burning characteristics of ap/htpb composite propellants prepared with fine porous or fine hollow...

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Burning Characteristics of AP/HTPB Composite PropellantsPrepared with Fine Porous or Fine Hollow Ammonium Perchlorate

Makoto Kohga*

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

DOI: 10.1002/prep.200600007

Abstract

Fine porous and hollow ammonium perchlorate (AP) particleswere prepared by the spray-drying method. Propellants preparedwith porous or hollow AP were found to have bubble contam-ination. The bubble in the propellant appeared inside the porousand hollow AP particles because the voids in porous and hollowAP cannot be completely filled with HTPB. The relationshipbetween the burning rate and the weight mean diameter, Dw, andthe specific surface area, Sw, is divided into two regions. Theburning rate was almost constant above the critical Dw andincreased with decreasing Dw below that. The burning rate wasalmost constant below the critical Sw and increased with increasingSw above that. These critical points did not depend on the voids inthe AP particles. The burning rate of the propellant prepared withspherical AP was dependent on Dw and Sw. The burning rates ofthe propellants prepared with porous or hollow AP were notassociated with Dw or Sw alone and were greater than that of thepropellant prepared with spherical AP at a constant Dw or Sw. Thevoids in porous and hollow AP particles thus had a positive effecton the burning rate.

Keywords: Ammonium Perchlorate, Fine Particle, HollowParticle, Porous Particle, Burning Rate, Bubble Contamination

1 Introduction

A propellant with a high burning rate is required tomanufacture high-performance rocket motors. A compositepropellant that uses fine AP has a high burning rate.However, it is difficult to make fine AP by grinding sinceAP combusts and explodes easily under friction and impact.Amethod for preparing fine AP safely was developed [1 –4].In contrast, Klager [5] and Hagihara [6, 7] prepared coarseporous AP using the thermal decomposition method andreported that porousAP is an effective oxidizer for preparinga composite propellant with a high burning rate, even if it is acoarse particle. These findings suggest that fine porous APwould be amore effective oxidizer than coarse porousAP forobtaining a propellant with a high burning rate.It is very difficult to prepare fine porous AP by the

thermal decomposition method. Some fine porous AP and

fine hollow AP were recently prepared by the spray-dryingmethod [1 – 3]. The weight mean diameters of those APsamples were 3.6 mm to 9 mm. The burning rate character-istics of propellants prepared with fine porous AP or finehollow AP were measured, and these AP particles werefound to be an effective oxidizer in preparing a compositepropellant with a high burning rate [8, 9]. Bimodal AP, amixture of coarse AP and fine porous APor fine hollowAP,was used for the oxidizer in these previous studies [8, 9].However, the burning rate characteristics of propellantsprepared with fine porous AP or fine hollow AP have notbeen sufficiently clarified.Twenty AP samples, including eight fine porous and

hollow AP samples, were used in this study as the oxidizer.The burning characteristics of the propellant prepared withunimodal AP were investigated to reveal the burningcharacteristics of the propellant prepared with fine porousAP or fine hollow AP.

2 Experimental

2.1 Samples

2.1.1 AP Samples

The 20AP samples used as oxidizer in this study are listedin Table 1. Samples A through K were prepared by thespray-drying method [1 – 3]. Samples L through P wereprepared by grinding a commercial AP with a vibration ballmill. The grinding times were 5, 10, 20, 30, and 40 minutes.Samples Q through Twere prepared by screening sample Pwith a sonic sifter. Sample Q was passed through a 38 mmsieve, R was caught between 10 mm and 16 mm sieves, S waspassed through a 10 mm sieve, and Twas passed through a5 mm sieve.Scanning electron microscope (SEM) photographs of

samples A, G, I, and L are presented in Figure 1. The weightmean diameter,Dw, and theAP shapeswere examined usingthe SEM photographs. The specific surface area, Sw, was* Corresponding author; e-mail: [email protected]

50 Propellants, Explosives, Pyrotechnics 31, No. 1 (2006)

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measuredby theBETmethod. Theparticle properties of theAP samples are listed in Table 1. Samples A through F wereporous [1, 2] and samples G andHwere hollow [3]. SamplesI through Twere spherical. The view of sample A indicatesthat porousAPhas some small holes at the crust and that thevoid in the porous particle connects to the outside. SampleGlooked spherical in the SEM photograph. However, sampleG had voids in the particles, indicating a hollow sample [3].TheDw and Swwere in the ranges of 2.9 mm to 130 mmand 60m2/kg to 3100 m2/kg, respectively.Differential thermal analysis (DTA), thermogravimetry

(TG), and X-ray diffractometry (XRD) were used in thepreliminary experiments. The DTA–TG thermograms andtheXRDpatterns of theAP inTable 1were almost the sameand closely coincided with the typical DTA–TG thermo-

gramandXRDpattern ofAP.These results indicate that thethermochemical behavior and the crystallographic proper-ties of the AP used in this study were almost the same.

2.1.2 Propellant Samples

The upper limit of theAP content was incorporated in thepropellant due to the requirements for the preparation ofthe AP/HTPB composite propellant [10, 11]. The upperlimit of the AP content incorporated in the propellantprepared with the AP sample in Table 1 was examinedaccording to Refs. [10] and [11]. The upper limit of the APcontent for samplesGandHwas 72wt%,which is the lowestvalue in this study. This indicates that a propellant at 72wt%APcanbepreparedwith all theAP samples listed inTable 1.The AP content was 72 wt% in this study. Three batches ofpropellants were prepared with the same AP sample andtheir burning rate characteristics were measured. Theprepared propellants were designated using the AP samplesymbols. For example, the propellant prepared with sampleAwas designated as propellant A.

2.2 Measurement of Propellant Density

The propellant strands were prepared with the APsamples listed in Table 1. Fourteen propellant strands wereprepared for one batch. Three batches of propellants wereprepared with the same AP sample, as described in Section2.1.2. The densities of all propellant strands were calculatedfrom the volume and weight. The weight of the strand wasdetermined with an electric balance with a minimumreading of 0.01 g. The size of the strand was measured withvernier calipers, for which the minimum scale was 0.05 mm.

2.3 Measurement of the Burning Rate

Each strand was 10 mm� 10 mm in cross section and40 mm in length. The side of each strand was coated bysilicon resin. The burning rate was measured in a chimney-type strand burner that was pressurized with nitrogen at288� 1.5 K. The pressure ranged from 0.5 MPa to 7 MPa.The strand sample was ignited by an electrically heatednichrome wire attached on the top of each strand sample.Two fuse wires were threaded through the strand sample25 mm apart. The fuse wire was cut as soon as the burningsurface passed through the fuse wire. The burning rate wascalculated using the cutoff period of two fuses.

3 Results and Discussion

3.1 Propellant Density

The propellant density was measured to estimate theamount of bubble contamination in the propellant. The

Table 1. Particle properties of AP sample.

Symbol Shape Dw

(mm)Sw(m2/kg)

A Porous 8.1 2100B Porous 4.7 1600C Porous 5.5 1900D Porous 4.5 2100E Porous 8.5 1500F Porous 9.0 900G Hollow 3.6 3100H Hollow 4.0 2600I Spherical 3.1 2000J Spherical 3.9 1800K Spherical 3.8 1700L Spherical 130 60M Spherical 90 80N Spherical 80 150O Spherical 70 190P Spherical 60 210Q Spherical 28 380R Spherical 15.3 500S Spherical 8.1 900T Spherical 2.9 1900

Figure 1. SEM photographs of sample A, G, I, and L.

Burning Characteristics of AP/HTPB Composite Propellants Prepared with Fine Porous or Fine Hollow AP 51

G 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim www.pep.wiley-vch.de Prop., Explos., Pyrotech. 31, No. 1, 50 – 55

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average density is tabulated in Table 2. The density meas-urements were reproducible to within 0.01 g/cm3. Thepropellant in this experiment was prepared at 72 wt% AP.TheAPandHTPBdensities were 1.95 g/cm3 and 0.93 g/cm3,respectively. The theoretical density of the propellant at 72wt% AP was 1.49 g/cm3. The densities of propellants AthroughHwere below the theoretical value. The densities ofpropellants I through T agreed with the theoretical value.The proportion of bubble contamination in the propellantwas estimated based on the propellant density. Table 2 alsoprovides the void fractions of the propellants. The voidfraction of propellants I through Twas zero. This indicatesthat these propellants did not have bubble contamination.The void fractions of propellants A through H ranged from6 vol% to 14 vol%and that for propellantGwas the greatestin this study.Bubble contaminationwas found in propellantsA through H, the propellants prepared with porous andhollow AP.Porous AP has some small holes at the crust, and the void

in the particle connects to the outside, as described inSection 2.1.1. Hollow AP has a void in the particle. Thebubble contamination in propellant preparedwith porous orhollow AP would be inside the AP particles since the voidsin porous or hollow AP cannot be completely charged withHTPB.

3.2 Burning Rate Characteristics

3.2.1 Burning Rate

Figure 2 presents the burning rate characteristics ofpropellants A, G, I, and L. A reproducible burning ratecould be obtained, and an approximately linear relationshipexisted between log(pressure) and log(burning rate) in thispressure range.The burning rate is influenced by bubble contamination

when the proportion of bubble contamination in a compo-site propellant contained at 80 wt% AP exceeds 2 vol%,and, consequently, a reproducible burning rate cannot beobtained [12]. The combustion of a composite propellant at80 wt% AP in which the proportion of bubble contami-nation is 10 vol% can be kept at a steady state when thediameter of the bubble in the propellant ranges from 10 mm

to 350 mm and the bubbles do not contact each other [12].Propellants A through H had reproducible burning rates, asindicated in Figure 2, even though the void fractions wereabove 2 vol% or 10 vol%. The bubble contamination inpropellantsA throughHwould be inside theAPparticles, asstated in Section 3.1. This suggests that the void was verysmall, less than the size of the AP particle, and numeroussmall voids would distribute independently. The burningrate decreases as the AP content decreases. TheAP contentof the propellant was 72 wt% in this study, which is less thanthe 80 wt%. Therefore, combustion of the propellantsprepared with porous and hollow AP can evidently bemaintained at a steady state even if the propellants havebubble contamination above 2 vol% or 10 vol%.

3.2.2 Relationship between the Burning Rate and Dw

The burning rate for a propellant at 83 wt% AP isdependent on Dw at a constant pressure [13]. Figure 3depicts the relationship between theburning ratesmeasuredat 1 MPa and 7 MPa andDw. This relationship is divided intotwo regions. The burning rate above 19 mm was almostconstant at each pressure, and the burning rates at 1 MPaand 7 MPa were 2.1 mm/s and 5.0 mm/s, respectively. Theburning rate below 19 mm increased with decreasingDw; therelationships can be approximated with the equationsbelow. These approximations are obtained using the least-squares method.

r1 ¼ 7:10D�0:41w at 1 MPa ð1Þ

r7 ¼ 16:23D�0:40w at 7 MPa ð2Þ

Here, r1 and r7 are the burning rates at 1 MPa and 7 MPa,respectively. The equations are specified by solid lines inFigure 3.

Table 2. Density and void fraction of propellant.

Propellant Density(g/cm3)

Void fraction(vol%)

A 1.34 11B 1.41 6C 1.41 6D 1.39 7E 1.39 7F 1.40 6G 1.31 14H 1.32 13I –T 1.49 0

Figure 2. Burning rate characteristics of propellants A, G, I, andL.

52 Makoto Kohga

Prop., Explos., Pyrotech. 31, No. 1, 50 – 55 www.pep.wiley-vch.de G 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim


The plot of the burning rate against Dw indicates widescattering below 19 mm. The relationship between theburning rate and Dw can be described by a smooth curvedline on a log-log graph for the propellant prepared withspherical AP at 83 wt% AP [13]. The burning rate of thepropellant prepared with spherical AP would most likelyalso be dependent on Dw at 72 wt%, similar to that for thepropellant at 83 wt% AP. The relationships for thepropellant prepared with spherical AP alone at less than19 mm can be approximated using the following equations.

r0

1 ¼ 4:56D�0:26w at 1 MPa ð3Þ

r0

7 ¼ 8:87D�0:19w at 7 MPa ð4Þ

These equations are designated by broken lines in Fig. 3.Equations (3) and (4) represent the relationships of thepropellant prepared with spherical AP. This indicates thatthe burning rate of the propellant prepared with sphericalAP is also dependent on Dw at 72 wt% AP. The criticaldiameter for the spherical AP sample was also 19 mm,similar to for all AP samples, including porous and hollowAP. This result suggests that the critical factor depends onthe size of the AP and not the void in the AP particle.The burning rates of the propellants preparedwith porous

andhollowAPare above the broken lines. Theburning ratesof propellants prepared with porous and hollow APexceeded that of the propellant prepared with sphericalAP at a constant Dw.The flame of AP-based composite propellant is called a

diffusion flame, and the multiple-flame model [14] isgenerally accepted as the flame structuremodel. Thismodelhas the flame structure of the AP-based composite propel-lant consisting of theAPmonopropellant 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 surface

but rather to extend out from the surface. The primary flameis a premixed flame with the oxidizer and binder decom-position productsmixing completely before reaction occurs.The final diffusion flame follows the primary flame. Thereare the fast gas flows of AP decomposition products, HTPBdecomposition products, and flame combustion products onthe burning surface.The relationship between the burning rate and Dw is

divided into two regions and the critical point ofDw is 19 mm,as indicated in Figure 3. It is conceivable that fine APparticles at the burning surface would tremble as a result offast gas flows. Therefore, the flame structure of thepropellant prepared with fine AP would become similar toa premixed flame. In contrast, coarse AP particles would befixed at the burning surface since coarseAP is so big that theparticles would be shaken only minimally by the gasesgenerated on the burning surface. The flame structure of thepropellant prepared with coarse AP would be a sufficientlydiffused flame. Therefore, the critical point is presumablydetermined by the difference in flame structure.The binder content in the propellant decreases as the AP

content increases. Consequently, the trembling of APparticles at the burning surface would weaken and thecritical diameter would become unclear. The relationshipbetween theburning rate andDw at a greaterAP content canbe described by a smooth curved line, as stated in Ref. [13].

3.2.3 Relationship between the Burning Rate and Sw

The Sw greatly affects the burning rate because the APparticles decompose at the surface. Figure 4 depicts therelationship betweenburning ratesmeasured at 1MPaand7MPa and Sw. This relationship is divided into two regions,similar to that forDw. The burning rate was almost constantat each pressure below 410 m2/kg. Above that, the burningrate increased with increasing Sw. The solid line and brokenline were approximated for the relationship for all APsamples and for spherical AP alone, respectively. Thebroken line represents the relationship of the propellant

Figure 3. Relationship between burning rate and Dw of thepropellant at 72 wt% AP.

Figure 4. Relationship between burning rate and Sw of thepropellant at 72 wt% AP.




prepared with spherical AP. The relationship between theburning rate and Sw for all propellants above 410 m

2/kg canapproximated using the following equations.

r1¼ 0.17 Sw0.42 at 1 MPa (5)

r7¼ 0.42 Sw0.41 at 7 MPa (6)

The relationships for the propellant prepared withspherical AP alone are approximated using the equationsbelow.

r1’¼ 0.34 Sw0.30 at 1 MPa (7)

r7’¼ 1.28 Sw0.23 at 7 MPa (8)

Theburning rate of thepropellant preparedwith sphericalAPdepends greatly onSw. Theburning rate of the propellantprepared with porous or hollow AP is greater than that ofthe propellant prepared with spherical AP at a constant Sw.The critical point for the spherical AP sample is the same asthat for all AP samples, including those with porous andhollow AP. These results agree with those for Dw, asdescribed in Section 3.2.2.

3.3 Influence of the Void Fraction on the Burning Rate

The burning rates of propellants prepared with porousand hollow AP were greater than that of the propellantprepared with spherical AP at a constantDw or Sw, as statedin Sections 3.2.2 and 3.2.3. The burning rate increases withan increasing temperature gradient in the vicinity of theburning surface. Themodel ofmultiple flames [14] indicatesthat each flame moves closer to the burning surface as theregression rate of AP particles is greater than that of HTPBat the burning surface, and, consequently, the temperaturegradient in the vicinity of the burning surface increases. Theflame propagation velocity in the voids inside porous andhollowAP particles would be higher than that of a non-voidAP particle [8]. This indicates that the regression rates ofporous and hollow AP would be greater than that ofspherical AP at a constantDw. Therefore, the flames wouldmove close to the burning surface, and the burning ratewould increase for the propellant prepared with porous or

hollow AP. On the other hand, Sw would greatly affect theburning rate since the AP particles decompose at thesurface. The burning rate of the propellant prepared withspherical AP is dependent on Sw, but that of the propellantprepared with porous or hollowAP is not associated with Swalone.These results signify that the voids in porous or hollowAP particles effectively increase the burning rate.Table 1 indicates that the smallest value ofDw (2.9 mm) for

the spherical AP sample was almost the same as that(3.6 mm) for porous or hollow AP, and the greatest value ofSw (1900 m2/kg) for the spherical AP sample was much lessthan that (3100 m2/kg) for porous or hollowAP. The burningrate of the propellant prepared with spherical AP with3100 m2/kg could not be estimated by Eqs. (7) and (8). Theinfluence of the void fraction on the burning rate wasinvestigated on the basis of the relationship between theburning rate and Dw, as illustrated in Figure 3.Theburning rate of thepropellant preparedwith spherical

AP was estimated by substituting Dw of porous and hollowAP into Eqs. (3) and (4). Table 3 presents the estimatedburning rate andmeasured values. The ratio of themeasuredburning rate to the values calculated byEqs. (3) and (4),R, isprovided in Table 3. TheR1 andR7 represent the values ofRat 1 MPa and 7 MPa, respectively. The range of R is 1.04 to1.88; R7 of propellant G was the greatest value in this study.

Table 3. Burning rates and R.

Propellant Measured burning rate (mm/s) Estimated burning rate (mm/s) R1 R7

1 MPa 7 MPa 1 MPa 7 MPa (�) (�)

A 3.8 10.1 2.6 6.0 1.44 1.69B 3.2 7.1 3.0 6.6 1.05 1.07C 3.6 7.6 2.9 6.4 1.23 1.18D 3.2 7.2 3.1 6.7 1.04 1.08E 3.5 7.8 2.6 5.9 1.34 1.32F 3.0 6.4 2.6 5.8 1.17 1.10G 4.6 13.1 3.3 7.0 1.41 1.88H 5.1 12.0 3.2 6.8 1.60 1.76

Figure 5. Influence of void fraction on R.

54 Makoto Kohga

Prop., Explos., Pyrotech. 31, No. 1, 50 – 55 www.pep.wiley-vch.de G 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim


Figure 5 illustrates the influence of the void fraction on R.The R7 was almost the same as R1 for each propellant in therangeof 6 to 7 vol%void fraction.TheR7was greater thanR1

above 11 vol%, and it increased with an increasing voidfraction. However, the definitive relationship between Rand the void fraction could not be obtained in this studybecause the voids in porousAP and hollowAP have variousshapes and sizes. More quantitative analyses into theinfluence of the voids in AP particles on the burning ratecharacteristics are necessary.

4 Conclusions

TwentyAP samples, including eightwith porous or hollowAP, were used as an oxidizer. The burning characteristics ofpropellants prepared with fine porous or hollow AP wereinvestigated in this study. TheAP content of the propellantswas 72 wt%.There was bubble contamination in the propellants

prepared with porous or hollow AP. The bubble contami-nation in the propellant exists in these AP particles becausethe voids in the particles could not be completely chargedwith HTPB.The relationship between the burning rate and the weight

mean diameter,Dw, is divided into two regions. The burningrate increases with decreasingDw below 19 mmand is almostconstant above that. The relationship between the burningrate and the specific surface area, Sw, agreedwith that forDw.The critical point of Sw was 410 m

2/kg. These critical pointswere not dependent on the voids in the AP particles. Thisrelationship is influenced by a difference in flame structure.Theburning rate of thepropellant preparedwith spherical

AP was dependent on Dw and Sw. The burning rates of thepropellants prepared with porous or hollow AP were notassociated only withDw or Sw and were greater than that ofthe propellant prepared with spherical AP at a constantDw

or Sw. The voids in porous and hollow AP particles werefound to have a positive effect on the burning rate.

5 References

[1] M. Kohga, Y. Hagihara, The Preparation of Fine PorousAmmonium Perchlorate by the Spray-drying Method, J. Soc.Powder Technol., Japan, 1996, 33, 273.

[2] M. Kohga, Y. Hagihara, The Preparation of Fine PorousAmmonium Perchlorate by the Spray-drying Method-TheEffect of Organic Solvents on the Particle Shape and Size, J.Soc. Powder Technol., Japan, 1997, 34, 437.

[3] M. Kohga, Y. Hagihara, The Spray-drying of AmmoniumPerchlorate by Ultrasonic Commination, J. Soc. PowderTechnol., Japan, 1997, 34, 522.

[4] M. Kohga, Y. Hagihara, Preparation of Fine AmmoniumPerchlorate by Freeze-Drying, Kagaku Kougaku Ronbunshu,1997, 23, 163.

[5] K. Klager, R. K. Manfred, E. L. Lista, Burning Behavior ofPorous Ammonium Perchlorate, 10th Int. Annual Conferenceof ICT, Karlsruhe, Germany, June 27 – 29, 1979, pp. 283.

[6] Y. Hagihara, T. Ito, Studies on Porous Ammonium Perchlo-rate (1), J. Ind. Expl. Soc. Japan, 1986, 47, 23.

[7] Y. Hagihara, M. Banki, Effects of Porous AmmoniumPerchlorate on Burning Rate and Slurry Viscosity of AP/HTPB Composite Propellant, J. Ind. Expl. Soc. Japan, 1990,51, 65.

[8] M. Kohga, Y. Hagihara, Burning Behavior of CompositePropellant Containing Fine Porous Ammonium Perchlorate,Propellants, Explos., Pyrotech. 1998, 23, 182.

[9] M. Kohga, Y. Hagihara, AP/HTPB Composite PropellantUsing Fine Hollow AP Prepared by Spray-drying Method,Sci. and Tech. Energetic Materials, Japan, 2003, 64, 75.

[10] 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.

[11] M. Kohga, Y. Hagihara, Estimation of Upper Limit of APContent in AP/HTPB Composite Propellant –A considera-tion based on Flow Characteristics of AP/HTPB, J. Ind. Expl.Soc., Japan, 2000, 61, 157.

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

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

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

(Received December 13, 2004; revised version August 3,2005; Ms 2004/047)




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