends on the particular requirements of the test material, e.g., backwall insula-

tion in the throat region.

An appropriate ablation sample or a water cooled steady state calorimeter can

be placed in the test section. The calorimeter and test sample both have the nominal

interior dimensions shown in Figure 3 so that the test conditions during an ablation

test can be inferred from a corresponding calorimeter test. The calorimeter instal-

lation is shown in Figure 4 and a view of the assembly is shown in Figure 5.

2.3 FUME COLLECTION SYSTEM

The APG for these tests is run on the atmospheric test stand with the test

gases exiting directly into the test bay. A fume collection system is employed to

collect, cool, clean, and exhaust the gases outsidL the test area. The system is

shown schematically in Figure 6.

The first com1,onent of the system is the heat exchanger section. The high

temperature of the test gases as they leave the APG requires a "cooldown" to less

than 250OF before they enter -he remainder of the system. This section is con-

structed of a high temperature alloy, Hastelloy Alloy C-276, and provides a set of

spray nozzles which "quench" or cool the gases with a water spray. Also included

in this section are two view ports to allow pyrom.ter viewing of 'the test section.

The gases are then ducted to the fume scrubber mounted outside the test bay.

This scrubber is of the packed tower type and is designed to remove all toxic fumes

(HCk, 11F) from the gas stream before they are exhausted to the atmosphere. The

scrubbing fluid is water used in the once-through mode.

7

Nozzle housing

Retaining

G-90 graphite

High tempinsulator

Test sectioninsert

Grafoil seal

Boundary layer~trip

PG washer

Grafoil seal

Cooling coil

-ounting flanqe

Pressure tap

Figure 2. Axisymretric nozzle assembly.

8

I

1. 250

.6g0 .350

200 Reff

2 Dia .350 Dia .503 Dia1. 998 .852

Dia

,,II-v" 200 Ref

Figure 3. Nominal test section insert configuration.

9

Thermocoupi e

Superonnc

Boundarylayer

tripDia 0.35 11

calorimeter

Mounting----flange

Pressure tap

Figure 4. Calorimeter nozzle assembly.

10

- -~ , - ,'.>,r~ -.

.- r~ft

LL+JCDi

ANN=__ C

LI 0

.43

Cke,

Le)

U-

ZVI,

Exhaust

Dut

" viewing~port

Ntat exChan~r

Figure 6. Fume collection system.

12

The final component of the system is the exhaust fan, mounted on the roof of

the test bay. This provides the positive draft required to draw the gases through

the heat exchanger section and the scrubber. The fan has been sized to provide a

slightly negative pressure in the system when it is used in the blanked-off mode.

This is necessary when hazardous or toxic test gases are used as it prevents the

release of such gases into the test bay and insures personal safety.

Due to the corrosive nature of certain of the test gases (HCL or HF), the fan,

scrubber and all ducting exc'dsive of tne heat exchanger section are constructed of

Rigedon 4837-AT-HF. This is a fire-retardant, fiberglass reinforced polyester plas-

tic resistant to cnrrosive attack by both acids and alkali and, in addition, is pro-

vided with a special Dynel veil for protection against flouride attack.

2.4 INSTRUMENTATION

The measurements to characterize the test conditions and material response

are

* Test Conditions

- Gas Total Enthalpy

- Chamber Pressure

- Cold Wall Heat Flux

* Material Response

-. Surface Temperature History

- Surface Recession

- Qualitative Surface Condition

The gas total enthalpy is defined by an energy balance on the arc heater

including the plenum chamber, i.e.,

ho "hamb =harc - Power In-Coolin Water Losses

0 - am LharcT1tal Gas .Flo6w Rate

0.948 x 10 Fl - cATc

mas

where hamb is the enthalpy of the test gases at room temperature. Voltage E and

current I are recorded continuously on a digit.,l data recording system; measure-

ments from panel meters are also taken as a check. The cooling water flow rate,

mc' is measured continuously during each test with a sharp-edge orifice and differ-

ential pressure transducer and its temperature rise, ATc, is measured continuously

13

with a differential thermopile. The total gas flow rate, mgas' is the sum of all gas

flow rates delivered to the APG. All gas flow rates are measured with ASME sharp-

edged orifices and differential pressure gauges, except hydrogen-chloride which is

measured with a rotameter with a magiier float follower.

The chamber pressure is measured continuously with strain gauge pressure

transducers. The pressure taps are located dt the downstream end of the plenum,-

mixing chamber (Figures 2 and 4). The chamber temperature is detennined from the

calculated net enthalpy addition due to arc heating, the measured chamber pressure,

the test gas composition, and an ACE computer code computation of chamber conditions.

Cold wall heat flux is measured at the throat of the water cooled copper cali-

bration nozzle with a steady state, water cooled calorimeter section. The coolant

water temperature rise ATc is measured with a single-pair copper-constantan differen-

tial thermopile, the output of which is recorded continuously. The calorimeter water

flow, ;c' is measured with a standard glass tube rotameter and the heat flux is then

calculated from the equation

mcATcqcw- A c

where Ac is the calorimeter heated area.

Surface temperature history is measured with an E2 Thermodot TD-9CH optical

pyrometer which is calibrated with a high temperature source. For each nozzle abla-

tion test, this pyrometer, which has a sensing wavelength of 0.8 microns, views the

nozzlj throat at an angle of approximately 40' from the APG centerline. Output data

is recorded both visually from the instrument meter and in digital form from the data

acquisition system. In some tests, a second pyrometer is used as a check on the pri-

mary unit. This secondary unit is a Thermodot TD-9FH optical pyrometer similar to

the primary instrument except calibrated in degrees Fahrenheit.

The test sample surface recession is obtained from pre- and post-test measure-

ments of the throat diameter. Measurements are made at three axial stations in the

throat region, namely, the entrance, center , and exit. In addition, at each station,

the diameters are determined at two angular positions 900 apart. The measurement

accuracy is approximately +0.0005 inch.

14

..............

SECTION 3

TEST GASES AND TEST CONDITIONS

3.1 TEST GAS SELECTION CRITERIA

The selection of gases for APG testing is very important since ideally one

would like to minimize the extent tc, wi ich experimental results must be extrapolated

in order to predict actual coadition;. Several questions must therefore be addressed

in selecting appropriate gases.

1. What rocket motor environments are antiLpaLcd?

2. What are the important surface reactions?

3. What information is required in the prediction procedure?

4. How sensitive is the kinetic response to predicted variables such as the

heat or mass transfer coefficient?

5. What are the operating limitations of the APG?

Test gases must be defined for two different kinds of tes'. On the one hand a

comprehensive set of gas mixtures must be defined to allow a full kinetic characteri-

zation of the test material. On the other hand a gas mixture or a set of gas mix-

tures must be defined for experimental screening or ranking of materials which are

similar to those which have rereived the full rharacterization treatment. Although

it is likely but not necessary, the screening gases arnd their test conditions will

be a subset of the full characterization test matrix.

3.1.1 Rocket Motor Environinents

Rocket motor environments will be based on tiree advanced MX propellants,

namely,

o XLDB

* HTPB

s PEG/FEFD

Representative elemental compositions and flame temperatures are given in

Table 1. For the purpose of studying surface kinetics only the elemental composition

of the propellant gas need be considered. The solid AZ.O3 does not enter into the

15

TPBLE 1. REPRESENTATIVE COMPOSITION AND FLAMETEMPERATURE OF ADVANCED MX PROPELLANTS

Propellant XLDB HTPB PEG/FEFOFlame Temperatures 3880 3690 3787

OrF 6524 6182 6360

Mass Fraction

H 2.5 4.0 2.6

C 13.5 8.4 12.5

N 24.0 9.0 23.0

0 39.5 40.0 37.9

F - - 1.5

Al 18.5 17.6 18.5

Cl 2.0 21.0 4.0

TABLE 2. PROPELLANT GAS COMPOSITION(A1203 REMOVED)

Propellant XLDB HTPB PEG/FEFO

Mass Fraction

H 3.8 6.1 4.0

C 20.8 12.7 19.2

N 36.9 13.6 35.4

0 35.5 35.2 32.9

F - - 2.3

Cl 3.0 32.4 6.2

16

surface kinetics problem although it probably contributes to surface erosion rates.Table 2 gives representative compositions of the propellant gases with all the At

and an appropriate amount of oxygen removed as AZ203. The ACE/GASKET program was

usd to determine the concentration of gas species which would exist at the carbon

surface for three conditions: (1) surface equilibrium, (2) very small surface abla-

tion, and (3) a nonreacting surface at typical surface temperatures (2200*K to

3300*K). Those species with significant concentrations would then be candidates for

reactants and/or poisons. A typnical distribution of surface species as a function

of temperature for an HTPB propeflanL is .hown in Figure 7. This solution represents

the kinetically controlled ablation of edge oriented pyrolytic graphite at a throat

pressure at 39.4 atm from the ACE/GASKET calculations. Those species considered as

possibly significant reactants (molar concentrations greater than 0.1 percent) are:

* Co

* H2 0

• H2

* N2

* CO2

* HCk

e HF (HF not a specie for the ITPB solution)

It should be noted that other species, such as CU and H, appear in represen-

tative amount and may also be important. Still other species, such as 0 and OH,

though present only in small quantities may have very fast reaction rates. These

latter species vanish very rapidly as ablation race increases so that they would

probably increase rates only a slight amount before they are depleted. Atomic hydro-

gen has been shown, at least on one c3 *, to react slower than H2 and since the con-

centrations of H2 are an order of magnitude greater than that of H, ablation due to

the latter will probably be insign~ficant. C9, a halogen, is a potential poison,

however, there is no firn evidence for this behavior. Thus, the species of interest

are those previously listed.

3.1.2 Important Surface Reactions

The most probable surface reactions can be identified by considering the

available species, the possible reactions with carbon, and the equilibrium constant

for each reaction. (The equilibrium rate serves as an upper limit to the surface

kinetic rate.) Of the reactions considered, only the following have sufficiently

large equilibrium constants in the temperature range of interest:

*Personal Communication. Professor D. Rosner, Yale University.

17

100 -E

101

10- 8

C* + H20 CO + H2

CA + CO2 - 2C0

2C*+ H2 C2H2

The reactions of carbon with CO, N2, HCU, and HF are not considered significant since

the equilibrium formation rates are too small. However, they may have innibitor

properties.

ihe mechanism of poisoning, or inhibition of surface reactions, is basically one

of active site competition. That is, a poison specie may occupy an active lattice

site and thus prevent a reactant fromi ,occupying that site. Of the seven species

listed as possible poisons, only N2 will not be considered because it appears to be

inert as far as surface kinetics are concerned.§ Although H20 and CO2 readily react

with the carbon surface they are, nevertheless, occupying lattice sites and thus,

in that sense, are poisons for each other.

3.1.3 Required Information for Predictions

The singular purpose of this experimental program is to provide empirical data

to be used in an analytical procedure for predicting rocket nozzle thermal performance.

The expression desired in Reference I for kinetic ablation rates of graphitic mate-

rials is

•MC (P 20 + ' 02) C 2p

F _7 _ _ (1)

where

01 [= + (Ap) 20 + (Ap)CO + (Ap)co.+ (Ap )Hj (2)

D 2 El[ + (Ap)Hc 91]

D3 = [I + (A"p)H + (A"p) o + (A"p)co 4 (Pp)I,

tAlthough C2H2 does not appear in the list of gas species, the reactions should notbe ruled out. Hydrogen is present in large concentrations (approximately 25 per-cent by mole) and the C2H2 coming off the surface may be eliminated by gas phasereactions.

§Kinetic rate data in Reference I substantiates the inert behavior of N2.

19

MCH 8 2/',e

Be'E'/RTw

The A, A', A", B, and E coefficients are th- terms that must be determined from

experimental data. Since temperature and a set of chemical species are involved,

the experiental data must vary these variables over a sufficient range so as to

allow the accurate definition in a data correlation procedure. As noted, the APG

provides wide latitude on gas composition over d limited temperature range. By

simultaneously including rocket motor data, the correlation procedure can be extended

over a relevant temperature range. This is done with tne current data analysis

scheme, however, for the present, only the fiIil data is of interest.

The term MC is the correlation pirJiweter and is presumed to be a function uf

temperature only. For any given test it is then necessary to define mcl TwI and pi

and for any given set of data, the correlation procedure must determine the coef-

ficients A, A', A", B, and [ sjch that in an MC versus Tw (or I/T w) plane, the data

has a minimumn scatter. A perimentally, it is not possible without great agony to

measure Pi in the vicinity of the hot ablating wall. In fact, it is not even neces-

sary if inc and Tw are measured and the elemental composition of the test gas is

known. Since, at the test pressures and temperatures, the gases are probably in

thermochemical equilibrium (hin-ogeneously but not necessarily heterogeneously) the

species composition and, hence, pi can be calculated from an open system thermochemi-

cal computation using i, and T V1.

Implicit in the thenrochertcal coirpitation it a knowledye of the mass transfer

coefficient (e ueCm). In an W P test hi< ra, be determiin:d by calibration with

appropriate heat transfer mea,,urt,-.ants; hoaev .e for rocket uotors this can only be

determined by analysis. Thus, -r, 'FG *t 'ecs cs all necessary variables (i.e.,

TwI , C , and e u eC m) whereas a motor firni yields oily total recession; Tw and Pe u eCm

must be deterrined or estimated by Dt'er mears.

For the measured r-silt', to ib rple;dnt in an analytic procedure, the test

gases and test conditiu.r, . 3,. m *Pd witl, cae. Thi- selection must be based

on the above discussio,i un vport.r5t ja: -ecies. Cut, in addition, the sensitivity

of the gases on predicte! r.;Su'" '.vd APG liriitatin, u 't also be considered. These

considerations are discuss,-.

3.1.4 Sensitivitv of Kinetic Response

Since gasket and ACE are the only computer codes that are currently in use by

manufacturers in th,, rocket cor-unity, the main concern here is to determine for a

given kinetic model, the sensitivity of predicted results to input parameters. The

20

Aerojet Solid Propulsion Company MX nozzle was selected as the baseline configura-

tion, and each parameter was studied to determine its influence on the surface

thermochemistry solution. The parameter., that were considered are: the thermodyna-mic state at the boundary layer edge; the mass transfer coefficient; and the kineticconstants at the surface reactions.

As a result of the sensitivity study, it was found that the sensitivity tothe input thermodynamic state at the boundary layer edg is negligibly smal. Sen-

sitivity to the input of the mass trenOrer coefficient however, is dependent upon

tne magnitude of the kinetic constants used in the Arrhenius type expression. For

the baseline case, at the location of the nozzle throat, the kinetic regimes of the

following materials were determined as:

* Layer P.G. Temperature .5900'R

* Edge P.G. TemperaLure <..7001R

• Bulk graphite Temperature < 3600R

At temperature below these thresnold temperature, it wds found that the mass trans-

fer coefficient had no significant role in the surface chemistry solution. Con-

versely, at temperature above the threshold temperature, the sensitivity to the in-

put value of the mass transfer coefficient increased monotonically with temperature

as the chemical reactions shifted fron kinetic limiting to diffusive limiting.

Qualitative Description of .vrfact; Themochemistry

The surface thermochemical process can be best described by the following

equation

C. eO i e (3)

where J is the diffusion ratc c.; the reacta4 n i, r. is tire reactant concentration

at the boundary layer edge, 'i 5 th. mass trtnslIM ,nafficient, and ki is the ki-

netic constant. Note that ;quatior, (3) only s,:'-is a imple first order reaction

and no inhibiting effect is coiiiderd. To account for, complex kinetics, further

modifications are required. Thc functional form of Equation (3) introduces a very

useful interpretation when compared to electric network theory. By analogy, Ciecan be considered as the driving potential, the reciprocals of the mass transfer

coefficient arid the kinetic constant as the surface resistance, and the flow of the

Reference 3

21

reactant as the current. It is interesting to note that when ki >> ai , the rate of

the overall process is then completely determined by the diffusion rate. Conversely,

when ki << si the rate of the overall process is completely determined by the actual

kinetics of the surface reactien. The former limiting regime is called the diffu-

sion regime, the latter is called the kinetic regime, and the intermediate regime is

called the transition regime.

In the sensitivity analysis, the effect of the following parameters was

assessed:

* One-dimensional flow versus two-dimensional flow (shows effect of the edge

thermodynamic state)

9 Equilibrium flow versus frozer flow

* Mass transfer coefficient for i..b-plane PG, C-Dlane PG, and bulk graphite

(shows the kinetic regimes of the three materials)

# Mass transfer coefficient versus erosion rate (wall temperature at 60000R)

The baseline case was a typical MX upper staqe nozzle contour with a PEG/FEFO pro-

pellant expanded from typical chamber conditions to the throat (sonic point). The

option of equal diffusion with a unity Lewis number was chosen in order to simplify

the analysis, however, the effect of unequal diffision, Lewis No. I will be addressed

later.

Table 3 shows the sonic point thermodynamic states at the boundary layer edge

as calculated for l-D equilibrium flow, 2-D equilibrium flow, and l-D frozen core

flow conditions. It was found that the I-D frozen flow predicted a higher pressure

and higher concentrations of the three major reactants (steam, carbon dioxide, hy-

drogen).

TABLE 3. MX BASELINE - PEG/FEFO PROPELLANT THROATTHERMODYNAMICS CONDITIONS AT GOUNDARYLAYER EDGE

Type of P T Reactant Concentration (lb-mole/ft3)Flow Field e I r

(atm) :)CO 2-1 2 "20

I-D Equil. 57.72 6467 13.93 % lOr 3.26 x 10- 5.89 x 10-

l-D Frozen 63.43 6343 10.51 x 0-' 3.44 x lO! 6.92 x lO4

2-D Equil. 47.74 6378 7.31 x lO" 2.70 x lO 3 4.80 x 10"'

22

Based on Equation (3) and the redctdnt compositions shown in Table 3, 1-1) flow as-

sumptions should result in higher erosion rates. However, as shown in Figure 8

for PG washers, thu'e is no dependence of che mass loss versus tempera-

ture relationsho, on bo'r,4ary l.iyer edgr state* for temperatures below 6500R andonly small differences %re observed foe higher temperatures. These results, though

unexpected, are not surprising sinc, tht: deriv.ation of Equation (3) does not accountfor inhibiting effects. In thc, :oo,',idered here, reactants are competing for

active sites at thp surrace. lienc, ,pecieF_ which serves as a reactant for a par-ticbhar surface reactant, re, rr 1 b,? -,r fhibitor to the other reactions

which simultaneously take place a, th- surface. This (ompeting phenomenon may bethe reason why no aprrecil ,-f , is ohserved.

Figure 8 thrnj7' 10 show t %, ,acrs graphitic material there is a regimewhere the ablation, rat e i inses,', " the magnitudc of the heat transfer coef-

ficent. This rec,-e as desrribed it, the above section is called kinetiz regime.

As can be seen in these tigure., the kinetical. y lirih.d regfi2s are as follows:

v Layer P.G. reit.I):re O1P

s Edge P.C. le,,,arat.jrc 4610 R

e Bult graphite ri pnrdtire e 30YR

It is apparent that nF'o. thrsh It v-;rperatures, the surface thermochemistryis independent of 'he ma!>s transfer coefficient. Hcwcver, at temperature above the

threshold tempera.urs, th' .p-nd:,.., icr,.a~es as tnrperature increases, At a

sufficiently hig'] tenper& , ,-. tIt utl.s reaction reches equilibrium, and theablation rate is rontrolled by dio-aIir, t ' s' lsr,,r dependency on the mass

transfer coeffic-r.t, %,f r d're then vc longer important and allcarbon, will iit& ;.,t s,; * : rte, iNiqu,- I nreseqrj the results which are pre-

dicted by ;urface squ,! ',,r;.A, -nr i

Figurv - prc,erts ,. -o expl 4(it po ._e ., 'r, : the erosion rate dependson the mass transfer efficiin.,. ;iip rn'-c , lY- N ,- si;rface ejuilibrium pre-dictions which show thr Vie ircv'iin ,i. * ' rc'r funct 4 on of the mass trans-

fer coefficient. e.,o. ut,, b, .A ti,,i (3, when .%eeCr - &,, the erosionrote becomes ind, %, nient i f i - tr "' c c n Though this is not asevident t e bul- qro,,_, ,t ... 41it .iLt nd edge P.G. solutions.

Tie Iinelqu . oiro o, a r-, ! - * ,,. itAc, lar weight differences

between species upors di ffusioa from tne voundary layer edge to the surface and vice

versa. The ligihtei -J$Ie ; diC'Fuse fx'tr- than the n-avier molecules since their

Note that this st fcments ,rv, only fnr ed e statei at the nozzle throat as pre-dicted by diff-rent gas .;ion assumntlos starting Wth the same chamberconditions.

23

41

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mean free paths are larger and hence allow them vo travel farther before collidingwith other molecules. Therefore, in a hydrogen rich environment, which is often

the case for propellant combustion gases, the equal diffusion assumption will under-

predict the diffusion rate. For layer P.G., due to the slow surface reactions, the

diffusion process does not affect the erosion rate at all. Figure 12 shows that

the erosion rate is constant at a typical wall temperature of 6000OR over a wide

range of peUeCm. With edge P.G. which has a stronger dependency on PeueCm, varia-

tions on the erosion rate prediction were found (see Figure 8) at temperature above

60000R. The determination of the kinetic regime, however, is not affected. Obvi-

ously, the bulk graphite predictions are going to be affected, as it also has a

strong dependency on PeueCm.

The Lewis number is the nondimensional group which characterizes the rela-

tionship between the energy and mass transports. The correlation frequently em-

ployed (Reference 4) is

CM = Le/ 3 (4)C H

Generally, Le j 1, and hence it has an effect on the erosion rate predictions.

Again, the predictions of layer P.G. are unlikely to be affected. Edge P.G. and

bulk graphite will undoubtedly be affected as shown in Figure 8. The erosion rates

are expected to decrease if Le < 1, and vice versa.

From the above discussion, one can conclude that both data analysis and abla-

tion predictions require a careful evaluation of the transfer coefficient. In addi-

tion, when large quantities of hydrogen are in the gas, unequal diffusion should be

included in all calculations.

3.1.5 Arc Plasma Generator Limitations

The design and operation of the arc plasma generator imposes restrictions on

the choice and use of certain test gases. There are two basic areas of concern;

first, the effect of a particular gas on the vital components of the APG (cathode,

anode, constrictor segments, etc.) and second, the stability of the arc when oper-

ating with a particular gas or combination of gases. The situation is further com-

plicated by the desire to produce a test gas at as high a temperature as possible.

This generally requires arc heating of the largest possible portion of the total

test gas flow to the maximum temperature achievable. i.e., maximum energy input,

while minimizing the energy losses to the cooled walls of the APG.

With the design of the APG currently being used, it is necessary to avoid

injecting any oxidizing species into the arc heater as the primary gas. This is due

29

to the tungsten material used in the cathode, which when rapidly removed through

oxidation processes can both limit APG run times to the order of seconds and cause

catastrophic failure of the arc heater. The normal solution employed is the injec-

tion of such gases several constrictor duct diameters downstream of the cathode.

This has been highly successful when the required test gas is simulated air, using

individually injected nitrogen and oxygen. However, in the case of propellant sim-

ulation, there is an additional prob'em. The reactive nature of the base species,

hydrogen, which for reasons of arc efficiency and maximum power input is the arc

heated gas, requires the injection of oxidizing species downstream of the arc

heater portion of the APG, in the plenum section (see Figure la). This is primarily

due to the combustion induced turbulence which adversely affects the stability of

the arc, resulting in failure of the constrictor segments. Therefore, the primary

or arc-heated gases must be either inert or nonoxidizing, the remainder of the test

gases required to make up the propellant simulation are injected in the plenum sec-

tion. This results in lower overall APG efficiences due to the portion of the test

gas which is not directly arc heated and the losses to the plenum section from both

the arc heated primary gases and the exothermic reactions which take place in the

plenum. The net effect is lower test gas temperature and hence lower test sample

surface temperature.

The "normal" APG limits of pressure, current and power input must also be

considered. These, in general, are less sever2 than those discussed above and

typically can be accommodated through arc heater and power supply configuration

changes. It should be noted that this is especially true with hydrogen, which is

very sensitive to the gas injection configuration and arc heater constrictor length.

The penalty for use of an improper configuration is usually very unstable arc

operation.

3.1.6 Potential Test Gases

As described in Section 3.1.2, the potentially important reactants are H2$

H20, and CO2 , typically in decreasing order of mole fractions (sep Table 3). In an

APG, various concentrations of these gases can be mixed and reacted to form the

test stream. By judicious sele ction various reactants and poisons could be isolated

in a systematic manner so that appropriate reaction rate constants could be deter-

mined. some kinetically appealing test gases are shown in Tpfjle 4. These gases are

separated into three groups, reactions which include H2, H20, CO2, and CO, reactions

with these gases and HCZ, and reactions with HF in lieu of HCX. The surface reac-

tion designations are shown in Table 5. The number of test gases to be used in a

material characterization test matrix would be selected as a subset of the gases

tested in Table 4. This selection will be based upon a trade-off between the degree

30

ai C0 (V If t Q " l t! 1.0 ll . "

4#.uI - C '.C.CC4' rl C ) CS. u C'.) c\J C\J (NJ) C\J C.

ui) LL.a) . coC 00 c ' *r00 -c

> )

WL 4-)

V.) 0V

41

cOJ to C C C'Jci .to JC\J I

w . r, Ci D q S.. C'. 0 C OJ 0 , O r j-T' r'.s

!2 CD C) Ca.C)

U.) cm) CJ'-0

00 0.0 l

4-. ui j 1.) C.S-

'-4 047 -

a4) C>I .

CL C) CO I. c k

C-'

0. i -

r0 4-

cm 0) co as tv. co c'.. ('. 0 40 40 0, C'J C'.) 0 ,- 1 co 4-' 46..- ,- - i , - - to

31

= (J 0 0 0

L) 4- CN1 40.0

Ls+ +C). 0 z L-

- ) (.3 C-) Un

32

to which a particular reactant (or poison) can be isolated and the operating limita-

tions of the APG. Note that the exhaust gas composition is only representative and

that all qases that contain CO will also have CO 2 in small quantities. At high

temperatures, it is not possible to have large concentrations of CO2 in the pre-

sence of H2 since the preferred species would be H20 and CO.

3.2 TEST GAS SELECTION

Of the gases shown in Table 4, those that contain HCZ and HF require special

toxic gas handling systems. The current Aerotherm APG facility is equipped to han-

dle HCZ, however, a number of iontrivial additions are required before HF can be

used. Since it is not at all clear that HF effects are really significant, the cur-

rent feeling is that a Lk , ;on on OF testing should bF deferred until sufficient

motor data with fluorinated propella, becomes available. In the event this data

can be analytically treated by an analog between IiF and HC then no further APG

testing will be necessary. However, should the data not correlate properly, then

the expense of the HF handling modifications would be justified. Thus for now,

reactions 14 through 18 of T;ble 4 will be eliminated frnm consideration.

3.2.1 Test Gas Evaluation

Test gases 1 through 8 have been evaluated under a wide variety of APG condi-

tions L.ing both water cooled calorimeters and carbon test sections. These tests

clearly show that te3t gases 2 and 4 resulted in anomalous heating conditions.

The probable cause can be defined by considering the schematic of the APG shown in

Figure la. In normal operations, H2 or an inert gas such as N2, Ar, or He is used

as the arc heated column and ill other gases ara injected between the arc column

and the plenum chamber. Ii we cinsider, dS dn example, test gas 2, the relative

moles of injection gan (02) to -rc olu= gas (H2) is 4/9. However, mixing of the

two gases will be dependcnt upon the relativc rrai rates of the two gases. A sim-

ple conversion shows that the relative mass of in.iection gas to arc column gas is

approximately 28/1. It was originally anticipated .hat combustion induced turbu-

lence would result in adequeat nixng in the plenum h.wever, neasured data suggests

a high concentration of low enthilpy injection qa!!s near the walls of che test

section. This rather poor n,.inq of arc heateJ cad r,,jectiun gases made the test

data impossible to adcqua:tly analyze. Subsequent trial and error experimentation

showed that ratios of itijectca y , tn arc heated gas of less than 5 (by mass) would

result in adequate plenum chamber mixing.* Thus test gar number 5 was also

eliminated.

*It is assumed that at least one of the injected gases will be 02 so that therewill be combustion induced turbulence.

33

Experimentation with test gases 4 and 10 revealed a second difficulty. The

kinetic reactioi rates of CO2 with H2 are much slower than those of H2 with 02. In

fact, some simple kinetic -.l1rulations revealed that there was insufficient residence

time in the plenum chai,,ier t, ttahin ther,.ocher ical equilibrium. Thus, all gases

which would normally inject CO,, suuld t. rtpidced by an equivalent combination of CO

and 02.

From the above dicusss;on, HC, t.,f t-es 12 and 13 can be eliminated out-

right, however ga- number 11 can bL mide aic-e,,table by reducing the relative moles

of HC from 8 to 2. Similarly, fur the ,4'F oises, ias number 16 can be eliminated

and gas numbers 14 and IS would be oore act:TtA le if relative ;.oles of CF4 were

reduced from 2 to 1. Finlly, te,;t g- 1' i unacceptable and needs revision after

the effects ef HF have been asJLssed.

3.2.2 Recor, mended Test t' .tr *l t -r, t 1 tL|z ontudies

Based upon the discusir, 3n Sect;on 1.?.1, the trst gases for material char-

acterization studies were redu~ed t% the suhsct sh wn in lable C. Note that CO2

will not be used as an injection (i, .; and that it has been replaced by an equivalent

quantity of 02 arQ CO. Note dlR,1 th: H$ iasts were not included since the advis-

ability of testing with HF h.- rgt vet 'Pen zsessed.

With the exception of the HF inhibitor all oet r s-,rface reactants are re-

presented by this set of ,'actiors. ft i, cleor1" not pcsible to isolate reactions

other H2 since oxygen bearinq spezcies (CO2, H 2 0)wll react witch solid carbon to

form CO and in gas phase equilihriup,, j small quantity of CO2 will also be present.

The reactions shown in Table 6 reprasent a iocd cot1romise bctween the desire to

isolate reactants and still sta,, within 'rNf' Opedtir limitations of the APG. In

the case of gas mixtures ', ?, 6, 7, dnd , te';, %%s ;:il, be eiposed at APG

conditions which will result in trir_- n), ina; urf),e f ,1'',-rtures, 4000'R, 4503CR

and 5500'R. Fur gas mixtures ? ;nd E, saml,. w I- be ,,.L,cs,!d to two nominal surface

temperatures, 4000F and 45 6'F. while for ms , 4, su-fie temperature of

40UO*R and 5000*F will be uzed An inert %;, wa5 il- ir-lu-ed in Table 5 to test

for shear removal affects. hiert q.s test-.; w:il be -r . th" highest heating con-

ditions compatible witti APG I riltatin-. With t: et- r five reacting gas

mixtures, two tests fvf thtei rec-ti..'cs, ard rn f,'r tre ilert gas test, a minimum of

twenty-two tests are req'iird. Six ,-1itior.il trt,, 3re planned is contingency or

repeat tests and will be perfornen a- required. Thus, a total of 28 tests are plan-

ned for each characterization -,aterial.

34

'a II~O I

4J- cu i - a~ cl * 4-)0.-. ii I-- (C ) CS U

44.)t ). CQ CJ ( .

-L -h .- '.V) CIJ - * L

~ 4J'A4J

o 4-

-U ) CDC

UU

f0 W-wC)O 0r

(I) C) d

C..9-

044JS. 9-

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35

SECiICN 4

CAL!8RAT!-O' :5T RESULTS

Considering the lack of acc-,-i:, ,,,alfttic~l tools for predicting arc heater

performance with the various gas mixijTe- .2'sc us.ed in Section 3.2.2., and the fur-

ther complication of the very laroce _7 thp plenum/mixing chamber on overall

operating efficiency, an effect which wor!' : nearly impossible to model analytical-

ly due to the combustion processes occuriinyj in this chamber, a comprehensive series

of calibration/checkout tests was run to define the operating limits for each test

gas. The following sections describe the results of this test series. Section 4.1

describes the test conditions for each test gas for all gas compositions/heater

configurations utilized. Section 4.2 presents the performance map for each test

gas while Section 4.3 presents the test nozzle response results of a checkout

series run with representative test materials.

4.1 TEST CONDITI OS

Arc heater operation was optimized through a series of calibration/checkout

tests using the calibration nozzle assembly to-measure cold wall heating rates.

Various test gas compositions, combined with different gas injection schemes and arc

heater lengths were used to define the .aaximum rupn condition possible for that par-

ticular test gas. The general criteria fr this test series can be summarized as

follows:

1. Maximum enerqy input te test aas

2. Maximum cold wall heating rate

3. Minimum losses in plentr!/irlirg cham-ber

4. Maximum test nozzle surf.ce termpe'aature

5. Reasonable arc heatci- clrrcnent lifetime

6. Reasonable arc heater running stability

Criteria 1, 2, and 3 generally lead to 4, maximum test nozzle surface temper-

ature; however, it was found that this temperature was a strong function of the gas

injection into the plenum chamber. 1t is felt this is due primarily to mixing and

combustion processes in this chamber and their affect on the test section boundary

layer. Certain conditions seemed to produce a stratified flow, a very hot core sur-

rounded by a cooler shroud. This produced very low surface temperatures as might

be expected.

36

Since the arc heated or primary gas in most cases was hydrogen, criterion 1

was the easiest to meet within the power supply limitations, the only complicating

factor being the unstable running characteristic of arc heating this gds with the

present electrode configuration of the arc heater. Criteria 5 and 6 then most often

determined the maximum run condition, usually limiting the maximum current that

could he reasonably used without seriously degrading arc heater stability and/or

lifetime. Various techniques were applied to overcome this limitation as will be

described below for each test gas.

Meeting criteria 3, minimum losses in the plenum/mixing chamber, proved to be

most difficult. The large size of this chamber, a result of the need to promote

complete mixing of the arc heated primary gas with the plenum injected gases, com-

bined with the turbulent nature of the flow in the chamber as induced by the com-

bustion processes and the energy released by these processes, resulted in very large

energy losses in this section. Most techniques to control this loss were either in-

consistent with the required test conditions, e.g., lower plenum pressure, or ad-

versely affected the flow quality, e.g., smaller plenum chamber. The emphasis there-

fore was on criteria I as discussed above.

The results of the calibration tests are summarized individually for each

nominal test gas in Tables 7 through 11. In most cases, the table is arranged in

the sequence in which the runs were actually made, showing the effect of test gas

composition, gas injection or arc heater configuration changes.

Test Gas 1

Hydrogen is the basic arc heated species for this test series and as such test

gas 1 represents the baseline performance for all the test gases. One basic arc

heater length was used for all test points shown in Table 7; this selection was

based on previous experience with hydrogen. This left the mass flow rate and in-

jection configuration as the only variables available for changing arc heater opera-

tion. The injection configuration was held constant, all flow being arc heated, as

this produced the highest efficiencies and there were no other reasons for trying

different configurations, e.g., gross arc instabilities or component failures.

Several different mass flow rates were tried, with the result that a maximum

of 0.007 Ibm/sec was found to be acceptable from an operational viewpoint. Higher

flow rates resulted in higher chamber pressures with attendant higher heating rates

(or recovery temperatures) demonstrating the need for the largest mass flow possible.

This flow rate also produced the high arc voltages necessary to lower the arc current

to acceptable levels, those which optimize electrode lifetime considering the sever-

ity of the test condition. Note that the last three points in Table 7 are the nomi-

nal test points for test gas 1.

37

%. 4 Un CQ -W C% -It ; kyl I.,j - C7E n m~ M) In fn en * 1

u 03 C~j IT (%J. C) k C) -tO CO4% Ln co 0% 0O0 CD~ l 1- -l Cs

-. N - LO CO L ol0- I O cc 0% ai 43% 47N CS L" C5 ON C) 0

LALA

Cie C3 c. 0 c0 0 0> C C) C > C) C)0

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S 0 0n to 0o to Cl I. I (I -,Cl

(A_ 0D -5 aJ c' cl 0, .4 r CO) m 1 0T

10 0 In 0 1 0 10 ;n C, 0 - N 3 - ra ) in W0 0 nC r

'a~: w1 - - In (\l rl l C) C.

4.)- C') C' - - N C C!) C- 471

0 0 01 0 0 ml " Cj 0l 0 0D 0 0o o 0om t~ o r- r,. 03 -m 4D C- 0C 0 0

39

1- N I 0 n wL

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42

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"43

Test Gas 2

Referring to Table 8, a large range of test gas compositions and injection

configurations were tried before achieving an acceptable balance between test gas

recovery temperature, chamber pressure and arc heater operation. In early tests, it

was found that injection of large amounts of oxygen, in relation to the primary hy-

drogen flow, into the plenum chamber had adverse effects on the arc stability to the

point of driving the arc termination at the anode (nearest the plenum) upstream into

the constrictor column. This usually resulted in early failure of the column seg-

ments. The solution to this problem was to inject varying amounts of an inert car-

rier (Argon) along with the hydrogen primary flow to force the arc termination into

the proper location on the anode, while at the same time reducing the oxygen flow to

the plenum. This necessitated a reduction in the hydrogen flow to maintain the

desired molar concentrations.

Several tests (2599-2506) were run with a completely inert primary flow of

Argon/Helium, adding all the reactants in the plenum. This resulted in very smooth,

stable arc operation but with very low arc voltage. As the arc current is limited,

primarily by the electrode design and secondarily by the power supply characteris-

tics, very low test gas enthaipies and hence low recovery temperatures were achieved.

The net result was that while measured cold wall heating rates were quite high,

achievable test nozzle surface temperature would be severely limited. For this

reason, this particular gas composition was abandoned.

The resulting test points for test gas 2 are the two final points of Table 8.

Test Gas 3

Unlike test gas 2, the primary flow of hydrogen for test gas (see Table 9)

was adequate for the relatively small amount of oxygen injected in the plenum. The

primary effect of the oxygen injection in this case was an increase in the insta-

bility of the arc operation limiting maximum arc current to significantly lower

levels. Efforts were therefore focused n increasing arc voltage through changes in

the oxygen injection configuration to maintain total energy input to the test gas at

the maximum level.

The final three points shown in Table 9 are the nominal test conditions for

test gas 3.

Test Gas 4

Test Gas 4 is similar to test gas 3 with the addition of carbon monoxide in-

jected in the plenum. The total mass flow is increased with a resulting increase in

arc voltage and accompanying decrease in overall efficiency.

44

The last three points shown in Table 10 are the nominal test conditions for

this test gas. Note that a performance limit appears to have been reached around

the 660 amp arc current level, further increases in arc current producing only

slightly higher recovery temperatures. For this reason, there may be effectively

only two test conditions for test gas 4.

Test Gas 5

Test Gas 5 is similar to test gas 2 with the addition of carbon monoxide

injected in the plenum. The primary effect is again an increase in arc voltage.

There is little chaoge in the arc stability, allowing the same maximum current level

as used for test gas 2 to be used, thus increasing the power input to the test gas

and overcoming somewhat the cooling effect of the increased mass flow. The final

results are shown in Table 11.

4.2 PERFORMANCE MAPS

The calibration data of Tables 7 through 11 are presented in graphical form

as performance maps in Figures 13 through 17. The average bulk enthalpy, H., is

plotted against the total gas flow rate, m, divided by the APG sonic area, A*,

(nozzle throat area) and the chamber pressure, Pc" The curve noted on the map is

the best fit for the limited data available for a particular test gas composition.

The various test gas compositions used for test gas 2 can be realistically

divided into four distinct compositions yielding four curves. In the case of test

gas 5, none of the three compositions used yielded sufficient data for accurate

curve fitting.

4.3 TEST NOZZLE RESPONSE

As a final calibration effort, a checkout series of tests were run at the

highest test condition for eaci test gas with three representative materials, c-plane

P.G., ATJ graphite and P03 graphite. The measured oeal surface temperatures for

this series is detailed in Table 12.

The test nozzle response in each case is presented in Table 13. The analyti-

cal and/or source of each iten, in this table are detailed in Appendix A, APG Data

Analysis Procedures.

45

04

II

4-1

6l u

46-

_____a a - F

0~.0

En

4-0

C- I0

C

4- ___ /w _________ _________ _________

Lo /

4- /

- 09- .fl.0

GJ __

C"

in

- .0

- S... 4Jin

43 0i

__ __ I 4.3___ 2

-0.

I .~ 0)U

-I 43Clii

_________ _________ __________ 1~0)

0.

'1~

___ __ ___ __ C'1~

LL

N----- ~~0

c..J - 0 o~

~.4 WW ~SS/q~ - .Y~d/N

47

00

.0 0, f

484.

jv _ _ _ _ _ _ _ _ _ _000_ _

4J

49,

0

49 ~

aj

05

TABLE 12. PRELIMINARY TEST RESULTS, SURFACETEMPERATURE COMPARISON, HIGHESTTEST CONDITION

Test Surface Temperature (OR) CalibrationGas C-Plane PG P03 Graphite ATJ Graphite Test

1 5220 5190 5040 2583,06

2 4540 4270 3970 2616,03

3 5120 5080 4840 2588,03

4 5100 - 4760 2595,04

5 - 4030 2618,01

51

uo sn p 0CcY C, u f--.. O4C'J C.J l El

c -) n 0r 71 Cl) CDC DD C

%00 0 c C.00 C 00 M

.- ,4 f r0 -~Q ;; (',

(as~C/ 9 CD , CDo In C)

C5 ) C' a C) .. -".r -

00 ou jjdaL-:ejn ~j)VC) ~rr~ cni r C7 :r m? 1

4) a 0~~~C .t..j~CU

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6- 0 C%-- c) 6 6 6 c c,.

'j 'ainwiadwaj -:AO0 c4 co~ r-.r r- , -

C) L

(qing M n C, C C )c ,

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.4as /q) 0ZL d "CDJc C C UC' a'

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I- c o C> --- mr %O O% %011;~~l(U 4A[ 040.C) un-z O hjfU 00 ~ CO co i

CJ\*\ n%(J (I)N' U'J) LoNr r

IC

SECTION 5

SUMMARY AND CONCLUSIONS

The process for selecting test gases and test conditions for arc plasma gen-

erator simulation of propellant environments was described. By considering the im-

portant reactants, anticipated environmental conditions, the sensitivity of surface

kinetics to these conditions, and arc plasma operating limitations, a set of test

gases was selected. Each of the test gases has been subjected to extensive calibra-

tion and checkout runs to define anticipated surface heating rates and temperatures

for graphitic materials. These gases permit a separation of the effects of various

reactants so that materials tested with these gases can be analyzed and correlated

to yield kinetic reaction rates which are appropriate for rocket nozzle environments.

53