experimental techniques for solid-propellant combustion research

7
AHA Journal VOLUME 5 JULY 1967 NUMBER? Experimental Techniques for Solid-Propellant Combustion Research RAYMOND FRIEDMAN Atlantic Research Corpora'ion, Alexandria, Va. I. Introduction T HE principal problem areas of solid-propellant combustion include: 1) maximization or minimization of burning rate; 2) minimization of burning-rate dependence on pressure and temperature; 3) control of transient phenomena, such as ignition, quenching, and combustion instability, both acoustic and nonacoustic; 4) maximization of combustion efficiency; 5) formation of condensed-phase products in finely divided form; and 6) predictability of combustion behavior in the presence of large crossflows (erosive burning) or large acceleratory fields. Each of these problems may be studied directly by means of the Crawford pressurized strand burner 15 or a recent refinement thereof, 49 instrumented static firings of test rockets, and instrumented test flights with telemetered data. Techniques utilized in these procedures will not be discussed in this paper. Instead, we are concerned with the more scientific approach to the preceding problem areas, namely, the basic understanding of the physicochemical processes occurring in the solid propellant combustion zone. This understanding, if available, would lead to rational rather than empirical procedures for attacking these problems. Clearly, a scientific approach to a complex process in which numerous condensed-phase, surface and gaseous rate processes such as chemical reaction, energy transfer, and mixing may have significant influence requires both theoretical and experi- mental work. Theories developed to date 20 are incomplete, apply only in limited regimes, or require unavailable knowl- edge of parameters. Experimental progress has been slow because of the very small thickness and high temperatures and pressures prevailing in the combustion zone, requiring either indirect or extremely sophisticated experiments. This paper will review experimental techniques developed to date. In view of the present state of knowledge, emphasis will be placed on the techniques themselves rather than on signif- icance of results. II. Steady-State Combustion Zone Structure of Propellants The characteristic thickness of the thermal wave penetrating the condensed phase during combustion is of the order of a/r, where a. is thermal diffusivity and r is burning rate. Thermal diffusivity of ammonium perchlorate has been measured by Rosser et al. 59 Taking a = 0.002 cm 2 /sec and r = 1 cm/sec, corresponding to high-pressure combustion of ammonium perchlorate, the wave thickness in the condensed phase is seen to be of the order of 20 /x. (Thermal difTusivities of binders are of the order of 0.001 cm 2 /sec.) The characteristic distance of the gas-phase flame above the surface may be estimated as follows. An energy flux from the flame of probably several hundred cal/g is required to vaporize the solid. (It is not possible to calculate the exact value because of uncertainty as to the degree of exothermic condensed-phase reaction.) Assuming 200 cal/g, or 400 cal/cm 3 if specific gravity is 2, then a flux of 400 cal/cm 2 -sec from flame to surface would be required to vaporize the pro- pellant at a rate of 1 cm/sec. Equating this flux to AAT 7 / Ao;, where X is the gas conductivity, say 0.0002 cal/cm-sec- 0 C, and AT is the difference between the maximum flame tem- perature and the surface temperature, say 2000°C, then we obtain A#, the height of the flame above the surface, to be 10 M- The previous calculations, although crude, show that the zone of interest is substantially less than 100 ^ thick when the burning rate is the realistic magnitude of 1 cm/sec. In com- posite propellants, particle sizes of ingredients are often in the Raymond Friedman is a Vice President of Atlantic Research Corporation and has been General Manager of its Research Division since 1963. He obtained his doctorate in chemical engineering, with a minor in physical chemistry, from the University of Wisconsin in 1948. He joined Atlantic Research Corporation after ten years at Westinghouse Research Labora- tories, where he carried out gaseous combustion research in support of Westinghouse's turbo- jet engine program. He has directed a wide variety of basic and applied research programs relating to combustion and fuel technology, especially of rocket fuels, at Atlantic Research Corporation. He is a past chairman of the American Chemical Society's Division of Fuel Chemistry, and is a director of the Combustion Institute. He is an Associate Fellow of the AIAA. Submitted January 4, 1967; also presented as Paper 67-67 at the AIAA 5th Aerospace Sciences Meeting, New York, January 23- 26,1967. [4.23] 1217 Downloaded by UNIVERSITY OF CAMBRIDGE on October 15, 2014 | http://arc.aiaa.org | DOI: 10.2514/3.4174

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Page 1: Experimental techniques for solid-propellant combustion research

AHA JournalVOLUME 5 JULY 1967 NUMBER?

Experimental Techniques for Solid-PropellantCombustion Research

RAYMOND FRIEDMANAtlantic Research Corpora'ion, Alexandria, Va.

I. Introduction

THE principal problem areas of solid-propellant combustioninclude: 1) maximization or minimization of burning

rate; 2) minimization of burning-rate dependence on pressureand temperature; 3) control of transient phenomena, suchas ignition, quenching, and combustion instability, bothacoustic and nonacoustic; 4) maximization of combustionefficiency; 5) formation of condensed-phase products in finelydivided form; and 6) predictability of combustion behaviorin the presence of large crossflows (erosive burning) or largeacceleratory fields. Each of these problems may be studieddirectly by means of the Crawford pressurized strand burner15

or a recent refinement thereof,49 instrumented static firingsof test rockets, and instrumented test flights with telemetereddata. Techniques utilized in these procedures will not bediscussed in this paper. Instead, we are concerned with themore scientific approach to the preceding problem areas,namely, the basic understanding of the physicochemicalprocesses occurring in the solid propellant combustion zone.This understanding, if available, would lead to rationalrather than empirical procedures for attacking these problems.

Clearly, a scientific approach to a complex process in whichnumerous condensed-phase, surface and gaseous rate processessuch as chemical reaction, energy transfer, and mixing mayhave significant influence requires both theoretical and experi-mental work. Theories developed to date20 are incomplete,apply only in limited regimes, or require unavailable knowl-edge of parameters. Experimental progress has been slowbecause of the very small thickness and high temperaturesand pressures prevailing in the combustion zone, requiringeither indirect or extremely sophisticated experiments. Thispaper will review experimental techniques developed to date.In view of the present state of knowledge, emphasis will be

placed on the techniques themselves rather than on signif-icance of results.

II. Steady-State Combustion ZoneStructure of Propellants

The characteristic thickness of the thermal wave penetratingthe condensed phase during combustion is of the order ofa/r, where a. is thermal diffusivity and r is burning rate.Thermal diffusivity of ammonium perchlorate has beenmeasured by Rosser et al.59 Taking a = 0.002 cm2/sec andr = 1 cm/sec, corresponding to high-pressure combustion ofammonium perchlorate, the wave thickness in the condensedphase is seen to be of the order of 20 /x. (Thermal difTusivitiesof binders are of the order of 0.001 cm2/sec.)

The characteristic distance of the gas-phase flame abovethe surface may be estimated as follows. An energy flux fromthe flame of probably several hundred cal/g is required tovaporize the solid. (It is not possible to calculate the exactvalue because of uncertainty as to the degree of exothermiccondensed-phase reaction.) Assuming 200 cal/g, or 400cal/cm3 if specific gravity is 2, then a flux of 400 cal/cm2-secfrom flame to surface would be required to vaporize the pro-pellant at a rate of 1 cm/sec. Equating this flux to AAT7/Ao;, where X is the gas conductivity, say 0.0002 cal/cm-sec-0C,and AT is the difference between the maximum flame tem-perature and the surface temperature, say 2000°C, then weobtain A#, the height of the flame above the surface, to be10 M-

The previous calculations, although crude, show that thezone of interest is substantially less than 100 ^ thick when theburning rate is the realistic magnitude of 1 cm/sec. In com-posite propellants, particle sizes of ingredients are often in the

Raymond Friedman is a Vice President of Atlantic Research Corporation and has beenGeneral Manager of its Research Division since 1963. He obtained his doctorate in chemicalengineering, with a minor in physical chemistry, from the University of Wisconsin in 1948.He joined Atlantic Research Corporation after ten years at Westinghouse Research Labora-tories, where he carried out gaseous combustion research in support of Westinghouse's turbo-jet engine program. He has directed a wide variety of basic and applied research programsrelating to combustion and fuel technology, especially of rocket fuels, at Atlantic ResearchCorporation. He is a past chairman of the American Chemical Society's Division of FuelChemistry, and is a director of the Combustion Institute. He is an Associate Fellow of theAIAA.

Submitted January 4, 1967; also presented as Paper 67-67 at the AIAA 5th Aerospace Sciences Meeting, New York, January 23-26,1967. [4.23]

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range of 10 to 100 /z. Thus we have an extremely small andnon-one-dimensional system to explore.

The experimental techniques developed to date to studythis problem may be divided into three groups: temperaturedistribution studies, chemical composition distributionstudies, and deductions from examination of suddenlyquenched surfaces.

The first experiments to measure temperature distributiondirectly were those of Klein et al.,36 who embedded thermo-couples made of 12.5- t̂ wire into nitrocellulose propellants.Further experiments of this type, using 7.5- and 12.5-/Z wire,were reported by Heller and Gordon.26 Sabadell etal.60 werethe first to apply these techniques to a composite propellantcontaining very finely ground (9 /x) ammonium perchlorate.They used 7.5- and occasionally 2.5-ju wire. These workers allconcentrated their experiments at the lower end of the pres-sure range of interest, so that burning rates would be lowerand the gradient less steep. These experiments, althoughrequiring extremely tedious procedures, have proved veryvaluable in providing the only available direct information ontemperature distribution through the combustion zone.However, they do not provide sufficiently accurate informa-tion to yield a precise value of surface temperature. Forexample, Klein et al. estimated a surface temperature of250° C for a nitrocellulose propellant at 350 psi, whereas Hellerand Gordon estimate 400-500°C from data for a nitrocellulose-nitroglycerine propellant at lower pressures (flameless com-bustion conditions), with surface temperature appar-ently increasing to 1000°C at 350 psi. Again, for com-posite propellants, Sabadell et al. found apparent surfacetemperatures varying randomly from 450° to 700° C in some70 experiments. This lack of precision is not surprising afterconsidering the steepness of the gradient relative to thethermocouple size, the nature of the "foam zone" for double-base propellants, and the heterogeneous nature of compositepropellants, even if finely ground. Sabadell et al., refer tomeasured gradients of at least 30°/ju; their thermocouplejunctions were "less than twice" the wire diameter of 7.5IJL used in most experiments. In summary, the precision ofthermocouple results is limited because the presence of thecouple distorts the thermal field and also because of thevariable time lag caused by the heat capacity of the couple.

An alternate experimental approach to combustion zonetemperature measurement, developed by Fowling andSmith,53"56 is based on the infrared emission from the burningsurface. Their studies have been concerned primarily withdeflagrating ammonium perchlorate crystals, either in pureform or in the presence of volatile organic fuels. The methodrequires selection of an infrared wavelength at which theammonium perchlorate is as opaque as possible, so that radia-tion from the cooler subsurface layers will not distort theresult to a low value. Furthermore, the wavelength shouldbe such that there is as little emission as possible from thehigher-temperature gaseous region above the surface, whichwould distort the result to a high value. It is easily seen thatboth these sources of error become more serious as pressure isincreased. Fowling and Smith feel that the method is notusable above 60 psia. In order to apply the method even atlow pressure it is necessary to know emissivity and trans-mittance of ammonium perchlorate at the surface tem-perature; Fowling and Smith measured these properties upto 360°C, finding emissivity peaks at 3.1 and 7.1 ju, so thatthese are favored wavelengths. Their results for ammoniumperchlorate burning either pure or in presence of fuel vaporsat 1 atm yield a surface temperature around 500°C. Theyfind surface temperatures of 300-400° C for various double-base propellants. (They also applied their technique topolymers burning in air and in oxygen, obtaining values rang-ing from 417° to 570° C for various conditions.)

It would appear that the infrared method is superior to thethermocouple method as a means of accurately determining

surface temperature. Unfortunately, however, it does notseem accurate enough to provide a definitive answer to a keyquestion: Does surface temperature of ammonium perchloratepropellants increase with increasing pressure, regardless ofburning rate, consistent with the variation of equilibriumsublimation pressure of ammonium perchlorate with tem-perature, or does it increase as burning rate increases, regard-less of the pressure, because of an Arrhenius dependence ofsurface gasification rate on temperature? In 1963 and 1964papers, Fowling and Smith reached the former conclusion,stating, "at any given pressure the concentration of fuel madelittle difference to the observed surface temperature althoughlarge changes in burning rate occurred." Their log-p vsl/Ts plot is also consistent in magnitude and slope with theequilibrium ammonium perchlorate sublimation experimentsof Inami et al.32 However, in Powling's 1966 paper, hereports some new data showing variation of Ts with burningrate at constant pressure, and states that he now favors thelatter (or kinetic) model of the burning surface. His datashow that plotting l/T8 vs either log-p or log-r (where r isburning rate) results in substantial scatter, perhaps ±40°in the log-p correlation and ±30° in the log-r correlation (mynumerical estimates). Clearly, a higher precision in meas-uring Ts would be very helpful in resolving this troublesomebasic question.

Some other ingenious techniques have been utilized to givebits of information about temperature distribution in thecombustion zone. Selzer62 burned thin propellant specimensbetween two crossed polarizing filters with strong back-light-ing, and was able to detect the subsurface phase change at240 °C due to the crystal transition of ammonium perchloratefrom orthorhombic to cubic. From the depth of this pointbelow the surface and assumed thermal properties, he calcu-lated surface temperatures of 430°-730°C, depending on pro-pellant type. Summerfield et al.65 added a trace of sodiumchloride to a composite propellant and observed the D-lineemission above the surface with an optical system havingspatial resolution of about 30 ju. They were able to deducethat the flame reached peak temperature at a distance of 100/x or less from the surface, at 50 psi and above, but flameinhomogeneities and tilted burning surfaces limited the pre-cision of their sjbudy. Povinelli51 monitored CN radicalemission at 3883 A for a composite propellant; the carbon iscontributed by the fuel, and the nitrogen by the oxidizer, sothe appearance of CN should mark the binder-oxidizer gaseousflame zone. He simultaneously located the surface by back-lighting of strands 3 mm thick. At 1 atm he found the CNemission to commence at 72 /z above the surface and to con-tinue strong emission over a zone 2 mm wide. Experimentsat higher pressures (50 to 100 psi) were unsuccessful becauseof a background continuum attributed to carbon.

Because of the great difficulties of in situ flame structureexperiments, attention has been given to examination of sud-denly quenched specimens, even though it is recognized thatone can never be sure of the validity of this procedure. In1946, Aristova and Leipunskii1 quenched nitrocellulose anddouble-base propellants originally burning at 1 atm, andcalorimetrically determined the subsurface heat content.From this they could deduce temperature distributions and,hence, Ts. Observations of quenched composite propellantsurfaces by Summerfield et al.65 reveal that for high combus-tion pressure and unimodal particle size distribution, i.e.,fuel-rich systems, binder projects above the mean surface andoxidizer crystals are found at the bottom of pits in the sur-face. McGurk45 has analyzed microtomed cross sections ofquenched composite propellant with a polarized light micro-scope and was able to determine depth below the surface atwhich the orthorhombic-cubic-orthorhombic transitions ofammonium perchlorate had taken place (240°C). Becksteadet al.6 applied the same technique to quenched deflagratingsingle crystals of ammonium perchlorate, and deduced in apreliminary report that the surface temperature was constant

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at about 560 °C over the combustion pressure range from 300to 1000 psi. The depth of the transition below the surfacewas about 11 ̂ at 1000 psi and varied inversely with burningrate at lower combustion pressures.

The writer is not aware of any experiments revealing detailsof chemical composition within the combustion zone of anormally burning composite solid propellant. Heller andGordon26 sampled the gaseous "induction" zone of a double-base propellant burning in a bomb. Samples were drawninto previously evacuated flasks via a Vycor capillary tubeand subsequently analyzed mass-spectrometrically. Thegases in this "dark zone" were found to be invariant withheight from 6 to 16 mm above the surface, at 200 psi, andconsisted mainly of CO, NO, C02, and H2. (Water was notmeasured.) Dauerman et al.,17 in more sophisticated experi-ments, induced double-base strands to burn at quite low pres-sures (10 torr) by imposing a high radiant flux from an arc-image furnace. They sampled the gases directly into a rapid-scanning mass spectrometer via a 51-ju-diam leak, and werethus able to detect unstable intermediates such as NOs(tentatively identified). Coates12 has described preliminaryexperiments wherein a platinum wire is coated with a thinfilm consisting of 5-ju ammonium per chlorate and an organicbinder; a capacitor discharge through the wire raises it to500-600°C in perhaps 50 jusec. The vapors pass directly tothe ionization region of a time-of-flight mass spectrometer.Hydrocarbon fragments, NH3, HC104, HC1, and C102 wereobserved.

Finally, it is necessary to mention direct motion-picturephotography of burning strands in window bombs. Thistechnique has been used by virtually every laboratory study-ing solid-propellant combustion and has provided interestingqualitative information. Perhaps the most striking findingby this diagnostic technique is the discovery that, for pro-pellants containing fine aluminum powder as an ingredient,agglomeration into much larger molten spheres of aluminumcan occur on the burning surface. Successful photographyrequires: 1) very thin samples, so that the combustion zoneis not optically opaque; 2) a flow of inert gas between theburning surface and the window to prevent obscuration bysmoke; and 3) illumination of the surface by a powerful lightsource. In some cases it will also be necessary to develop anoptimum technique of "inhibiting" the sides of the strand toprevent "flashing" in such a way that the charred inhibitorresidue does not block a view of the surface.

III. Idealized Experiments Relating toComposite Solid-Propellant Combustion

In view of the composite nature of a fuel-oxidizer mixtureand of the difficulties of performing quantitative experimentsthereon, investigators have devoted considerable effort toperforming idealized experiments intended to simulate certainaspects of composite propellant combustion. One class ofthese experiments involves studies of the oxidizer self-deflagration in the absence of binder, of the binder pyrolysis inthe absence of condensed-phase oxidizer, or of combustion ofsingle particles of metal ingredients in oxidizing gases.

Friedman et al.21»37 found that pure ammonium per-chlorate, in pressed-strand form, would self-deflagrate atpressures above 22 atm, at a measurable rate increasing withpressure up to the highest pressure studied, 340 atm. Irwinet al.33 reported continued increase of burning rate up to 1500atm. This phenomenon makes it possible to obtain informa-tion not only on burning rate vs pressure but also on tempera-ture and composition of the product gases from self-deflagrat-ing ammonium perchlorate. Measurements with 13-̂thermocouples21 showed the product gas temperature to be900-950°C, increasing slightly with pressure and beingseveral hundred degrees below the thermochemically calcu-lated value. Sampling experiments37 were in agreement with

this, showing that most of the nitrogen from ammonium wasconverted to nitric oxide rather than the thermodynamicallymore stable molecular nitrogen. The technique for collectingthe product gases, the corrosiveness of which is comparableto aqua regia, at pressures up to 2000 psi is described.37

However no technique for determining unstable intermediatesat these pressures has been developed. Levy and Friedman37

showed that combustion of pure ammonium perchlorate at1 atm can be maintained by an imposed radiant flux of about10 cal/cm2-sec, whereas Fowling and Smith54 have accom-plished the same result by preheating to about 300°C. High-tower and Price28 have performed combustion experimentswith large single crystals of ammonium perchlorate.

The hot-plate technique first developed at Aerojet-GeneralCorporation4 can be used with ammonium perchlorate toobtain curves showing regression rate vs temperature of thehot plate. This is somewhat different from self-deflagration.The technique has been criticized10'46 because of the uncertaineffect of the temperature gradient across the gas film betweenthe hot plate and the specimen. Coates13 and Guinet22 havemade measurements with a porous hot plate, with the productgases aspirated through the plate to a vacuum. In order toachieve uniform temperature in the hot plate, Coates found itnecessary to use a focused radiant flux from an arc-imagefurnace rather than resistance heating. Coates's results forammonium perchlorate were not radically different from theprevious results with the solid hot plate. Guinet's resultsgenerally agreed with those of Coates, but upon extending hisdata to lower temperatures (below 470° C) he found the regres-sion rate to decrease much more rapidly with decreasingtemperature than would have been expected.

Pyrolysis studies of organic binders have also been per-formed by the hot-plate technique.61 Another more versatiletechnique has been developed by McAlevy and Hansel23'41

which permits variation of the gas composition over the poly-mer; they heat the surface of a porous plug made of plasticspheres by exposing it to a flow of hot gas from a rocket motor,while simultaneously maintaining a much smaller counterflowof an inert or reactive cold test gas through the porous plug.

Two techniques have been developed for study of metalparticle combustion. Macek38 and others have refinedtechniques for injecting single particles of aluminum orberyllium into laminar combustion product gases of controlledtemperature, oxygen content, and water vapor content, ob-serving both ignition and combustion. The technique islimited in regard to the range of ambient compositions andpressures which can be achieved. Nelson47 has used intenselight pulses to ignite metal particles in free fall through a glasstube with a controlled atmosphere. This permits studies ofcombustion but not ignition; also, the particle is burning in acold ambient gas, as contrasted with the rocket environment.The ambient pressure to date has been restricted to 1 atm.Attention is being given to develop a flash-heating apparatusthat functions at higher pressure. Attempts to use a laserlight source for metal combustion studies are under con-sideration.

The foregoing experiments have dealt with behavior ofoxidizer, or organic fuel, or metal additive, in the absence ofthe other ingredients. Idealized studies have also been madeof fuel-oxidizer interactions. For example, Fowling andSmith,53~56 Barrere and Nadaud,3 and McAlevy and Lee42

have all developed different geometrical arrangements forburning ammonium perchlorate in an atmosphere of fuel gassuch as hydrogen or methane. The experiments permitobservation of regression rate as a function of gas velocity,and provide an environment in which temperature or com-position studies could be made. Hightower and Price27 haveprepared "sandwiches" consisting of a thin binder layer (ca.25 ju) between two single crystals of ammonium perchloratetypically 8 X 8 X 1 mm thick. The sandwiches are partiallyburned in a window bomb under nitrogen, suddenly quenchedby depressurization, and then examined microscopically.

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Other investigators have studied entirely gaseous combus-tion systems intended to simulate aspects of solid-propellantcombustion. Cummings and Hall16 and Heath and Pearson25

have investigated premixed flames of perchloric acid vaporwith methane by classic low-pressure flat-flame burner tech-niques. They could not study premixed ammonia-perchloricacid vapor systems because of the high reactivity. Burger,van Tiggelen, and Poncelet9 have constructed a diffusion-flame burner consisting of a water-cooled bundle of parallelcapillary tubes, each 0.5-mm diam, half of which are fed withfuel and the other half with oxidant. Although their experi-ments were with methane and oxygen, the technique isobviously amenable to more reactive combinations of fuel andoxidizer.

IV. Ignition Studies

The first objective of solid-propellant ignition studies is todetermine the time lag before ignition as a function of the heatflux applied to the surface, with the proviso that the "proper"definition of ignition is not obvious. A more basic objectiveis to determine the relative contributions of gaseous, con-densed-phase, and surface reactions to the preignition energyrelease.

To simulate realistic rocket conditions and to avoid pre-heating the propellant to an excessive depth, ignition timesranging from milliseconds to tens of milliseconds are required.Experiments show that this may require heat fluxes of theorder of 100 cal/cm2-sec. Such fluxes may be achieved in thelaboratory either by convection or by radiation.

Beyer and Fishman7'58 describe an arc-image furnacecapable of a maximum radiant flux of 75 cal/cm2-sec. Thepressure over the sample may vary from vacuum to 35 atm,and the chemical composition of the atmosphere may beadjusted. The energy is applied as a pulse of variable dura-tion (from 3 to 1000 msec) achieved by a rotating shutterassembly. Ignition is determined on a go no-go basis byapplying pulses of increasing duration to a series of samplesand observing the first emission of visible light from the sur-face after the pulse. Quite often, there is a time delay. Asimilar apparatus has been described by Fleming and Flem-ing.18 Beyer et al.8 have described a flux-calibrating calorim-eter.

Baer and Ryan2 have described a much simpler radiant-fluxapparatus, which, however, can only achieve a flux of 13 cal/cm2-sec, corresponding to ignition delays of not much lessthan a second. They rapidly thrust the sample into a furnace .(requiring 20-30 msec) and then detect ignition with aninfrared-sensitive photocell that views the surface.

Although useful data are obtained in such experiments,there are uncertainties as to the applicability of the results toignition by convectively supplied flux, because of basic dif-ferences in the experimental conditions. Firstly, the ambientgas immediately above the surface is cold in the radiant fluxexperiment and hot during convective ignition; this couldhave an important effect on gaseous preignition reactions.Secondly, the propellant or ingredients thereof may be some-what transparent to the radiant flux, so that the subsurfacetemperature distribution at the instant of ignition may dependon whether radiant or convective flux was used. Thirdly,the radiation may cause photochemical decomposition.

Convective heating apparatus suitable for basic studies ofignition has not proven easy to develop, although several goodtechniques are now available. Early attempts to sting-mount solid propellant samples in a shock tube were unsuc-cessful, presumably because the supersonic velocity past thesample swept away the pyrolyzing gases before self-sustainingflame reactions could begin. When samples were flush-mounted in the end-wall of a shock tube40-44 and exposed tothe stagnant hot gas of the reflected shock, ignition could bereproducibly obtained if the ambient gas contained a sub-stantial proportion of oxygen, but not otherwise.

Keller, Baer, and Ryan35 have successfully adapted a shocktube to ignition studies without requiring oxygen. Thedriven end of their shock tube narrows to a constant-areaflow channel terminated by a critical-flow orifice; the sampleis flush-mounted in the wall of this channel and is observedthrough an opposing port. Gas velocities of 50 to 800 m/see,temperatures of 1000° to 2600° K, and pressures from 14 to25 atm are achieved as the hot gas behind the reflected shockis vented through the test channel. Fluxes up to 160 cal/cm2-sec are achieved, giving ignitions with nitrogen within afew milliseconds. One interesting result of this work is thatsample surface roughness favors ignition at relatively highflux levels and low flow velocities, but not otherwise. This isexplained by the authors as follows. At high flux levels thethermal penetration is small relative to the roughness. Atlow velocity, local exothermicity is encouraged since thedecomposing gases are not swept away soon enough. Hence,extreme care in sample preparation may be much more im-portant under some experimental conditions than others.A limitation of the Keller-Baer-Ryan apparatus is that theywere not able to obtain ignition at pressures below 14 atmbecause of insufficient shock-tube test time.

A somewhat similar apparatus has. been developed byBastress et al.5 which differs in that the hot gas is generatedby combustion rather than by shock heating. The ruptureof a diaphragm initiates flow from the combustion chamberover the sample. Particulate matter can be added to thecombustion gases, and effects of surface condensation ofmolten particles on ignition can be studied.

In the preceding experiments, ignition is defined by thefirst emission of light, detected either with a phototube or aninfrared sensor. There is a question as to whether this coin-cides with the time at which the exothermic processes becomeself-sustaining, but no more refined techniques have beendeveloped. It will be noted that, while the radiant sourcecan be terminated just before ignition, the convective flux inpresently available apparatus continues after ignition, leavingunanswered the question of whether one has achieved a self-sustaining condition when light emission first occurs.

In addition to direct experiments in which thermal flux isapplied to propellants, other tests relevant to solid-propellantignition have been reported in which perchloric acid vapor50

or oxygen43 is allowed to suddenly come into contact withsolid organic fuels at elevated temperature and the resultingevents observed.

V. Extinguishment Studies

There are two distinct phenomena related to extinguish-ment of solid propellants: 1) the lowest steady-state ambientpressure at which combustion will maintain itself; and 2) theextinguishment resulting from very rapid rate of pressuredecrease. In neither case is there a complete theoreticalunderstanding of the controlling factors, so design andinterpretation of experiments must be done with care.

Friedman et al.21>37 found a low-pressure limit of 22 atm forcombustion of pure pressed ammonium perchlorate. Thisvalue was confirmed by an experiment of Horton and Price,30

yielding 23 atm as the limit. Hightower and Price28 reporteda limit of 19 atm for single-crystal ammonium perchlorate.However, when trace quantities of additives are present, orless effective ignition procedures are used, the lower limit ofpressed perchlorate may be raised to about 100 atm. On theother hand, addition of heat or of small additions of organicfuel lowers the limit to below atmospheric pressure. Thisgreat sensitivity raises a question as to the ultimate nature ofthis 22-atm limit. It was found37 that the limit is insensitiveto the ignition energy as long as this is above a threshold value,insensitive to substitution of helium for nitrogen as thepressurizing gas, and insensitive to the sample diameter.Levy and Friedman37 proposed that this limit is due to radia-

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tive loss; however more recent experiments30 involving com-bustion on the inner surface of a cylinder of ammoniumperchlorate gave the same limit value, which casts doubt onthe radiation loss theory. Furthermore, mathematicalstudies of the limit by Johnson and Nachbar34 indicate thatthe radiation loss would not be large enough in magnitude toaccount for the limit. Until this phenomenon is understood,it is hardly possible to design meaningful limit experimentswith more complex systems in which ammonium perchlorateis only an ingredient.

It may be noted the combustion wave thickness parametera/r when calculated for pure ammonium perchlorate at 1atm is 0.1 cm, if Levy and Friedman's extrapolated value37

of 0.015 cm/sec for burning rate is used. Von Elbe hasshown that, over a wide range of pressures, the quenchingdiameter of another solid-monopropellant oxidizer, hydrazinenitroform (which burns at quite low pressures), is about 20 to30 times the characteristic length a./r. If the same ratioholds for ammonium perchlorate, then a minimum samplediameter of 2 to 3 cm would be required to achieve self-deflagration at 1 atm. Perhaps more experiments withlarger specimens, better ignition techniques, and more con-trolled heat losses are needed to determine the nature of thelower limit.

A more amenable phenomenon is the upper pressure limitfor ammonium perchlorate21'37 which has been traced to freeconvective heat loss. Buoyancy of the hot products inducesmixing with the cold ambient nitrogen in the test bomb, thebuoyancy increasing with increasing pressure while viscosityis nearly pressure-independent. Changing the sample geom-etry or shielding with asbestos allows combustion to con-tinue above the "upper limit."

Transient extinction due to very rapid pressure decrease isalso of great interest. An approximate theoretical analysisof this phenomenon, based on the variation of subsurfacesteady-state heat storage with burning rate r, and hence withpressure p, as derived by von Elbe,66 and subject to variousrestrictive assumptions, shows that the required rate ofpressure decrease, — d Inp/dt, must be greater than r'2/2nafor extinction to occur. If r is 1 cm/sec; n, the burning-ratepressure exponent, is 0.5; and a, the propellant thermal dif-fusivity, is 0.001 cm2/sec; then the critical value for — dInp/dt is 1000 sec"1; i.e., the pressure must drop to l/e of itsvalue in a millisecond.

Ciepluch11 has described an apparatus for suddenly ventinga combustion chamber containing burning propellant at ratesup to 250,000 psi/sec into a vacuum chamber at 3.5 torr;monitoring the flame luminosity with a phototube. He foundthat a sufficiently rapid decay rate ("critical rate") gavepermanent extinction, whereas much lower decay rates (downto one-fourth the critical rate) caused luminosity to decreaseto perhaps 5% of the original value and then increase again ascombustion reestablished itself at a lower pressure. He alsofound that venting to atmospheric pressure rather than tovacuum occasionally led to reignition.

Fletcher and Bunde19 have performed related experimentsin which they sheared out a diaphragm at the head end of aninternally perforated grain motor during combustion, andmonitored the rate of pressure decay. They compared themeasured decay rate with a calculated decay rate assumingcontinuing gas generation in accordance with a burning ratedecreasing with decreasing pressure as specified by the steady-state burning law. The results were opposite to those ex-pected from Ciepluch's observed luminosity decrease, in thatthe transient gas evolution rates were often found to be greaterthan called for by the steady-state burning law; i.e., pressuredropped more slowly than calculated.

This result is probably due to the great increase in transversegas velocity over the propellant surface during depressuriza-tion, resulting in abnormally high burning rates. Clearly,grain geometry and corresponding flow patterns must beconsidered carefully in planning venting experiments.

VI. Acoustic Interaction with theCombustion Zone

Empirical study of solid-propellant rocket motors hasshown that, for certain propellant compositions, the com-bustion zone can amplify an acoustic wave, leading to dis-astrous consequences. If the combustion zone is thought ofas a region of gas generation, it is easily seen that a variationof either pressure or transverse velocity caused by a standingacoustic wave may produce a sympathetic variation of gasgeneration rate, although perhaps with a phase lag becauseof the finite time constants of heat and mass transfer andchemical processes occurring in the combustion wave.Whether or not amplification will take place depends on therelative magnitudes of the time-dependent sensitivity of thecombustion process to an acoustic wave of given frequency(acoustic admittance of the flame) and the acoustic dampingwhich may be present.

Rather extensive theoretical studies of this problem havebeen carried out and are reviewed by Hart and McClure.24

The theory highlights the necessity of having an experimentalmeasurement of acoustic admittance vs frequency for thepropellant burning at the pressure of interest, since it isgenerally conceded that the combustion process is too compli-cated and insufficiently understood to permit an a prioriprediction of this admittance, except qualitatively, even ifthe problem were mathematically tractable. Over the pastdecade, numerous attempts have been made to design anexperiment for measuring acoustic admittance of a burningpropellant sample. For example, shock-tube techniques havebeen used to impinge a single pulse on the burning surfaceand compare its shape before and after reflection. Again,attempts have been made to pass a wave into the flame fromthe propellant side. Experiments involving a conventionalacoustic approach, a cavity resonator, have been attempted.An electromechanically driven resonance tube has been tried.Sirens have been installed near a burning strand. None ofthese experimental efforts have been shown to be reasonableways of obtaining the desired values. However, one approachinvolving a self-excited center-vented tubular end burner withpropellant at one or both ends has been successful in produc-ing data. This device, designated a T-burner, will be dis-cussed in some detail.

The T-burner was developed in 1960-1963 in parallel pro-grams at the University of Utah, the Naval Ordnance TestStation, and the Army Ballistic Research Laboratories.14'29'31,57,64 jt js shown in Fig. 1, in the apparently more successfulversion with propellant at both ends. The diameter isgenerally about 4 cm, and the length is varied to vary theoperating frequency by the relationship / = c/2L, where cis sonic velocity in the combustion products. Frequenciesfrom about 150 to about 5000 cps can be studied.

In this type of system, for many propellant compositions,exponentially growing pressure oscillations build up immedi-ately after ignition, until they level off at a high value. After

Sample .

xf/Sample

1

Sonic Nozzles

Transducer forMean PressureMeasurement

Transducer forhf Pressure -

Measurement

Fig. 1 Schematic drawing of T-burner. (Means for pre-press urizing with inert gas and simultaneous ignition of

both samples not shown.)

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1222 R. FRIEDMAN AIAA JOURNAL

burnout, the oscillations decay exponentially. It is assumedthat the same attenuation effects are acting during build-upand decay. Thus, the sum of the measured oscillation growth-rate constant and the decay-rate constant should equal theoscillation growth which would have occurred in the absenceof attenuation. This quantity can be related to the real partof the acoustic admittance if nonlinear effects are ignored.

A number of variations of the T-burner design are possible.For one thing, the burner can vent to a surge tank at essen-tially the same pressure, instead of one or several symmetricallyarranged sonic nozzles at the midpoint of the tube (pressurenode). The pressure transducer can be mounted at the sideof the tube instead of in the end-wall under the propellant.It js not believed that such changes affect the results. Oscil-lation can also be obtained if propellant is placed in only oneend.of the tube; however Oberg48 found that the oscillationamplitude at the nonburning end was, unexpectedly, abouttwice as large as at the burning end. Mathes39 has reportedwork with a T-burner that is 15 cm in diameter and up to 18m long, to permit acoustic studies at lower frequencies (10-100 cps). His burner includes a loosely fitting mechanicaldamper which is not turned to the "open" position until theburner is full of combustion gases, thus giving a cleaner start-ing point for the oscillation growth measurement.

In general, double-base and nonmetallized compositepropellants are self-exciting in the T-burner. However, ifmore than 1 or 2% of aluminum is present in the propellant,excitation does not occur. Oberg48 has shown that oscilla-tions can be induced in a T-burner loaded with highly alu-minized propellant by firing a squib near one end of the tubeduring combustion. The rate of decay of the squib-inducedoscillation is observed and compared with that resulting froma second squib fired just after burnout.

It would of course be highly desirable to be able to compareacoustic admittance of propellants measured by the T-burnermethod with measurements obtained by some other technique.It is not feasible however for the reviewer to discuss the ex-perimental difficulties of all these other techniques.

The foregoing discussion has dealt with pressure-coupledinstability, the propellant samples in the T-burner beinglocated at velocity nodes. Velocity-coupled instability,although believed to be less common, is also a possiblephenomenon. Price57 has described a center-vented tubularburner with propellant lining the inner cylindrical surfaceexcept at the vent, which exhibits velocity-coupled instabil-ity. Other arrangements under investigation for studyingthis type of coupling include a vortex burner52 and mountinga third propellant sample near the middle of a T-burner.63

References1 Aristova, Z. I. and Leipunskii, O. I., "On the surface heating

of burning powder," Compt. Rend. Acad. Sci. U.R.S.S. 54,503-505 (1946).

2 Baer, A. D. and Ryan, N. W., "Ignition of composite pro-pellants by low radiant fluxes," AIAA J. 3, 884-889 (1965).

3 Barrere, M. and Nadaud, L., "Combustion of ammoniumper chlorate spheres in a flowing gaseous fuel," Tenth Symposium(International) on Combustion (Combustion Institute, Pitts-burgh, Pa., 1965), pp. 1381-1394.

4 Barsh, M. K., Andersen, W. H., Bills, K. W., Moe, G., andSchultz, R. D., "An improved instrument for measurement of thelinear pyrolysis rates of solids," Rev. Sci. Instr. 29, 392-395(1958).

5 Bastress, E. K., "Test methods for solid propellant rocketigniter development," Interagency Chemical Rocket PropulsionGroup Second Combustion Conference, Applied Physics Lab.,Silver Spring, Md. Chemical Propulsion Information AgencyPublication 105, pp. 551-561 (May 1966); also Allan, D. S.,Bastress, E. K., and Smith, K. A., "Heat transfer process duringignition of solid-propellant rockets," AIAA Preprint 66-66 (1966);also J. Spacecraft Rockets 4, 95-100 (1967).

6 Beckstead, M. W., Hightower, J. D., and Murphy, D. W.,"On the surface temperature of deflagrating ammonium per-

chlorate crystals," Interagency Chemical Rocket PropulsionGroup Third Combustion Conference, Applied Physics Lab.,Silver Spring, Md., Chemical Propulsion Information AgencyPublication 118 (1966).

7 Beyer, R. B. and Fishman, N., "Solid propellant ignitionstudies with high flux radiant energy as a thermal source," SolidPropellant Rocket Research (Academic Press Inc., New York,1960), pp. 673-692.

8 Beyer, R. B., McCulley, L., and Evans, M. W., "Measure-ment of the energy flux density distribution in the focus of anarc image furnace," Appl. Opt. 3, 131-135 (1964).

9 Burger, J., van Tiggelen, A., and Poncelet, J., "Technique deBruleur analogique en vue d'une application aux propergolscomposites," Astronaut. Acta 11, 57-64 (1965).

10 Cantrell, R. H., "Gas-film effects in the linear pyrolysis ofsolids," AIAA J. 1, 1544-1550 (1963).

11 Ciepluch, C. C., "Effect of rapid pressure decay on solidpropellant combustion," ARS J. 31, 1584-1586 (1961).

12 Coates, R. L., "Final report: Research on combustion ofsolid propellants," Lockheed Propulsion Co., Redlands, Calif.,Rept. 641-F (December 1965).

13 Coates, R. L., "Linear pyrolysis rate measurements of pro-pellant constituents," AIAA J. 3, 1257-1261 (1965).

14 Coates, R. L., Horton, M. D., and Ryan, N. W., "T-burnermethod of determining the acoustic admittance of burning pro-pellants," AIAA J. 2, 1119-1122 (1964).

15 Crawford, B. L., Jr., Huggett, C., Daniels, F., and Wilfong,R. E., "Direct determination of burning rates of propellantpowders," Anal. Chem. 19, 630-633 (1947).

16 Cummings, G. A. McD. and Hall, A. R., "Perchloric acidflames. I. Premixed flames with methane and other fuels,"Tenth Symposium (International) on Combustion (CombustionInstitute, Pittsburgh, Pa., 1965), pp. 1365-1372.

17 Dauerman, L., Salser, G. E., and Tajima, Y. A., "Evidencefor nitrogen trioxide in the combustion of a double-base pro-pellant," J. Phys. Chem. 69, 3668-3669 (1965).

18 Fleming, R. O., Jr. and Fleming, R. W., "Determination ofpropellant properties by the arc image furnace technique,"AIAA J. 2,117-118 (1964).

19 Fletcher, E. A. and Bunde, G. W., "Gas evolution from asolid rocket propellant during depressurization to produce aquench," AIAA J. 4, 181-182 (1966).

20 Friedman, R., "Mechanism of composite solid propellantcombustion," Applied Mechanics Surveys (Spartan Books,Washington, D. C., 1966), pp. 1171-1176.

21 Friedman, R., Nugent, R. G., Rumbel, K. E., and Scurlock,A. C., "Deflagration of ammonium per chlorate," Sixth Sym-posium (International) on Combustion (Reinhold PublishingCorp., New York, 1957), pp. 612-618.

22 Guinet, A., "Vitesse lineaire de pyrolyse du perchlorate d'am-monium en ecoulement unidimensionnel," Rech. Aerospatiale109,41-49(1965).

23 Hansel, J. G. and McAlvey, R. F., "Energetics and chemi-cal kinetics of polystyrene surface degradation in inert andchemically reactive environments," AIAA J. 4, 841-848 (1966).

24 Hart, R. W. and McClure, F. T., "Theory of acoustic in-stability in solid propellant rocket combustion," Tenth Sym-posium (International) on Combustion (Combustion Institute,Pittsburgh, Pa., 1965), pp. 1047-1065.

25 Heath, G. A. and Pearson, G. S., "Perchloric acid flames:Part III, chemical structure of methane flames," EleventhSymposium (International) on Combustion (Combustion Insti-tute, Pittsburgh, Pa., to be published).

26 Heller, C. A. and Gordon, A. S., "Structure of the gas phasecombustion region of a solid double base propellant," J. Phys.Chem. 59, 773-777 (1955).

27 Hightower, J. D. and Price, E. W., "Two-dimensional ex-perimental studies of the combustion zone of composite pro-pellants," Interagency Chemical Rocket Propulsion GroupSecond Combustion Conference, Applied Physics Lab., SilverSpring, Md., pp. 421-432 (May 1966).

28 Hightower, J. D. and Price, E. W., "Combustion of am-monium perchlorate," Eleventh Symposium (International) onCombustion (Combustion Institute, Pittsburgh, Pa., to be pub-lished).

29 Horton, M. D., "Use of the one-dimensional T-burner tostudy oscillatory combustion," AIAA J. 2, 1112-1118 (1964).

30 Horton, M. D. and Price, E. W., "Deflagration of pressedammonium perchlorate," ARS J. 32,1745 (1962).

31 Horton, M. D. and Price, E. W., "Dynamic characteristics

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of solid propellant combustion," Ninth Symposium (International)on Combustion (Academic Press Inc., New York, 1963), pp. 303-310.

32 Inami, S. H., Rosser, W. A., and Wise, H. "Dissociationpressure of ammonium perchlorate," J. Phys. Chem. 67, 1077-1079 (1963).

33 Irwin, O. R., Salzman, P. K., and Andersen, W. H., "De-flagration characteristics of ammonium perchlorate at highpressures," Ninth Symposium (International) on Combustion(Academic Press Inc., New York, 1963), pp. 358-365.

34 Johnson, W. E. and Nachbar, W. "Deflagration limits inthe steady linear burning of a monopropellant with applicationto ammonium perchlorate," Eighth Symposium (International)on Combustion (Williams and Wilkins, Baltimore, 1962), pp.678-689.

35 Keller, J. A., Baer, A. D., and Ryan, N. W., "Ignition ofammonium perchlorate composite propellants by convectiveheating," AIAA J. 4, 1358-1365 (1966).

36 Klein, R., Mentser, M., von Elbe, G., and Lewis, B., "De-termination of the thermal structure of a combustion wave byfine thermocouples," J. Phys. Colloid Chem. 54, 877-884 (1950).

37 Levy, J. B. and Friedman, R., "Further studies of pureammonium perchlorate deflagration," Eighth Symposium (Inter-national) on Combustion (Williams and Wilkins, Baltimore,1962), pp. 663-672.

38 Macek, A., "Fundamentals of combustion of single aluminumand beryllium particles," Eleventh Symposium (International) onCombustion (Combustion Institute, Pittsburgh, Pa., to bepublished).

39 Mathes, H. B., "Experimental techniques in low frequencysolid propellant acoustic instability," Interagency ChemicalRocket Propulsion Group Third Combustion Conference, AppliedPhysics Lab., Silver Spring, Md., Chemical Propulsion Informa-tion Agency Publication 118 (September 1966).

40 McAlevy, R. F., Cowan, P. L., and Summerfield, M., "Themechanism of ignition of composite solid propellants by hotgases," Solid Propellant Rocket Research (Academic Press Inc.,New York, 1960), pp. 623-652.

41 McAlevy, R. F. and Hansel, J. G., "Linear pyrolysis ofthermoplastics in chemically reactive environments," AIAA J.3,244-249(1965).

42 McAlevy, R. F. and Lee, S. Y., "A porous plug burner tech-nique for the study of composite solid propellant deflagrationon a fundamental level and its application to hybrid rocketpropulsion," AIAA Progress in Astronautics and Aeronautics:Heterogeneous Combustion, edited by H. G. Wolf hard, I. Glassman,and L. Green Jr. (Academic Press Inc., New York, 1964), Vol.15, pp. 583-608.

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46 Nachbar, W. and Williams, F. A., "On the analysis of linearpyrolysis experiments," Ninth Symposium (International) onCombustion (Academic Press Inc., New York, 1963), pp. 345-357.

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48 Oberg, C., "Acoustic instability in combustion," Ph.D.thesis, Univ. of Utah (1965).

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measurement of solid propellant burning rates," Rev. Sci. Instr37, 86-92 (1966).

50 Pearson, G. S. and Sutton, D., "Ignition of composite pro-pellant fuels by perchloric acid vapor," AIAA J. 4, 954-956(1966).

51 PovmeHl, L. A., "A study of composite solid-propellantflame structure using a spectral radiation shadowgraph tech-nique," AIAA J. 3, 1593-1598 (1965).

52 Povinelli, L. A., "Velocity sensitivity in transverse mode solidpropellant combustion instability," Interagency ChemicalRocket Propulsion Group Third Combustion Conference,Applied Physics Lab., Silver Spring, Md., Chemical PropulsionInformation Agency Publication 118 (September 1966).

53 Fowling, J., "Experiments relating to the combustion of am-monium perchlorate composite propellant," Eleventh Symposium(International) on Combustion (Combustion Institute, Pittsburgh,Pa., to be published).

54 Powling, J. and Smith, W. A. W., "Measurement of theburning surface temperatures of propellant compositions byinfra-red emission," Combust. Flame 6, 173-181 (1962).

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56 Powling, J. and Smith, W. A. W., "The surface temperatureof ammonium perchlorate burning at elevated pressures,"Tenth Symposium (International) on Combustion (CombustionInstitute, Pittsburgh, Pa., 1965), pp. 1373-1380.

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58 Rosser, W. A., Fishman, N., and Wise, H., "Ignition of simu-lated propellants based on ammonium perchlorate," AIAA J.4,1615-1622 (1966).

59 Rosser, W. A., Inami, S. H., and Wise, H., "Thermal dif-fusivity of ammonium perchlorate," AIAA J. 4, 663-666 (1966).

60 Sabadell, A. J., Wenograd, J., and Summerfield, M., "Meas-urement of temperature profiles through solid-propellant flamesusing fine thermocouples," AIAA J. 3, 1580-1584 (1965).

61 Schultz, R. D. and Dekker, A. D., "The absolute thermaldecomposition rates of solids," Fifth Symposium (International)on Combustion (Reinhold Publishing Corp., New York, 1955),pp. 260-267.

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