2004_3821[1]

8/10/2019 2004_3821[1]

1/12

1

American Institute of Aeronautics and Astronautics

Characterization of Nano-Sized Energetic Particle

Enhancement of Solid-Fuel Burning Rates in an X-Ray

Transparent Hybrid Rocket Engine

Brian Evans*, Nicholas A. Favorito, Eric Boyer, Grant A. Risha, Robert B. Wehrman, and Kenneth K. Kuo**

The Pennsylvania State University

University Park, PA 16802

Conventional polymeric fuels for hybrid rocket propulsion have relatively low regression

rates. Two possible solutions were examined: utilization of energetic nano-sized particles,

and adoption of non-polymeric paraffin fuel. Addition of 13 wt.% of nano-sized tungsten

powder to HTPB-based fuel resulted in an increase of 38% in fuel regression rate compared

to the pure HTPB fuel. The use of nano-sized tungsten powders in solid fuels for volume

limited propulsion systems is greatly beneficial due to its high density, high heat of oxidation,

and low oxidation temperature. SEM/EDS micrographs of the newly processed energetic

paraffin-based solid fuels have shown that the nano-sized Silberline

aluminum flakes arehomogenously mixed in the fuel matrix. Paraffin-based solid fuels containing aluminum

flakes showed a significant increase in regression rate over the non-aluminized paraffin fuel.

A real-time X-ray radiography system enables the measurement of the instantaneous radius

of the solid fuel grain. The radial increment of the regressing fuel surface can be correlated

with time in a power-law form. An implicit relationship showing the dependency of

instantaneous fuel regression rate on the total mass flux was obtained. The functional

relationships for aluminized HTPB and paraffin fuels were obtained in graphical forms.

Results show that the conventional power-law relationship between the average regression

rate and average oxidizer mass flux cannot be applied to the instantaneous regression rates

of solid fuel burning in hybrid rocket motor conditions.

I. IntroductionHybrid rocket motors (employing a solid fuel and a gaseous, liquid, or gel oxidizer) posses many advantages

over conventional solid-propellant rocket motors including on/off capability, thrust tailoring operability, minimal

environmental impact, and also an inherent safety.1,2,3 With the ever growing concern of safety and reliability in thespace-propulsion industry, the hybrid rocket can fill a void due to the inherent safety stemming from the physical

separation of the fuel and oxidizer and the ability to control the combustion event by regulating the oxidizer flow

rate as a function of time. In hybrid rocket combustors, the regression rate of solid polymeric fuels is dictated by the

total mass flux in the port region. This mass flux is the local sum of the burned products and the unconsumed

oxidizer in the flow stream. Since the oxidizer and fuel-pyrolysis products are not initially premixed in the hybridrocket engine, the rate of the combustion is diffusion-limited. In other words, the combustion processes of the

reactants in hybrid rockets are dominated by the mixing and reaction rates of the pyrolyzed fuel species with the

turbulent oxidizer stream flowing through either the center port or multiple ports of the solid-fuel grain.4,56,7For thecase of paraffin fuels (e.g., C32H66), the combustion process is still diffusion limited; however, the fuel droplets are

shed from the surface waves of the molten fuel layer generated during the combustion process. The mixing of thefuel components and the oxidizer involves the evaporation of fuel droplets and oxidizer gas in the two-phase flow

*M.S. Student, AIAA Student Member.M.S. StudentPh.D. Candidate, AIAA Student MemberPh.D. Research Associate, AIAA Member

**Distinguished Professor of Mechanical Engineering, AIAA Fellow

40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit1-14 July 2004, Fort Lauderdale, Florida

AIAA 2004-382

Copyright 2004 by Brian Evans , Nicholas A. Favorito , Eric Boyer , Grant A. Risha , Robert B. Wehrman, and Kenneth K. Kuo. Published by the American Institute of Aeronautic

8/10/2019 2004_3821[1]

2/12

2


field. In addition, the mixing of the oxidizer species with those generated from surface pyrolysis/evaporation of the

molten fuel also controls the combustion rate.

Due to the relatively low mass-burning rate of polymer-based solid fuels, such as hydroxyl-terminated

polybutadiene (HTPB) and hydroxyl-terminated polyether (HTPE) compared to solid propellants, a large fuel

burning surface area is required to attain a given thrust level. The utilization of various energetic nano-sized powderadditives in the fuel formulation is one way to alleviate this problem by increasing the fuel regression rate. Previous

studies conducted by Risha, et al.8,9,10,11have evaluated 19 different HTPB-based fuel formulations using a Long-

Grain Center-Perforated (LGCP) hybrid rocket motor. In addition to the 19 fuel formulations previously tested,nano-sized tungsten powders and nano-sized rod shaped aluminum have been acquired and tested as performance

enhancing additives in HTPB-based solid fuels.

Another means of increasing the mass-burning rate of solid fuels is by the use of paraffin wax-based solid fuels,

which have shown to increase regression rates to 3 times that of traditional polymeric fuels such as HTPB. 12Thecombination of paraffin-based solid fuels and nano-sized energetic particles drastically increases the regression rates

of solid fuel formulations. The large increase in fuel regression rate that is attained helps to alleviate the need for

complex port geometries required with current polymer-based fuel formulations.

A. Objective of Research

Existing solid propellant missiles have relatively low specific impulses, but high density impulses. In addition,the burning rates of the current solid propellant grains in the existing systems are limited due to the less efficient

burning-rate modifiers that have been utilized in the conventional solid propellants. Several other undesirable

features of the solid-propellant rocket systems are: 1) not being capable of thrust modulation; 2) the propellant graincannot be extinguished once the grain is ignited; 3) their susceptibility to mechanical damage and subsequent flame

spreading into the damaged regions and potential catastrophic motor failure; and 4) solid fuel mechanical propertiesare sacrificed because of relatively low loading densities. As mentioned, the major disadvantage of the existing

solid-fuel grain formulations is the relatively low mass-burning rate of the fuel, requiring a relatively large fuel

surface area for a desired thrust level. Low burning rates of solid fuels have been shown to be enhanced by theaddition of energetic metal powders. In addition the use of non-polymeric fuels such as paraffin wax increase the

burning rate of solid fuels by many hundred percent.

The objective of this research is to find means of further enhancement of the mass-burning rate of solid fuel

formulations for use in hybrid rocket motor propulsion systems. To accomplish this goal there have been two mainmethods of fuel burning rate enhancement:

1) Nano-sized particle addition for increasing the heat that is released near the surface of the fuel. This

will increase the linear regression rate of the fuel and thus the mass flow-rate of propellant.

2) Paraffin-based solid fuel formulations have an inherently high mass-burning rate due to thehydrodynamically unstable liquid layer that is formed on the surface of the fuel during combustion.13,14

Evaluation of various fuel formulations with either and/or both of these mass-burning rate enhancement

techniques has led to the development of novel solid fuel formulations with greatly enhance mass-burning rates.With the X-ray Translucent Casing (XTC) hybrid rocket motor the surface of the fuel is imaged throughout the

duration of the combustion test. This allows for the instantaneous regression rate to be determined as a function of

time. The regression rate can than be correlated to the instantaneous oxidizer mass flux.

II. Method of Approach

B. Hybrid Rocket Motor Casting Procedures1. HTPB-Based Solid Fuel Processing

The XTC hybrid rocket motor was used to burn two baseline solid fuel formulations, one of which is the cured

HTPB. The HTPB was formulated using R45-M resin with either Isonate 143L

methylene diphenyl isocyanate(MDI) or isophorone diisocyanate (IPDI) curing agent with T-12catalyst. It is advantageous to use IPDI curingagent in casting fuel grains that contained particles with a high specific surface area, such as the nano-sized

tungsten, because of the ability to vary the working time of the solid fuel. The casting process for solid-fuel

formulations for the XTC motor grain is similar to the LGCP motor.15The solid fuel was mixed following the same

procedure for introducing particles into the resin and degassing. Also the grains were cast utilizing a previouslydeveloped novel low-melting point wax mandrel process, in order to create a center-perforated grain. Grains were

cast in paper phenolic tubes that have an outer diameter of 114.3 mm (4.5 inches) and an inner diameter of 63.5 mm

(2.5 inches) in lengths up to 457.2 mm (18 inches). Typical test durations for the XTC ranged from 5 to 7 seconds

depending on the fuel formulation and the injected oxidizer mass flux.

8/10/2019 2004_3821[1]

3/12

3


2. Paraffin-Based Solid Fuel Casting

The casting procedure for paraffin-based solid fuels differs from that of HTPB-based solid fuels significantly. To

cast paraffin-based fuels a centrifugal casting procedure is used. This will cause the wax to be pressed outward on to

the paper phenolic grain tube as it cools. This is necessary because of the nature of paraffin wax to shrink as it cools

which results in gap formation between the inner surface of the phenolic tube and the outer surface of the fuel. Toprevent in-depth radiation absorption, small percentages of opaque particles were added to the solid fuel. These

particles can either be carbon black or aluminum powders. In addition to this radiation absorption blockage effect,

nano-sized aluminum has been shown by the authors to increase the burning rates of solid fuels by significantamounts.

Two slightly different procedures were implemented for casting solid-fuel grains for the LGCP and the XTC

hybrid rocket motors. To achieve the smooth bore in the center port of the LGCP fuel grain, the casting is completed

in two steps to overcome the significant shrinking effect. First, the tube was only filled partly with the liquid paraffinand then spun on a lathe until the grain cooled (~1 hour) to create a large center port. Then, this partially loaded

grain was again filled with liquid paraffin and spun until it was cooled. The final internal surface was machined to a

very smooth port of desired dimension.

Casting of paraffin-based (C32H66) solid-fuel grains for the XTC hybrid rocket motor varies slightly from thecasting procedure for the LGCP motor. A horizontal casting procedure like the one described above has been used

for casting XTC grains. For the XTC motor with much larger diameter fuel grains, the important factor in

controlling the fuel quality was found to be the restriction of the particle movement during the cooling/solidificationprocess. A multiple step casting and solidification process was used for both the baseline paraffin fuel formulation

(paraffin wax with 3% carbon black by weight) and fuel grains containing significant percentages of nano-sizedaluminum flakes. The speed of the lathe was selected to limit the centripetal forces applied to the particles, while

still maintaining good adhesion between the interfaces. In order to help with the adhesion of the fuel to the phenolic

grain tube or to previously cast fuel layers two hydrocarbon-based additives have been acquired whose physicalproperties aid in this. Both materials are completely soluble in paraffin wax and have good compatibility

characteristics with paraffin. One of these additives also is very strong and will impart its strength onto the solid fuel

through the physical structure of the material.

Energy Dispersion X-ray Spectroscopy (EDS) was utilized to check the homogeneity of the cast fuel samples.EDS enables various elements to be detected and gives a qualitative analysis of the homogeneity of the fuel. To

check for particle migration, samples were taken at the inner and outer surface of a fuel layer. Figures 1 and 2 show

the sample characterization micrographs for the inner and outer regions of a cast fuel, respectively. Figure 1a is ascanning electron microscope (SEM) of the sample taken from the inner radius region of the paraffin solid fuel

containing 13% Silberline aluminum flakes by weight. Figures 1b and 1c show the distribution of carbon and

aluminum atoms in the fuel matrix, respectively. The size of these three micrographs are identical with 250 m inboth vertical and horizontal directions (at 500x magnification). These EDS micrographs indicate the highly uniform

nature of the dispersion of the aluminum particles. Figure 2 shows the SEM and EDS micrographs of the fuel

sample taken from the outer radius region of the fuel grain. Again the uniformity of the aluminum particle

distribution is verified through the EDS micrographs. The similarity between Figures 2c and 1c indicates the radialdistribution of the aluminum concentration is very uniform. The thickness of the Silberlinenano-sized aluminum

flakes is approximately 100 nm. However the width and height of the flake are approximately 27 and 10 m,respectively. These flakes are relatively large particles in the width and height dimensions. Using the presently

developed method for fuel cooling/solidification, the distribution of aluminum particles is already very uniform.Applying the same method to solid fuels containing smaller aluminum particles such as Alex one can be sure that

the particle distribution will be very uniform.

8/10/2019 2004_3821[1]

4/12

4


C. Hybrid Rocket Motor

Experimental Setup

A variable-throat

venturi with upstream

pressure and temperaturemeasurements was

employed in the gaseous

oxygen feed system inorder to attain the desired

constant mass flow rates.

From previous experiments

using the LGCP motor, therange of oxidizer mass flux

was determined and scaled

accordingly so that the

larger scale XTC motor(see Fig. 3) had similar

initial operating conditions.

Flow rates of up to 0.42kg/s can be obtained

through the use of thisventuri, which is sufficient

for the desired oxidizer

mass fluxes. Oxidizer mass fluxes ranging from60-290 kg/m2-s have been tested in the XTC

hybrid rocket motor to obtain a range of data for a

given fuel formulation.

The dominating parameter that dictates themass-burning rate in a hybrid rocket motor is the

oxidizer mass flux. Since there is no discernable

pressure effect, the steady-state chamber pressureof the XTC can be similar to or much greater then

that of the LGCP while still maintaining

approximately the same linear regression rate.Because of this, the chamber pressure has beenselected to range from approximately 2.17 to 4.24

MPa (300 to 600 psig), which is similar to the

range used for the LGCP and will allow for

comparison in performance.

D. Fuel Formulation Consideration and Thermochemical Calculations of Paraffin-Based Fuels with

Silberline and Nano-Sized Tungsten Particles

For examining the burning rate enhancement by a new type of particle, the smaller quantity of nano-sized

particles required for a series of evaluation test firings in the LGCP hybrid rocket motor is highly beneficial. For this

reason, 20 types of nano-sized particles have been evaluated using the LGCP motor. 18 fuel formulations were

evaluated previously and a complete list of the particle type, percent by weight added to the solid fuel, andperformance enhancement can be found in References 8 and 9. In addition, two new types of nano-sized tungsten

particles have been evaluated as well: one is a neat tungsten particle with no oxide coating and the other with a 5%oxide coating by weight. A comparison of the effectiveness of these two types of particles and their burning

characteristics is included below. These particles were obtained from Nano-Mat, Inc. for evaluation using the LGCP

hybrid rocket motor. The solid-fuel regression results of the HTPB-based fuels with these two new particles can be

compared to our previous data to determine the benefit on combustion performance using tungsten particles.To further enhance the burning rate of new fuel formulations, the use of a non-polymeric fuel formulation has

also been implemented. Paraffin-based solid fuels have been shown to increase the burning rate several times over

that of traditional polymeric fuels such as HTPB. Testing and analysis of paraffin-based fuel formulations have been

Figure 1. Micrographs of unburned inner surface of paraffin fuel layer a)SEM

micrograph, b) carbon elemental map, c) aluminum elemental map. All

micrographs are 250 mm in height and width and are at 500x magnification

Figure 2. Micrographs of unburned outer surface of paraffin fuel layer a)SEM

micrograph, b) carbon elemental map, c) aluminum elemental map. Allmicrographs are 250 mm in height and width and are at 500x magnification

Figure 3. X-ray imaging setup used with XTC hybrid

rocket motor

a b c

ba c

X-ray

Source

Image

Intensifier

Blast Shield

8/10/2019 2004_3821[1]

5/12

5


considered in both the LGCP and XTC hybrid rocket motors. A wide range of average oxidizer mass fluxes (60-290

kg/m2-s) have been evaluated for comparison to results obtained in the LGCP hybrid rocket motor for particle

addition to HTPB-based solid fuels and to

further extend the range of data.

The addition of Silberline aluminumparticles has shown to increase the linear

regression rate of HTPB-based solid fuel

formulations by approximately 30% overthe baseline pure HTPB fuel. Burning rate

enhancement is also expected for paraffin-

based solid fuel formulations containing

different amounts of aluminum particles.Besides the increase in the regression rate

of the solid fuel, the density impulse of the

metallized fuel is also higher than the

baseline case. Figure 4 shows a comparisonof density impulses for varying percentages

of Silberlinealuminum nano-sized flakes.

All data presented is for liquid oxygen asthe oxidizer and a chamber pressure of

1000 psia. As can be seen the addition ofaluminum to paraffin based fuels can

indeed be highly beneficial in volume

limited systems as the density impulseincreases very significantly (8%) with a

20% aluminum addition by weight.

Figure 5 shows a comparison of density

Isp of pure Paraffin to various weightpercentages of nano-sized tungsten particles.

The number of particles in a 13% by weight

tungsten addition is much lower than that ofa 13% by weight aluminum addition due to

the significantly higher molecular weight of

tungsten. The large increase in density Ispthat is observed, even over comparableweight percentages of Silberline

aluminum, is also very advantageous in

volume-limited systems.

III. Discussion of Results

The energetic powder additive to theHTPB-based fuel that has shown the largest

increase in burning-rate enhancement inprevious series of tests is the Viton-A

coated Alexaluminum nano-sized powders

with an increase of 120% in mass-burning

rate and 123% increase in the average linearregression rate at an oxidizer mass flux of

112 kg/m2-s. A number of other candidates

also exhibited significant performance

increase compared to the baseline solid fuel

HTPB. For these candidates, which includeSilberlinealuminum flakes, Viton-A coated Silberlinealuminum flakes, Technanogies aluminum, an average

increase in linear regression rate of 60-70% was seen.8,9,11The addition of Viton-A has proven to be beneficial in

increasing the average mass burning and linear regression rates.

220

240

260

280

300

320

340

1 1.5 2 2.5 3 3.5 4 4.5 5

PF w/ 20% SILBALPF w/ 17% SILBALPF w/ 13% SILBAL

PF w/ 10% SILBALPF w/ 7% SILBALPF w/ 4% SILBALPure Paraffin Fuel

DensityImpulse[g-s/cm

3]

O/F

Figure 4. Thermochemical calculations of density Isp of varying

percentages of Silberline aluminum nano-particles added to

paraffin

220

240

260

280

300

320

340

1 1.5 2 2.5 3 3.5 4 4.5 5

PF w/ 20% W additionPF w/ 13% W addition

PF w/ 17% W additionPF w/ 10% W additionPF w/ 7% W additionPF w/ 4% W additionPure Paraffin Fuel

Dens

ity

Impu

lse

[g-s

/cm

3]

O/F

Figure 5. Thermochemical calculations of density Isp of varying

percentages of tungsten nano-particles added to paraffin

8/10/2019 2004_3821[1]

6/12

6


E. Solid Fuels Containing Nano-Sized Tungsten Particles

The addition of nano-sized tungsten powder to solid-fuel formulations is beneficial due to the high volumetric

heat of oxidization (88.9kJ/cm3), which is approximately 6% higher than that of aluminum. For a volume limited

system this is extremely beneficial. The combustion of tungsten is also suitable for a rocket motor application due to

the strong heterogeneous reactions that begin at approximately 400-500 C and at higher temperatures, rapidoxidation occurs, based upon information from reference 1616. If not for these strong heterogeneous reactions

occurring at relatively low temperatures, tungsten would be difficult to melt and vaporize due to its high melting

temperature (3,380 C) and extremely high boiling temperature (5,927 C).As mentioned above, nano-sized tungsten powders were cast in an HTPB-based solid fuel formulation using an

IPDI curing agent with T-12 catalyst. To ensure that the IPDI curing agent did not alter the performance of the

baseline HTPB solid fuel formulation, a series of three baseline HTPB solid fuels were cast and tested using the

IPDI and T-12 in similar amounts as the tungsten nano-sized powder containing grains. This pure HTPB fuelformulation is named SF20 and the data is contained in Fig. 6 and shows little variation in burning rate from SF1,

baseline HTPB fuel cast with MDI curing agent.

Comparison of the burning rate enhancement of the two newly developed fuels containing nano-sized tungsten

powders with the data of HTPB-based fuels containing other energetic powders is shown in Fig. 6. Even though theHTPB with 13% tungsten powder showed only moderate increase in regression rate (about 38% at the average

oxidizer mass flux level of Gox~140 kg/m2-s) with respect to the pure HTPB, the 13% of tungsten by weight has

much fewer particles than 13% aluminum particles. At the equivalent amount of tungsten particles having the samevolume as the aluminum particles, one can expect a much higher burning rate enhancement. A fair comparison

should be based upon the equal molar percentage rather than equal weight percentage. The comparison of data inFig. 6 is still based upon the traditional weight

percentage consideration of fuel/propellant

formulation. It is noted that fuels containing thetungsten particles with oxide coating showed a

slightly lower mass-burning rate than those with

the pure tungsten powder. Their slopes are quite

similar, though. This is expected since the activetungsten content is less than that of the neat

particles, resulting in slightly lower energy

release.The number of particles contained in a 13% by

weight tungsten fuel formulation is much less than

that of a 13% by weight aluminum or boronbecause the molecular weight of tungsten is somuch higher. Comparing a 13% by weight

tungsten fuel formulation with an equimolar

concentration solid fuel formulation with

aluminum, the aluminum fuel would only contain~2.14% by weight of aluminum. If an equimolar

concentration of tungsten were to be added to

equal the number of moles contained in a 13% byweight aluminum fuel grain, a 50.4% by weight

tungsten fuel formulation would be needed. When

taking this factor into consideration, it is

anticipated that the solid fuels with 50.4%

tungsten particles by weight should show aconsiderable increase in mass-burning rate, since

equimolar basis is a more suitable means of

comparison. Higher percentage particle addition is

possible based upon test data obtained. Theobserved plume jet did not show particle streaks,

which shows complete particle combustion within the rocket motor chamber. Combustion efficiency data showsvalues from 77-87%, which falls in the upper half of previously reduced data. Since the surface temperature of

HTPB, which is approximately 660 C, is higher than the temperature for onset of oxidation of tungsten the particles

will start to under go heterogeneous combustion as soon as they are exposed to the oxidizing atmosphere of the

Average Oxidizer Mass Flux [kg/m2-s]

60 80 100 120 140 160 180

Average

R

egress

ion

Ra

te[mm

/s]

0.8

1

1.5

2

2.5

3

3.5

Pure Paraffin w/ 2% Carbon Black (SF23)

Pure HTPB (SF1)

13%ALEX (SF2)13%B

4C (SF3)6.5%WARP-1 (SF4)6.5%B

4C+6.5%WARP-1 (SF5)13%Boron (SF6)13%SILBAL (SF7)

13%CLAL (SF8)13%WARP-1 (SF9)

13%Cat-B4C (SF10)

13%TECHAL (SF11)

13%Al325 (SF12)

13%NMATWPure (SF21)

13%NMATW5 (SF22)

Pure HTPB/IPDI (SF20)

5.65%Boron (SF19)

13%C-ALEX (SF13)13%C-Boron (SF14)

13%AVAL (SF15)13%IHD-AR (SF16)13%NTECH80 (SF17)

13%NTECH50 (SF18)

Figure 6. Comparison of average linear regression rate

data for various solid-fuel formulations

8/10/2019 2004_3821[1]

7/12

7


motor combuster. Based upon this, complete combustion is anticipated even at much higher particle loadings

because of the means of ignition and heterogeneous combustion.

F. Paraffin-Based Solid Fuel

To evaluate the burning rate characteristics of paraffin-based solid fuels, both the LGCP and the XTC hybrid

rocket motors have been utilized. Initial baseline paraffin solid fuel testing, containing 2% carbon black by weight,

was conducted in the LGCP motor. The earlier tests of the paraffin fuels in this series exhibited certain amounts ofdelamination of the fuel from the paper phenolic grain tube or between interfaces of consecutive casting layers.

Later on this delamination problem was solved by improvement of processing procedures. Further testing was

completed in the XTC hybrid rocket motor with paraffin-based fuel containing 13 wt. % of Silberline nano-sized

aluminum flakes. The measured chamber pressure and resultant burning rate data is given in Table 1. The resultant

linear regression rate data obtained from LGCP motor tests with the baseline paraffin solid fuel is also included inFigure 6 for direct comparison with the HTPB-based solid fuels containing different types of nano-sized energetic

particles. It is quite obvious that the baseline paraffin solid fuels in the LGCP hybrid rocket motor demonstrated asignificantly higher regression rate than any of the HTPB-based fuel formulations containing nano-sized particles.

For clarity purpose, the data obtained from paraffin-based solid fuels containing Silberline aluminum flakes is not

included here, but is shown later in the text.

The testing of paraffin-based solidfuels in the XTC motor focused on

paraffin-based fuel formulations

containing 13% by wt. Silberline

aluminum particle addition. Of the XTC

rocket motor firings that were conducted,five of the tests produced useful results.

The five aluminized paraffin solid-fuel

tests firings exhibited very high regressionrates in comparison to baseline paraffin

solid fuels. Table 1 shows the average

oxidizer mass fluxes and linear regression

rates measured for these tests. The datapresented in Table 1 are plotted in Fig. 7

against the data obtained for the baseline

paraffin-based solid fuel formulations. The

paraffin solid-fuel data of Karabeyoglu, et.al.12 of the Stanford University are also

plotted on the same figure for comparison

purposes. The Stanford data curve fit has thesame slope as the PSU data for nonaluminized

paraffin fuel; however, the regression rates of

PSUs data are higher due possibly to the

difference in processing techniques and theslight difference in fuel formulations. The

difference in the amount and size of carbon

black powder of these two fuels could cause the

surface energy release amount to be different. It

is evident that there is a noticeable increase in

the linear regression rate of the solid-fuelformulations containing Silberline

nano-sized

aluminum flakes compared to the baseline

paraffin fuel formulations if the data wereextrapolated to a higher mass flux level. By

extrapolating the curve fit obtained for the

baseline paraffin fuel formulation to the Gox,avevalue of 190 kg/m2-s, an increase of ~30% is

seen for the formulations containing 13% by wt.

Silberlineparticles. An increase of about 60%

Table 1. Paraffin-based solid fuel formulation test conditions and

resultant average regression rates

Test No.Gox,ave

[kg/m2-s]

rb

[mm/s]

AverageChamber

Pressure

[MPa]

Average

Thrust[N]

NS-115-PFC2-100 72.4 2.05 3.11 131.1

NS-116-PFC2-100 122.4 3.13 3.54 168.1

NS-117-PFC2-100 108.9 2.79 2.09 162.1

NS-121-PFC2-100 64.9 2.22 3.14 153.0

XTC-17-PF-SILBAL-13 190.0 4.84 2.34 1081.9

XTC-20-PF-SILBAL-13 148.4 4.56 1.69 806.9

XTC-21-PF-SILBAL-13 166.0 4.42 2.21 1041.6

XTC-22-PF-SILBAL-13 290.3 7.16 3.10 1458.6XTC-23-PF-SILBAL-13 217.4 6.58 3.31 1899.8

Average Oxidizer Mass Flux [kg/m2-s]

60 80 100 150 200 250 300125 175Average

LinearRegressionRate[mm/s]

1.5

2

3

4

5

2.5

6

7

8Paraffin Fuel w/ 2% Carbon Black (SF23)

Paraffin Fuel w/ 13% Silberline Aluminum (SF24)

Stanford Paraffin Regression Rate Data12

Figure 7. Comparison of average linear regression rates of

baseline paraffin fuel, paraffin fuel containing 13% by wt.

Silberline aluminum flakes, and Stanford paraffin burn-

rate curve fit

8/10/2019 2004_3821[1]

8/12

8


is seen for the difference between the aluminized paraffin and the Stanford curve fit results. Another observation

that can be made regarding the burning trends of the paraffin-based fuel formulations is the difference in slope of the

curve fits. This difference in slope is caused by the aluminum particles in the paraffin fuel combustion process. Inbaseline paraffin fuel formulations, one of the governing parameters is the oxidizer flow velocity. This dictates the

size of the wave structure that occurs on the fuel surface (see Fig. 8) and also the size of the particles that are

stripped from the surface waves of the molten fuel layer and entrained into the combustion zone.

G. Instantaneous Regression Rate Determination

Analysis of the visual data obtained from four XTC test firings with HTPB-based and paraffin-based solid fuel

formulations containing 13% by wt. Silberline

aluminum were analyzed in order to get a correlation of theinstantaneous fuel port radius near the aft end of the grain as a function of time. This was accomplished by

processing the video images from the real-time X-ray radiography system using a commercially available special

effects package that did not alter the actual fuel surface location. The special effect inverts the image density of theoriginal picture; this inversion greatly enhances the clarity of the fuel grain surface. Images are analyzed at ~0.25

second increments, which was chosen to be small enough to give sufficient resolution for the curve fit and largeenough to show the radial variation of the fuel surface. This time step is altered if an image is difficult to measure a

distinct fuel grain surface location. The deduced instantaneous radius is fit as a power law of time according to:

0( ) ( )n

ignr t r a t t = (1)

Tests XTC-05 and XTC-07 were conducted

with HTPB-based solid fuel grains containing

13% by weight Silberline aluminum flakes.

Figure 9 shows the deduced instantaneous portradii (in terms of change of port radius) versus

time of these two tests. The difference between

the oxidizer flow rates of these two tests isapproximately 13% with Test XTC-05 being

higher. The power-law curve fit parameters aand

nfor Tests XTC-05 and XTC-07 are presented in

Table 2. The exponent of the power lawcorrelation for the higher flow rate test is slightly

greater than that for the lower flow rate test. The

slopes of the two Silberlinealuminum fuel grain

tests expressed by parameter aare quite similar. It

was noted that both curve fits have a high R2

value greater than 0.98 indicating a good match tothe data.

A similar plot for the two aluminized paraffin

fuel tests (Tests XTC-21 and XTC-22) is shown

in Fig. 10 using the same power law curve fit

function. The fitting parameters aand nare also

shown in Table 2. Again it was found that theexponent nis higher for the higher oxidizer flow

rate case. The flow rate of Test XTC-22 is about

25% higher than that for Test XTC-21.

Figure 8. Grain surface of a recovered paraffin-based solid fuel with 13% Silberline aluminum

showing wrinkled pattern (Test No. XTC-PF-10-SILBAL-13)

Table 2. Power-law curve fit parameters for XTC

motor tests with aluminized HTPB and paraffin fuels

Test No. a n

XTC-05-SILBAL-13 1.758 0.9207

XTC-07-SILBAL-13 1.793 0.8772

XTC-21-PF-SILBAL-13 4.640 0.752

XTC-22-PF-SILBAL-13 7.762 0.9708

Time from Ignition [sec]

0 2 4 6

ChangeinPortRadius[mm]

0

2

4

6

8

10

XTC-05 Instantaneous Change in Port Radius


Figure 9. Deduced instantaneous port radii as functions of

time for test firings XTC-05 and XTC-07

O2flow40 mm

8/10/2019 2004_3821[1]

9/12

9


To determine a correlation of the regression

rate with respect to important operating

parameters (such as the instantaneous oxidizer

mass flux, port diameter, and total mass flux), a

data analysis procedure has been developed. Sincethe instantaneous regression rate of the solid fuel

is governed by the local energy transfer rate and

mass diffusion processes occurring in the port ofthe hybrid motor, it should depend strongly upon

the total mass flux at the given station. By

differentiating Eq. (1) with respect to time, we

have

( ) ( )1 1

*n n

b ign

drr an t t an t

dt

= = = (2)

where: ( )* .ignt t t

From this equation it can be seen that if the exponent n is less than unity, the regression rate decreases as timeincreases. The total mass flux at an axial location is the total mass flow rate divided by the instantaneous port area,

hence

( )2

*

totaltotal

n

o

mG

r a t

= +

& (3)

where the total mass flow rate is the sum of the instantaneous oxidizer mass flow rate and the fuel flow rate from

surface pyrolysis, namely

,total ox fuel py ox p b sm m m m r A= + = +& & & &

( ) ( )1

2 * *n n

ox p om L an t r a t = + +

& (4)

Substituting Eq. (4) into Eq. (3) yields,

( ) ( )

( )

1

2

2 * *( *)

*

n n

ox p o

totaln

o

m L an t r a t G t

r a t

+ + = +

&

(5)

Inverting Eq. (2) to express t* in terms of regression rate, we have1

1( *)*

nbr ttan

= (6)

Eliminating *t from Eq. (5), we obtain the following relationship between ( *)br t and ( *)totalG t

2

11

2 ( *)( *)

( *)( *)

p boxtotal nn

nn bboo

L r tmG t

r tr t r ar aanan

= + + +

& (7)

This equation is in an implicit form of regression rate dependency on the total mass flux, oxidizer mass flux, initial

port radius, the axial distance and the power-law parameters. In order to express regression rate in terms of the total

mass flux, one has to make certain order-of-magnitude approximations of exponent n. Even though this expressionis much more complex than the conventional expression of regression rate in terms of power-law of oxidizer mass

flux, it is derived without major assumptions.

Time from Ignition [sec]

0.0 0.5 1.0 1.5 2.0 2.5

ChangeinPort

Radius[mm]

0

2

4

6

8

10



Figure 10. Deduced instantaneous port radii as functions of

time for test firings XTC-21 and XTC-22

8/10/2019 2004_3821[1]

10/12

10


Using Eq. (7), the deduced instantaneous regression rates of the two aluminized HTPB-based fuel test runs

(XTC-05 and XTC-07) are plotted versus the instantaneous total mass flux in Fig. 11. Even though the slopes (a) of

the fuel radius change versus time are nearly identical (see Table 2 and Fig. 9), the functional relationships between

regression rate and total mass flux are quite different as indicated by the difference of curvature and location of these

two sets of data in Fig. 11. We have looked into the possibility for using instantaneous oxidizer mass flux to replacethe total mass flux; however, the results shown in Fig. 12 also indicated the non-collapsing situation of the two

curves. Furthermore, the data do not follow the conventional power-law relationship between the measured

regression rate and the instantaneous oxidizer mass flux when the data are given in a log-log plot.

The aforementioned differences between the

regression rate versus total mass flux curves obtained

under different operating conditions is further

illustrated in Fig. 13, from the results of two

aluminized paraffin-based fuel test runs (XTC-21 and

XTC-22) conducted under different test conditions. Inthis figure the two curves are even further apart

showing the strong dependency of instantaneousregression rate on the instantaneous total mass flux.

IV. Conclusions

(1) Nano-sized tungsten powder addition to HTPB-based fuel formulation in 13% by wt. shows an

increase of 38% in fuel regression rate compared to

the baseline HTPB fuel formulation. Volume limited

propulsion systems would greatly benefit from the use

of nano-sized tungsten powders in solid fuelformulations due to its high density, high heat of

oxidation, and low oxidation temperature viaheterogeneous reactions. The benefit derived in high

density impulses and combustion efficiency makes this energetic powder a viable additive to advanced energeticfuels.

(2) SEM/EDS micrographs of the newly processed energetic paraffin-based solid fuels have shown that the nano-

sized Silberline aluminum flakes are homogenously mixed in the fuel matrix. It was verified that the recentlydeveloped mixing and casting procedures do not cause particle migration in the fuel matrix, producing grains that

have fairly uniform distribution of aluminum particles throughout the fuel.

Instantaneous Oxidizer Mass Flux [kg/m2-s]

100 150 200 250 300 400

Linear

Regress

ion

Ra

te[mm

/s]

1

1.25

1.5

1.75

2

rbvs. Gox,instTest XTC-05

rbvs. Gox,instTest XTC-07

Figure 12. Dependency of the instantaneous

regression rate of aluminized HTPB fuels on

instantaneous oxidizer mass flux

Total Mass Flux [kg/m2-s]

100 200 300 400 500 600

LinearRegressionRate[mm/s]

1.2

1.4

1.6

1.8

2.0

XTC-05

XTC-07


regression rate of aluminized HTPB fuels on

instantaneous total mass flux

Total Mass Flux [kg/m2-s]

200 400 600 800 1000 1200 1400 1600 1800

LinearRegressionRate[m

m/s]

3

4

5

6

7

8

9

XTC-21

XTC-22


regression rate of aluminized paraffin fuels on

instantaneous total mass flux

8/10/2019 2004_3821[1]

11/12

11


(3) Paraffin-based solid fuels containing Silberlinealuminum flakes showed an increase in linear regression rates

of about 30% over the baseline paraffin fuel formulation. An increase of about 60% is seen for the difference

between the aluminized paraffin and the Stanford curve fit results.

(4) The real-time X-ray radiography system enables the measurement of the instantaneous radius of the solid fuel

grain with a cylindrical center port. The radial increment of the regressing fuel surface can be correlated with time ina power-law form. The exponent for time was found to be higher as the oxidizer mass flow rate is increased.

(5) An implicit relationship showing the dependency of instantaneous fuel regression rate on the total mass flux was

obtained. The functional relationships for aluminized HTPB and paraffin fuels were obtained in graphical forms.Results show that the position and the curvature of these curves depend strongly upon the oxidizer mass flow rate

and initial port radius of the fuel grain. This shows that the conventional power-law relationship between the

average regression rate and average oxidizer mass flux cannot be applied to the instantaneous regression rates of

solid fuel burning in hybrid motor conditions.

V. Acknowledgements

The authors would like to acknowledge Dr. Richard E. Bowen of NAVSEA and Mr. Carl Gotzmer and Mrs.

Nancy Johnson of the Naval Surface Warfare Center-Indian Head Division for their sponsorship of this research

project through CPBT corporation (under contract number N00174-02-C-0024) with a subcontract to PSU.

VI. References

1Kuo, K. K., Importance and Challenges of Hybrid Rocket Propulsion Beyond Year 2000, Invited von Krmn

Lecture in the Proceedings of the 37th Israel Annual Conference on Aerospace Sciences, pp. II-1 to II-31, February26-28, 1997.2Chiaverini, M. J., Serin, N., Johnson, D., Lu, Y. C., Kuo, K. K., and Risha, G. A., "Regression Rate Behavior of

Hybrid Rocket Solid Fuels,"Journal of Propulsion and Power,Vol 16, No. 1, pp. 125-132, 2000.3Risha, G. A., Harting, G. C., Kuo, K. K., Peretz, A., and Koch, D. E., "Pyrolysis and Combustion of Solid Fuels in

Various Oxidizing Environments," AIAA Paper No. 98-3184, 34th AIAA/ASME/SAE/ASEE Joint Propulsion

Conference, Cleveland, OH, July 13-15, 1998.4Strand, L. D., Ray, R. L., and Cohen, N. S., Hybrid Rocket Combustion Study, AIAA Paper 93-2412, June 1993.5Strand, L. D., Ray, R. L., Anderson, F. A., and Cohen, N. S., Hybrid Rocket Fuel Combustion and Regression

Rate Study, AIAA Paper 92-3302, June 1992.6Teague, W., Wright, A., Balkanli, D., and Hybl, L, Effect of Energetic Fuel Additives on the Temperature of

Hybrid Rocket Combustion, AIAA 99-2138, AIAA/SAE/ASME/ASEE 35th

Joint Propulsion Conference andExhibit, Los Angeles, CA, 20-23 June 1999.7Risha, G. A., Harting, G. C., Kuo, K. K., Peretz, A., Koch, D. E., Jones, H. S., and Arves, J. P., Surface Heat

Release of HTPB-Based Fuels in Oxygen Rich Environments, Combustion of Energetic Materials, Eds. K. K. Kuo,L. T. DeLuca, Begell House, Inc., pp. 101-115, 2002.8Risha, G. A., Evans, B., Boyer, E., Wehrman, R. B., and Kuo, K. K., Nano-Sized Aluminum- and Boron- Based

Solid-Fuel Characterization in a Hybrid Rocket Engine, AIAA 2003-4593, AIAA/SAE/ASME/ASEE 39th Joint

Propulsion Conference and Exhibit 20 23 Huntsville, AL, July 20039Risha, G. A. Enhancement of Hybrid Rocket Combustion Performance Using Nano-Sized Energetic Particles,Ph.D. Dissertation, The Pennsylvania State University, August 200310 Evans, B., Risha, G. A., Favorito, N., Boyer, E., Wehrman, R. B., Libis, N., Kuo, K. K., Instantaneous

Regression Rate Determination of a Cylindrical X-Ray Transparent Hybrid Rocket Motor, AIAA Paper No. 03-

4592, AIAA/SAE/ASME/ASEE 39thJoint Propulsion Conference and Exhibit 20 23 Huntsville, AL, July 2003.11Risha, G.A., Boyer, E. B., Wehrman, R. B., and Kuo, K. K., Performance Comparison of HTPB-Based Solid

Fuels Containing Nano-Sized Energetic Powder in a Cylindrical Hybrid Rocket Motor, AIAA 2002-3576,

AIAA/SAE/ASME 28thJoint Propulsion Conference, Indianapolis, IN, July 7-10, 2002.12 Karabeyoglu, M. A., Zilliac, G., Cantwell, B. J., DeZilwa, S., and Castellucci, P., Scale-Up Tests of High

Regression Rate Liquefying Hybrid Rocket Fuels, AIAA Paper No. 03-1162, 41 st AIAA Aerospace Sciences

Meeting and Exhibit, Reno, NV, January 6-9, 2003.13Karabeyoglu, M. A., Altman, D., and Cantwell, B. J., Combustion of Liquefying Hybrid Propellants: Part 1,

General Theory,Journal of Propulsion and Power, Vol 18, No. 3, pp. 610-620, 2002.14Karabeyoglu, M. A., and Cantwell, B. J., Combustion of Liquefying Hybrid Propellants: Part 2, Stability of

Liquid Films,Journal of Propulsion and Power, Vol 18, No. 3, pp. 621-630, 2002.

8/10/2019 2004_3821[1]

12/12

12

15 Risha, G. A, Ulas, A., Boyer, E. B., Kumar, S., and Kuo, K. K., Combustion of HTPB-Based Solid Fuels

Containing Nano-sized Energetic Powder in a Hybrid Rocket Motor, AIAA 01-3535, AIAA/SAE/ASME/ASEE

37thJoint Propulsion Conference and Exhibit, Salt Lake City, UT, 8 11 July 2001.16Weast, R. C.(ed), CRC Handbook of Chemistry and Physics, 67thEdition, p. B-40, 1987.

Documents

2004_3821[1]