Full Paper

Effect of Voids inside AP Particles on Burning Rate of AP/HTPBComposite Propellant

Makoto Kohga*

Department of Applied Chemistry, National Defense Academy, Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686(Japan)

Received: June 21, 2007; revised version: June 23, 2008

DOI: 10.1002/prep.200700234

Abstract

Bubble contamination in an ammonium perchlorate (AP)-based composite propellant has a positive effect on the burningrate. However, the quantitative effect of the bubble contamina-tion on the burning rate has never been revealed. In order toclarify the relationship between the increase in the burning rateand the void fraction of the propellant, propellants were preparedwith fine porous AP particles (PoAP) or fine hollow AP particles(HoAPs), and their burning rate characteristics were investigated.The voids inside AP particles have the effect of increasing theburning rate. The increase in the burning rate is enhanced linearlyas the void fraction increases. The effect of the void fraction onthe burning rate for a propellant containing PoAP is not identicalwith that for a propellant containing HoAP. It was found that theeffect of the void fraction on the burning rate could be estimatedby the void fraction when the bubble contamination is uniform insize and shape.

Keywords: Ammonium Perchlorate, Bubble Contamination,Burning Rate, Composite Propellant, Void Fraction

1 Introduction

In order to accommodate a diversity of rocketmotors, it isnecessary to expand the range of the burning rate of thepropellant, especially in the higher burning rate region.Recently, ammonium perchlorate (AP)/hydroxyl-terminat-ed polybutadiene (HTPB) composite propellant is widelyused as a solid propellant. It is generally known that theburning rate of the AP/HTPB composite propellant in-creases with the decreasing particle diameter of the APparticles contained in the propellant. Fine AP particles arerequired to obtain a high burning rate AP/HTPB compositepropellant. However, it is difficult to safely prepare fine APparticles by grinding [1, 2] because fine AP is easily ignitedand explodes with any slight impact or friction. It wasreported that some fine porous or hollow AP particles

(HoAPs) were safely prepared by a spray-drying method[3 – 7].

It is possible that bubble contamination is scattered in thesolid propellant grains during their manufacture. Bubblecontamination in the solid propellant grains loaded in arocket motor might induce a serious problem. Someinfluences of cracks or voids in solid propellant grains oncombustion have been investigated [8 – 10]. Yamaya et al.[10] caused bubble contamination in the binder by addingsome microspheres, e.g., hollow carbon, hollow glass, andhollow plastic microspheres, to the uncured AP-basedcomposite propellant, and they investigated the influencesof the bubble contamination on the burning characteristics.It was reported that the bubbles, of which the averagediameter was in the range of 43 – 200 mm, in the binder havea positive effect on the burning rate [10].

The fine hollow and porous AP particles (PoAPs) haveempty spaces in these particles. It can be considered that apropellant prepared with PoAP and HoAP would havebubble contamination because voids inside the PoAP andHoAP cannot be completely charged with HTPB. In theprevious paper [11], twenty AP samples including eightkinds of PoAPs and HoAPs were used as an oxidizer, andthe burning characteristics of the propellant preparedwithfine PoAPs or HoAPs were investigated. It was found thatthere was bubble contamination in the propellant pre-pared with PoAPs and HoAPs, and the voids in the PoAPsand HoAPs had a positive effect on the burning rate.However, the quantitative effect of the bubble contami-nation on the burning rate was not revealed. In this study,an attempt was made to clarify the relationship betweenthe increase in the burning rate and the void fraction of thepropellant.

* Corresponding author; e-mail: kohga@nda.ac.jp

249Propellants, Explosives, Pyrotechnics 33, No. 4 (2008)

C 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

2 Experimental

2.1 Sample

2.1.1 AP Samples

In this study, three kinds of AP particles were used as anoxidizer: PoAP, HoAP, and a ground commercial AP(GrAP). PoAP and HoAP were prepared by spray-dryingmethods [5, 7]. GrAP was prepared by grinding a commer-cial AP with a vibration ball mill for 5 min. The scanningelectron microscope (SEM) photographs of PoAP andHoAPare shown inFigure 1.According to the external viewof these AP particles, PoAP has some small holes in thecrust, and the voids in the porous particles are connected tothe outside [5]. The shape of HoAP appears spherical in theSEM photograph. However, HoAP has voids in theparticles, indicating a hollow sample [7]. GrAP has aspherical shape. The weight mean diameter (Dw) of APwasmeasured using the SEMphotographs. The Dw of PoAP,HoAP, and GrAP are 9, 4, and 240 mm, respectively.

The thermochemical behavior and crystallographic prop-erties of a particle would be altered by quick recrystalliza-tion. Differential thermal analysis (DTA), thermogravim-etry (TG), andX-ray diffractometry (XRD)were employedin preliminary experiments. The DTA–TG thermogramsand the XRD patterns of AP samples were almost the sameand closely coincided with the typical DTA–TG thermo-gram and XRD pattern of AP, respectively. These resultsindicate that the AP samples used in this study have almostthe same thermochemical behavior and crystallographicproperties.

2.1.2 Propellant Sample

In this study, the propellants were prepared with 80%AP.HTPB was used as a binder. HTPB was cured withisophorone diisocyanate. Eight percent of isophoronediisocyanate related to HTPB was added.

The upper limit of AP content incorporated in thepropellant exists due to limitations in the formulation ofAP/HTPB composite propellant, and the upper limit of APcontent decreases with decrease in size of the AP [12, 13].The propellant with 80% AP could not be prepared with

PoAP or HoAP alone because these AP samples are fineparticles.

The upper limit of AP content incorporated in thepropellant prepared with fine particles increases on replac-ing fine particles with coarse particles [14]. GrAP was acoarse AP sample. An attempt was made to prepare thepropellant containing 80%APwith bimodal APmixed withPoAPor HoAP and GrAP. PoAP could not be contained atmore than 48% in the propellant with 80% AP, and HoAPcould not be contained atmore than 40%. Table 1 shows thecomposition of the propellant used to investigate the effectof the void fraction in thepropellant on theburning rate.TheDw of AP included in the propellant was calculatedtheoretically from the proportion of AP and each Dw ofthe AP sample. The propellants prepared in this study weredesignated by the symbols shown in Table 1. Three batchesof propellants were prepared with the sameAP sample, andthe burning rate characteristics of these propellants weremeasured.

2.2 Measurement of Propellant Density

The propellant strands were prepared using the APsamples shown in Table 1. Approximately 14 propellantstrands were prepared for one batch. As mentioned inSection 2.1.2, three batches of propellants were preparedwith the same AP sample. The densities of all propellantstrands were calculated from the volume and weight. Theweight of the strandwas determinedwith an electric balancewith aminimum reading of 0.01 g. The size of the strand wasmeasuredwith a vernier caliper, theminimumscale ofwhichwas 0.05 mm.

2.3 Measurement of Burning Rate

The size of each strand was 10 mm � 10 mm in cross-section and 40 mm in length. Theburning ratewasmeasuredin a chimney-type strand burner which was pressurized withnitrogen at 288� 1.5 K. The range of pressure was from 0.5to 7 MPa. The strand sample was ignited by an electricallyheated nichrome wire attached on the top of each strandsample. Two fuse wires were threaded through the strandsample at 25 mm distance. The fuse wire was cut as soon asFigure 1. SEM photographs of AP samples.

Table 1. Compositions and properties of propellants.

Symbol HoAP PoAP GrAP HTPB Dw Density Void% % % % mm g · cm�3 fraction

A1 – 8 72 20 220 1.60 0A2 – 24 56 170 1.59 0.004A3 – 40 40 130 1.56 0.024A4 – 48 32 100 1.55 0.033B1 8 – 72 220 1.57 0.018B2 24 – 56 20 170 1.51 0.058B3 40 – 40 120 1.44 0.097C – – 80 20 240 1.60 0

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the burning surface passed through the fuse wire. Theburning rate was calculated using the cutoff period of twofuses.

3 Results and Discussion

3.1 Void Fraction

APoAPhas some small holes in the crust, and the voids inthe particle are connected to the outside, as described inSection 2.1.1. HoAP has a void in the particle. The bubblecontamination in a propellant prepared with porous orhollow AP would be inside the AP particles because thevoids in porous or hollowAP cannot be completely chargedwith HTPB. In this section, the propellant density wasmeasured and the void fraction of the propellant wasestimated on the basis of the density.

The propellant density is tabulated in Table 1. Thedeviation of the density measurements remained within0.01 g · cm�3. The density decreases with the increase incontent of PoAP or HoAP. The propellant was preparedwith 80% AP in this experiment. The densities of AP andcured HTPB were 1.95 and 0.93 g · cm�3, respectively.The theoretical density of the propellant at 80% AP was1.60 g · cm�3. The densities of propellants A2 –A4 and B1 –B3 were below the theoretical value. The densities ofpropellants A1 and C agreed with the theoretical value. Theproportion of the bubble contamination in the propellantwas estimated based on the propellant density. Table 1 alsoprovides the void fractions of the propellants. The voidfraction of propellants A1 and C was zero. This indicatesthat these propellants did not have bubble contamination.Despite the fact that propellantA1 contained 8%PoAP, thevoids inside this propellant were undetectable by thetechnique used in this experiment, while the voids insidethe propellant B1 prepared with 8% HoAP were detected.This was because of the difference in the particle properties,especially the size and shape of the voids in the AP particlesand the particle diameter, between PoAP and HoAP. Thevoid fraction was in the range of 0.004 – 0.097, and that forpropellant B3 was the greatest in this study.

Figure 2 shows the influence of the content of PoAP orHoAP on the void fraction. The void fractions increaselinearly with the increase in content of PoAPor HoAP. Thisindicates that the void fractions can be estimated by thecontent of PoAP or HoAP.

3.2 Burning Rate

Figure 3 presents the burning rate characteristics. Areproducible burning rate was obtained, and an approx-imately linear relationship existed between log(pressure)and log(burning rate) in this pressure range. The burningrate increases with the increase in concentration of PoAPorHoAP.

When the proportion of the bubble contamination in thecomposite propellant containing 80% AP exceeds the voidfraction of 0.02, the burning rate is influenced by the bubblecontamination and, consequently, a reproducible burningrate cannot be obtained [10]. However, the combustion of acomposite propellant, in which the proportion of bubblecontamination is 0.10, can be kept at a steady state when thediameter of the bubbles in the propellant ranges from 10 to350 mmand the bubbles do not contact each other [10]. Eventhough the void fraction of propellants A3, A4, B2, and B3were above 0.02, these propellants had reproducible burn-ing rates as indicated in Figure 3. The bubble contaminationin propellants A2 –A4 and B1 –B3 would be inside PoAPand HoAP particles as stated in Section 3.1. This suggeststhat the voids were very small, less than the size of the APparticles, and numerous small voids would be distributedindependently. Therefore, the combustion of the propel-lants prepared with PoAP and HoAP can evidently bemaintained at a steady state even if the propellants havebubble contamination above a void fraction of 0.02.

3.3 Effect of Void Fraction on Burning Rate

Table 2 shows the burning rates (r) at 1 and 7 MPadetermined on the basis of the burning rate characteristics.At constant AP content and combustion pressure, the

Figure 2. Relationship between content of PoAP or HoAP andvoid fraction.

Figure 3. Burning rate characteristics.

Effect of Voids inside AP particles on Burning Rate of AP/HTPB Composite Propellant 251

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burning rate of the propellant without bubble contamina-tion is dependent on Dw [11, 15, 16]. Figure 4 depicts therelationship between the burning rates measured at 1 and 7MPaandDw. For eachpropellant includingPoAPorHoAP, rrelates to Dw as well. The r increases with decrease in Dw.However, the relationship between r and Dw of thepropellant containing HoAP is not identical to that of thepropellant containing PoAP. The r of the propellantcontaining HoAP is higher than that of the propellantcontaining PoAP at constant Dw.

At 72% AP, the voids in PoAPs and HoAPs have apositive effect on the burning rate [11]. The relationshipbetween the burning rate for propellant with 80% AP andDwwas reported inRef. [16]. The relationship between r andDw of the propellant prepared with spherical AP [16] is alsoshownby the broken line in Figure 4. The burning rate of thepropellant containing PoAP or HoAP is greater than thatshown by the broken line. This indicates that the burningrates of propellants prepared with PoAP and HoAPexceeded that of the propellant with spherical AP at aconstant Dw. It is confirmed that the voids inside APparticles also have the effect of increasing theburning rate at80% AP.

Table 2 shows that the burning rate (r*) of the propellantpreparedwith spherical AP, the Dw of whichwas the same asthat of AP shown in Table 1, was determined on the basis ofthe broken line. In order to clarify the difference in theburning rate, the ratio of r to r* (r/r*)was calculated, and theresults are also shown in Table 2. The relationship betweenr/r* and the void fraction is plotted in Figure 5. The r/r*increases linearly with increase in void fraction at eachcombustion pressure.

The burning rate increases with an increase in temper-ature gradient in the vicinity of the burning surface. Theflame of an AP-based composite propellant is called adiffusion flame, and the multiple-flame model [17] isgenerally accepted as the flame structuremodel. Thismodelhas the flame structure of AP-based composite propellantconsisting of the AP monopropellant flame, the primaryflame, and the final diffusion flame. The AP monopropel-lant flame, which is composed of AP decompositionproducts, is not considered to occur at the propellant surfacebut rather to extend out from the surface. The primary flame

is a premixed flame with the oxidizer and binder decom-position productsmixing completely before reaction occurs.The final diffusion flame follows the primary flame.According to this flame model, because the regression rateof the AP particles is larger than that of HTPB, the locationof each flame is brought closer to the burning surface and,consequently, the temperature gradient in the vicinity of theburning surface increases. The regression rate of porous andhollow AP would be greater than that of spherical AP at aconstant Dw because the flame propagation velocity in voidsinside porous and HoAPs would be higher than that innonvoid AP particles [11]. Therefore, the location of eachflame would be moved close to the burning surface, and theburning rate would be increased because of the use of PoAPand HoAP contained in the propellant, compared with thepropellant using a spherical AP alone. The flames wouldmove close to the burning surface as the concentration ofPoAP and HoAP in the propellant increased. According toFigure 2, the void fraction was enhanced as the content ofPoAP and HoAP increased. Therefore, r/r* would increasewith the increase in void fraction.

The relationship between r/r* and the void fraction of thepropellant containing HoAP did not agree with that of thepropellant containing PoAP. For HoAP, the increase in r/r*versus the void fraction at 1 MPa is definitely larger than

Figure 4. Influence of Dw on burning rate.

Table 2. Burning rate characteristics of propellant.

Propellant 1 MPa 7 MPa

r r* r/r* r r* r/r*mm · s�1 mm · s�1 – mm · s�1 mm · s�1 –

A1 3.9 3.9 1.00 8.6 8.6 1.00A2 4.4 4.1 1.07 9.9 9.7 1.02A3 6.2 4.8 1.29 13.8 10.9 1.27A4 7.1 5.2 1.37 16.3 12.2 1.34B1 4.7 4.1 1.15 9.3 8.9 1.04B2 5.4 4.3 1.26 11.4 9.7 1.18B3 7.6 4.9 1.55 14.5 11.1 1.31C 3.9 3.9 1.00 8.5 8.5 1.00

Figure 5. Effect of void fraction on r/r* (R squared values areover 0.98).

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that at 7 MPa. For PoAP, the increment at 1 MPa is slightlylarger than that at 7 MPa. The influence of the void fractionon the r/r* of PoAP is greater than that of HoAP.

When the burning surface of a propellant, extinguished byrapid depressurization, was observed, at lower pressure, theAP particles protrude above the exposed surface of thebinder to a greater height, and at higher pressure, they recess[18, 19]. This fact indicates that the regression rate ofAP at ahigher pressure is greater than that at a lower pressure. Theflame propagation velocity in the voids inside PoAPs andHoAPs would be higher than that of a non-void AP particle[11]. The overall regression rate of AP at a lower pressurewould show a remarkable increase due to the PoAPs andHoAPs present, compared to that at a higher pressure,because the regression rate of AP at a higher pressure issufficiently great even with non-void AP particles. There-fore, r/r* at 1 MPa would be greater than that at 7 MPa.

HoAP has voids in the particles and the voids in thehollow particles are isolated from the outside. On the otherhand, PoAP has some small holes in the crust, and the voidsin the porous particle are connected to the outside. Someentries of the voids in PoAP would be clogged with HTPB.Because the combustion process of the propellant preparedwith HoAP was different from that of the propellant withPoAP, the relationship between r/r* and the void fraction ofthe propellant containing HoAP would not be identical tothat of the propellant containing PoAP. It was found that theinfluence of the bubble contamination on the burning ratewas not presumed due to the void fraction alone. Thecombustion process of the AP-based composite propellantis greatly dependent on the particle properties of AP. Theinfluence of the void fraction on the burning characteristicswould vary due to the difference in the particle propertiesbetween PoAP and HoAP, e.g., the mean diameter of APand the size and shape of voids in AP particles.

As mentioned in Section 3.1, the void fraction increaseslinearly with the increase in content of PoAP and HoAP.Also, r/r* is enhanced with the increasing void fraction ateach combustion pressure as shown in Figure 5. It was foundthat the effect of the void fraction on the burning rate wouldbe determined by the content of PoAP and HoAP. Thesefacts suggest that the effect of the void fraction on theburning rate would be estimated by the fraction when thebubble contamination is uniform in size and shape.

4 Conclusion

In order to reveal the effect of void fraction on the burningrate, the propellants were prepared with fine PoAP and fineHoAP, and their burning rate characteristics were inves-tigated. From this study, the following conclusions wereobtained: (i) the propellants prepared with PoAP andHoAP have bubble contamination. The void fractions ofthese propellants are in the range from 0.004 to 0.097. (ii)The combustion of propellants prepared with PoAP andHoAP was steady even if these propellants have bubblecontamination. This is because of the fact that the void size

in the propellant with PoAP and HoAP is very small, lessthan the particle diameter of AP, and numerous fine voidsare distributed independently of one another. (iii) The voidsinside the AP particle have the effect of increasing theburning rate. The effect of the void fraction on the burningrate for the propellant containing PoAP is not identical withthat for the propellant containing HoAP. The effect wouldbe estimated by the fractionwhen the bubble contaminationis uniform in size and shape.

5 References

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[9] K. K. Kuo, A. T. Chen, T. R. Davis, Convective Burning inSolid Propellant Cracks, AIAA J. 1978, 16, 600.

[10] T. Yamaya, A. Iwama, H. Tokui, Combustion and Detonationof Porous Composite Propellants(1) Microspheres includedPropellant, J. Ind. Expl. Soc. Jpn. 1982, 43, 381.

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[12] M. Kohga, Y. Hagihara, AP/HTPB Composite PropellantUsing Fine Hollow AP Prepared by Spray-Drying Method,Sci. Technol. Energetic Mater. Jpn. 2003, 64, 75.

[13] M. Kohga, Y. Hagihara, Experimental Study on Estimation ofUpper Limit of Ammonium Perchlorate Content in Ammo-nium Perchlorate/Hydroxyl-Terminated Polybutadiene Com-posite Propellant, Trans. Japan Soc. Aero. Space Sci. 1998, 41,74.

[14] M. Kohga, Y. Hagihara, Estimation of Highest Burning Rateof AP/HTPB Composite Propellant with ExperimentalApproarch(1) – Burning Rate Characteristics of AP/HTPBComposite Propellant, Sci. Technol. Energetic Mater. Jpn.2003, 64, 153.

[15] N. Tsujikado, Y. Ohyumi, I. Ohmura, T. Harada, M. Aboshi,Burning Rate Control of HTPB Propellants, J. Ind. Expl. Soc.Jpn. 1980, 41, 287.

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[17] M. W. Beckstead, R. L. Deer, C. F. Price, A Model ofComposite Solid- Propellant Combustion Based on MultipleFlames, AIAA J. 1970, 8, 2200.

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[18] T. L.Boggs, R. L.Deer, M. W.Beackstead, Surface Structureof Ammonium Perchlorate Composite Propellants, AIAA J.1970, 8, 370.

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Surface Structure and Profile Characteristics of BurningComposite Solid Propellant, Combustion Sci. Technol. 1970,1, 369.

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