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39 AIAA/ASME/SAE/ASEE JointPropulsion Conference and Exhibit

20 23 July 2003Huntsville, AL

Instantaneous Regression Rate Determination of a Cylindrical X-Ray Transparent Hybrid

Rocket Motor

Brian Evans*, Grant A. Risha, Nick Favorito

, Eric Boyer

, Robert B. Wehrman

,Natan Libis, and

Kenneth K. Kuo

The Pennsylvania State UniversityUniversity Park, PA 16802

Abstract

The determination of the solid fuel regression rate is one of the key steps in hybrid rocket combustion studies.

Historically, there is lack of direct regression rate measurements for validation of theoretical models. In practice

most mass-burning rates were determined by the net burned mass divided by the test duration that yields an average

rate. However, this method does not capture the instantaneous regression behavior. To achieve this, a newlydesigned X-Ray Transparent Center-perforated (XTC) hybrid rocket motor system has been fabricated and tested to

provide the ability to measure the instantaneous solid fuel regression rate using a high-powered real-time X-ray

radiography system. Tests have been conducted using hydroxyl-terminated polybutadiene (HTPB) as the baselinesolid-fuel formulation. The solid-fuel regression rate can be enhanced by the addition of energetic metal powders.

Tests have been conducted using a 13% Silberline aluminum flakes solid fuel formulation in order to evaluate ametalized fuel with the X-ray radiography system. The capability of the visual analysis system to capture

instantaneous X-ray radiography images has been demonstrated. Time variations of port dimensions have showngood comparison with calculated regression results from developed numerical code. Differences in recovered fuel

burning surfaces were observed from SEM photographs. Large surface roughness, exhibited on the burned surfaces

of fuels containing nano-sized aluminum particles, indicates high potential for introducing stronger heat feedback to

substrate of solid-fuel and enhance burning rate.

Introduction

A traditional hybrid rocket motor employs a

solid-fuel grain with a gaseous, liquid or gel oxidizer

injected at the head end of the motor. Hybrid rocketsposses many advantages over conventional solid- orliquid-propellant engines including on/off capability,

improved operability of motor performance, minimal

environmental impact, and also an inherent safety.1,2

The inherent safety of hybrid rockets is due to theseparation of the fuel and oxidizer physically and also

by phase. For hybrid rockets, the combustion of solid

fuels can be controlled by the supply rate of oxidizer tothe combustion chamber. The rate-limiting process of

the combustion of hybrid rockets is the mixing and

combustion of the fuel grain pyrolysis products with the

oxidizer flowing through the center port of the solid-

fuel grain.

3,4,5

Correlations between the instantaneouslinear regression rate and the instantaneous oxidizer

mass flux are unavailable and most investigations

address the dependency of the averaged linear

regression rates on averaged oxidizer mass fluxessupplied to the rocket motors. The instantaneousregression rate of the solid fuel in a hybrid rocket motor

could differ significantly from the averaged value. It is

highly desirable to have the capability for measuring

the instantaneous fuel regression rate as a function oftime and position. One of the objectives for this paper

is to develop an X-Ray Transparent Center-perforated

(XTC) hybrid rocket motor system. Using the XTChybrid rocket motor allows correlations to be

established that describe the regression behavior of

various solid fuels burning under different operating

conditions.

One of the disadvantages of the existing HTPB-based solid fuels is the relatively low mass burning rate,

requiring a relatively large fuel burning surface area for

S

D

* M. . Student, AIAA Member

Ph. . Candidate, AIAA Student Member

Undergraduate Assistant

Visiting Scholar Distinguished Professor of MechanicalEngineering, Fellow AIAA

1

American Institute of Aeronautics and Astronautics

39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit20-23 July 2003, Huntsville, Alabama

AIAA 2003-459

Copyright 2003 by Brian J. Evans, Grant A. Risha, Nick Favorito, Eric Boyer, Robert B. Wehrman, Natan Libis, and Kenneth K. Kuo. . Published by the American Institute of Aer
http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/

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4. Flexible oxidizer system to allow the use of

either gaseous or liquid oxidizers; and

producing a given thrust level. Using energetic nano-

sized powder additives, developed by various

manufacturers, this disadvantage can be alleviated.6,7 5. Capability for instantaneous thrust

measurements for performance evaluation.A previous study conducted by Risha, et al.8,9has

evaluated 19 different fuel formulations using a Long-Grain Center-Perforated (LGCP) hybrid rocket motor.

The energetic additives that have shown the greatest

increase in mass-burning rate performance have beenselected for further study using the XTC hybrid rocket

motor. In addition to continued combustion studies of

energetic additives, a parallel material characterizationstudy has been conducted in order to further understand

which physical properties of energetic particles are

important to the combustion of solid fuels containing

these energetic additives.

The predecessor to the XTC hybrid rocket motor,known as the Long Grain Center Perforated (LGCP)

hybrid rocket motor, has provided much useful

information as to the effects of various energeticadditives.12 Based on the findings of the LGCP, current

research is focused on understanding the effects of the

addition of energetic particles with two main focuses:

Material characterization to analyze physical

properties (eg. particle size distribution,percent oxide vs. active content, etc.) and their

effect on combustion.Various techniques have been adopted in recent

studies to deduce the instantaneous regression rate of

solid fuel. Strand, et al.10 used a small low-pressure

slab motor with optical viewing ports to determine boththe instantaneous and also the average regression rates.

Chiaverini, et al.2used a similar slab configuration butwith a larger rocket motor and higher steady-state

chamber pressure. Both ultrasound pulse-echo systemand a real-time X-ray radiography system were

employed to make instantaneous measurements at

several axial locations along the fuel surface. The

ultrasound technique was also used by Russo Sorge, et

al.11 to determine the regression rate at a single axiallocation, but in a cylindrical center perforated grain. A

cylindrical grain has also been utilized in a small motor

in previous studies at PSU by Risha et al. 12 toexperimentally find average regression rates of various

solid fuel formulations containing different types of

nano-sized energetic particles. Lengelle, et al.13

andRisha, et al.

14 have used other methods involving a

convective flow to measure the ablation rates of various

solid fuels.

Experimental combustion research, via

instantaneous linear regression rate

determination, to evaluate the performanceenhancement when energetic particles are used

in solid-fuel formulations.

With the capability of real-time X-ray radiography,

the instantaneous regression rate of the various solid-fuel formulations can be measured as a function of time

and axial location. The regression rate can then be

correlated to the instantaneous oxidizer mass flux as afunction of time and position.

In addition to studying the benefits of addition of

various nano-sized energetic particles, research will

also focus on the effect of various oxidizers as well in

hopes of finding an optimum fuel/oxidizer combination.The test facilities enable the use of various oxidizers

that can be gaseous, liquid, or gelled materials. The test

facility is also capable of heating the gaseous oxidizers

to elevated temperatures for simulating a ramjetcombustion process.

With the XTC and the LGCP hybrid rocket motors(both capable of being operated with minimal transition

and turn-around time), a comparison of similar fuel

formulations can be made between them. Comparisons

between these two motors with different geometric

sizes can help us to assess any scaling effects that are

present.

Objective of Research

In the current study, an X-ray transparent lab-scale

hybrid motor has been designed to allow the testing andcharacterization of various fuels and additives in a

hybrid rocket system. In order to obtain the maximum

amount of information from limited supplies of

experimental materials, a number of unique features

have been implemented. Specific design objectives forthe motor design include:

Method of Approach

Motor Component Design

The design of the X-Ray Transparent Center-perforated (XTC) hybrid rocket motor was based upon

the design strategies of its predecessor, the LGCP

hybrid rocket motor. Figure 1 shows a photograph of

the XTC test rig installed on the test deck with major

motor components noted. In order to enable X-rays topass through the motor without significant attenuation,

the motor casing has been designed from thick walled

1. Cartridge loading of the solid-fuel grain to

facilitate motor assembly and achieve rapid

turn-around for test firings;2. Center-perforated grain to simplify casting and

simulate real motor port;3. X-ray transparent case to allow instantaneous

measurements to be made over entire motor

operation;

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paper phenolic tubing, which is commercially available,

instead of a traditional steel casing for the LGCP motor.

Figure 1.Photograph of the assembled XTC hybridrocket motor

Both ease of construction and simplicity of usewere taken into account as part of the design process in

order to allow quick fabrication of the rocket motor and

rapid turn-around between static test firings. The newdesign provides several desirable capabilities such as:1) ability to change injector to swirl, annular, impinging

jet, etc.; 2) various nozzle throat sizes to tailor the

chamber pressure; and 3) potential to install a water-cooled nozzle to avoid nozzle erosion. In contrast to

the LGCP motor that employs a stainless steel chamber,

the paper phenolic casing allows direct imaging using a

real-time X-ray radiography system. Fine-wirethermocouples and ultrasonic devices can be installed

in any axial locations along the solid fuel grain, using

the newly developed diagnostic assemblies as shown

below in Fig. 2 in the XTC motor layout diagram.This design also offers a maximum flexibility in

operation and maintenance of the nozzles to be used in

the experiments. After the graphite nozzle is insertedinto the nozzle plug, it is retained in place by a threaded

retainer ring. This design makes it very easy to use

different types of nozzles with little dismantling of the

nozzle assembly. Tungsten or other types of high-

temperature metallic nozzles can be used instead ofgraphite nozzles. This selection is based upon the

operating conditions of the experiment. An existingwater-cooled nozzle can be directly mounted onto theXTC motor to provide cooling of the nozzle during the

experiment. The injector face can also be quickly

replaced if a different injection pattern is desired.

The motor is equipped with two Setra pressure

transducers to monitor the pressure near the oxidizerinjector and the exit nozzle. The XTC motor is

mounted on a precision linear guide platform allowing

free axial movement during test firing and the

instantaneous thrust can be measured using a 1000-lbf

load cell.

Since the XTC motor grain is new, eachcomponent was hydrostatically tested. According to the

Environmental, Safety and Health Pressure Vessel and

System Design documentation used by LawrenceLivermore National Lab (LLNL)15, the components of

any high pressure testing apparatus used in remote

operation must be hydrostatically tested to at least1.25PMAWP

(maximum allowable working pressure).

The design equations for maximum operating pressure,

maximum allowable operating pressure, and the

hydrostatic test pressure are as follows:

MOP

MOP

MAWP

HYDRO MAWP

Max. Operating Pressure:

P 1, 850 psig = 12.86 MPa

Max. Allowable Operating Pressure:

P P 2, 050 psig 14.24 MPa

0.90

Hydrostatic Test Pressure:

P 1.32 P 2, 700 psig 18.72 MPa

=

= = =

= = =

Both the solid fuel paper phenolic cartridge and the

diagnostic assembly successfully passed the hydrostatic

pressure tests conducted up to 2,700 psig.

In order to study the effects of various liquid

oxidizers on the combustion of solid-fuel formulations,a liquid-oxidizer feed system was designed and

constructed. The liquid oxidizer feed system was

designed for a 10-second run of the XTC motor with

mass fluxes up to 0.42 kg/s. This time is longer thenthe anticipated test time and allows for sufficient

oxidizer to assure steady flow in the feed lines

throughout the test. A volume of approximately 4.2liters was determined to be sufficient to accommodate

for a 10-second test time. Therefore, based on this

information a piston tubular series reactor from High

Pressure Equipment Company (HiP) was selected thatallowed for the needed volume of the oxidizer and

operating pressure. The tubular reactor has a working

volume greater than needed and a working pressure

rating of 34.6 MPa (5,000 psig). However, the results

covered in this paper are related to hybrid rocketoperation using gaseous oxygen as the oxidizer.

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Solid Fuel

Paper Phenolic Cartridge

Oxidizer

Diagnostic Assembly

Graphite SleeveHead-end Closure

Assembly

Injector Assembly

Retainer

Injector Assembly

Injector

Face

Injector Face

Retainer

Linear Guide

Deck SupportLinear Bearing

Linear Guide Rod

Support

Graphite

Nozzle

Graphite

Sleeve

Nozzle-end Closure

Assembly

Nozzle Assembly

Nozzle

Retainer

Load Cell

Figure 2. XTC motor layout with major components noted

In order to compare results obtained from the XTC

motor and the smaller LGCP motor, oxidizer mass

fluxes were controlled to range from approximately 80

to 140 kg/m2-s. In addition to mass fluxes in this range,

the XTC motor has the capability to be operated atmuch higher fluxes.

XTC Hybrid Rocket Motor Test Setup

The XTC hybrid rocket motor was used to burn

solid-fuel formulations consisting of hydroxyl-

terminated polybutadiene (HTPB) as the baseline solidfuel (SF1). When Silberline energetic nano-sized

aluminum flakes were added for performance

enhancement, the solid fuel sample is named as SF7.

The HTPB was formulated using R45-M resin withIsonate 143L methylene diphenyl isocyanate (MDI)

curing agent. The casting process for solid-fuel

formulations for the XTC motor grain is similar to thepredecessor, the LGCP. The solid fuel was mixed

following the same procedure for introducing particles

into the resin and degassing. Also the grains were castutilizing a previously developed novel low-melting

point wax mandrel process, in order to create a center-perforated grain. Further information pertaining to the

exact process through which this is completed can be

found in Reference 16. Grains were cast in the paperphenolic 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). Typicaltest durations for the XTC ranged from 5 to 7 seconds

depending on the fuel formulation and the injected

oxidizer mass flux.

Since hybrid rocket motors have shown that the

oxidizer mass flux is the dominant parameter on the

linear regression rate with no discernable pressureeffect8, the steady-state chamber pressure of the XTC

can be similar to or much greater then that of the LGCP

while still maintaining approximately the same linearregression rate. Because of this, the chamber pressure

has been selected to range from approximately 2.17 to

4.24 MPa (300 to 600 psig), which is similar to therange used for the LGCP and will allow for comparison

in performance.The LGCP motor has been used to evaluate 19

different fuel formulations. A complete list of fuel

formulations with the additive type and percent byweight addition can be found in References 8 and 9.

From the results obtained from testing various nano-

sized particles with the LGCP motor a group of front-running candidates have been selected for continued

investigation with the XTC motor. The energetic

additive that has shown the most improvement in

performance is the Viton-A coated Alex with an

increase of 120% in mass-burning rate and 123%increase in the average linear regression rate at an

oxidizer mass flux of 112 kg/m2-s. At this same

oxidizer mass flux, Alexpowder without any coating

has also proven to be a leading candidate. Averagemass-burning and linear-regression rates increased by

61% and 105%, respectively. A number of other

candidates exhibited significant performance increasecompared to the baseline solid fuel HTPB. For these

candidates, which include; Silberline aluminum flakes,

A high capacity variable-throat venturi with

upstream pressure and temperature measurements wasemployed in the gaseous oxygen feed system in order to

reach the desired mass flow rates. From previous

experiments using the LGCP motor, the range of

oxidizer mass flux was determined and scaledaccordingly so that the larger scale XTC motor had

similar initial operating conditions. Flow rates of up to

0.42 kg/s can be obtained through the use of thisventuri, which is sufficient for the desired oxidizer

mass fluxes.

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Viton-A coated Silberline aluminum flakes,

Technanogies aluminum, an average increase in linear

regression rate of 60-70% was seen. The addition of

Viton-A has proven to be beneficial in increasing the

average mass burning and linear regression rates ascited by Reference 16.

Due to the fact that more (~3 to 4 times) energetic

powder mass is needed to cast an XTC grain than theLGCP grain with the same percentage of additive

loading, the LGCP motor has been used to evaluate the

effectiveness of the nano-sized energetic additive.Based on the LGCP motor results, the larger XTC

motor grains were prepared for studying the

combustion behavior in larger motor with detailed

diagnostics.

Instantaneous Regression Rate Measurement Method

The main feature of the XTC hybrid rocket motoris the capability to image the instantaneous surface of

the regressing fuel grain using a real-time X-rayradiography system. During test firings, the regression

of the surface can be determined at any instant in timeand at any axial position. The experimental setup of the

XTC motor with the real-time X-ray radiography

system in place is shown in Fig. 3 below.

Figure 3. X-ray imaging setup used with XTC hybridrocket motor

The X-ray radiography system employed in this

study uses a Phillips 320 kV X-ray tube in conjunctionwith MGC 03 controller. The X-ray system has the

capability to be operated in any one of three test cells

within the lab. In order to assure safe operation of the

X-ray system an interlock system has beenimplemented. When the interlock system is armed no

one may enter the test cell in which the X-ray is being

operated or into the fenced area surrounding the testcell. If someone does breech this perimeter the X-ray

system power supply will automatically cut off the

power to the X-ray tube. The X-rays are detected by a

Hamamatsu 9/6 image intensifier and captured by

camera and recorded.

For X-ray cinematography, an X-ray source is

positioned adjacent to the XTC motor with an imageintensifier on the opposite side of the test setup, as close

to the motor as possible. Keeping the image intensifier

as close to the motor as possible reduces the amount ofX-ray scatter, which allows as sharp an image as

possible. Attached to the image intensifier is a right-

angle adapter that reflects the output image from the

image intensifier. A video camera is setup off to theside of the image intensifier to capture the image while

not being in line with the high-power X-ray beam to

protect the camera electronics.

Images obtained from the X-ray system wereanalyzed to determine the location of the surface of the

solid fuel as a function of time. Initial imaging was

taken on a calibration grain (having a 13% Silberlinealuminum flake composition) that was machined with

precision steps cut into the solid fuel at knowndiameters (see Fig. 4). Imaging the calibration grain

with the X-ray system allowed the determination of thesurface clarity from the image density gradient at the

surface. An alumunized fuel formulation was chosen

since it would attenuate the largest amount of the

signal. It was found that the gradient in image density

at the surface was sufficient to distinguish the port fromthe surface of the fuel grain as shown in the calibration

grain.

Blast ShieldX-ray

Source

Image

Intensifier

Solid Fuel Surface

Figure 4. Calibration grain image with surface of fuel

noted.

Discussion of Results

Six XTC hybrid rocket motor test firings were

conducted with two different fuel formulations, pure

HTPB (tests XTC-01, XTC-03, XTC-04, XTC-06) and13% Silberline aluminum flakes (tests XTC-02, XTC-

05). Two of the six test firings were run using the real-

time X-ray radiography system to image the surface of

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the solid-fuel grain. All tests have been conducted

using gaseous oxygen as the oxidizer with varying

average oxidizer mass fluxes from 100 to 150 kg/m2-s.

The pressure-time trace from XTC-02-SILBAL-13

(SF7) is given in Fig. 5. Noted on the plot are thespecific times corresponding to the initiation of oxidizer

flow, onset of ignition, oxidizer flow cutoff, and

initiation of nitrogen purge. On this plot, the time t=0corresponds to the onset of ignition. The oxidizer flow

began approximately 2 seconds prior to ignition in

order to assure a steady flow in the feed line. Shortlyafter the small pyrotechnic igniter is energized, the

chamber pressure raised sharply indicating successful

ignition followed with gradual pressure decay. 0

50

100

150

-5 0 5 10

0

80

160

240

320

400

480

560

640

Thrus

t[lb

f]

Time [s]

O2Flow Off

Ignition

O2Flow On

N2Purge On

Thru

st[N]

Figure 6. Typical thrust-time profile using XTC rocket

motor (XTC-02-SILBAL-13)

0

50

100

150

200

250

300

-5 0 5 10

0

0.5

1

1.5

2

ChamberPres

sure[psig]

Time [s]

O2Flow Off

Ignition

O2Flow On

N2Purge On C

hamber

Pressure[MPa]

Burning Rate Comparisons to LGCP Results

One of the objectives of the XTC hybrid rocketmotor was to study the effects of geometric scaling

between the XTC and the smaller LGCP grain.

Geometric scaling effects arise since the XTC fuelgrains are approximately 3-times the size of its

predecessor the LGCP grain. Initial average burning

rate results show that a power-law curve fit with very

close values of exponents (0.647 vs. 0.698) fits both

sets of data as seen in Fig. 7. Comparison between thetwo sets of data shows agreement to within 6%. From a

simplistic qualitative point of view, the larger XTC

motor will demonstrate a smaller ratio of surface

roughness to diameter (/D) assuming similar surfaceroughness between the LGCP and XTC motor. FromMoodys chart for pipe flows, it can be deduced that

there will be a lower friction coefficient (f) and

therefore a lower heat-transfer coefficient hcaccording

to Reynolds analogy shown below:

Figure 5. Typical pressure-time profile using XTCrocket motor (XTC-02-SILBAL-13)

The pressure decreases throughout the steady burntime of the grain, which is caused by the increase in

port area that creates a decrease in oxidizer mass flux.For test XTC-02, the initial oxidizer mass flux was 341

kg/m2-s with a final oxidizer mass flux of

approximately 113 kg/m2-s yielding an average oxidizermass flux of approximately 170 kg/m2-s based upon the

time-averaged port area at the nozzle-end of the grain.

The thrust trace for this test is given below in Fig. 6

including specific times corresponding to motoroperating condition changes as given in Fig. 5. Test

XTC-02 produced a consistent thrust level trace for the

pressure-time profile that was obtained. A gradual

decrease in the thrust is seen as the pressure decreasesdue to the increase in port area. A maximum thrustlevel of this test was approximately 120 lbf(534 N) and

the average thrust level was approximately 100 lbf(445

N).

2 23 3

f Nu= St Pr = Pr Pr

2 Pr Re

where

c

p

tot ox fuel

h

u c

u G G G

=

= = +

23

The resultant decrease in heat-transfer coefficient

causes a slight decrease in solid-fuel regression rate that

is seen with the XTC motor when compared to the dataof the LGCP motor. Thorough investigation into the

detailed cause of the regression rate difference will berequired to facilitate an in-depth understanding of the

physical phenomena involved. These will include:

curvature effects, heat loss to inert motor components,

as well as the influence of surface roughness and portgeometry on turbulent kinetic energies distribution.

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0.5

0.6

0.7

0.8

0.9

1

2

70 80 90 100 200

LGCP Hybrid Rocket Motor

XTC Hybrid Rocket MotorAverageLinearRegress

ionRate,rb[mm/s]

Average Oxidizer Mass Flux, GOX

[kg/m2-s]

rb[mm/s] = 0.0401*(G

ox,ave[kg/m

2-s])

0.698

rb[mm/s] = 0.0541*(G

ox,ave[kg/m

2-s])

0.647

7


Figure 7. Comparison of average regression rates for

pure HTPB (SF1) grains in XTC and LGCP motors

Instantaneous Regression Rate Determination

Tests XTC-05-SILBAL-13 and XTC-06-HTPB-

100, which used SF7 and SF1 formulations

respectively, were the first two tests to utilize the real-

time X-ray radiography system. Both tests obtained

successful visual data allowing the determination of theinstantaneous fuel surface. A location near the nozzle

was chosen for the measurement of the instantaneous

regression rate, since the average linear regression rate

is determined by the average of the port area at thenozzle end of the grain. With continued testing other

axial locations will be imaged to gain further

understanding into the regression behavior at different

locations of the grain. Figure 8 shows an image

captured from XTC-05-SILBAL-13 test firing.

Specific features are labeled in the image such as the

diagnostic assembly feedthrough screw that housed a

25 m fine-wire thermocouple.

Nozzle End of

XTC Motor

Diagnostic Assembly

Feedthrough Screw

Oxidizer Flow

Measurement Location

Figure 8. Captured image of the viewing section from

XTC-05-SILBAL-13 test firing using a real-time X-ray

radiography system

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Time [s]

Solid

FuelPortRadius[mm]

Deduced From Measured P-t Trace

Calculated Port Radius Variation Prediction

Final Port Radius-After Testing

Adapted LGCP Code, Deduced From P-t Trace

X-ray Image Data

34

1

2

1

2

3

4

Figure 9. Comparison of measured and calculated time variations of port radius at nozzle-end of solid fuel grain

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An XTC motor test using SF1 fuel grain was

conducted at an average oxidizer mass flux level around

100 kg/m2-s to directly compare linear regression rate

data with those obtained from the LGCP motor. At this

oxidizer flux level almost zero throat erosion wasnoticed in the nozzle. Under such condition, C*

predictions are more accurate when throat erosion

effect is absent.The measured instantaneous port radius data at

nozzle-end of the solid fuel grain from the analysis of

X-ray images are plotted on Fig. 9. These data werecompared to the predicted results from several different

analysis codes developed by this research team. Curve

1 represents the deduced time variation of port radius

using the pressure-time trace obtained from the test

firing. Curve 2 shows the predicted results using datafrom NASA-CEA code (C* curve fit, chamber gas

temperature curve fit) as input and solved the

conservation of mass equation as a function of time.The output of this analysis is the time variations of

chamber pressure, O/F ratio, etc. Curve 3 shows themeasured final port radius after the test firing. Curve 4

shows the deduced radius variations with respect totime by solving both conservation of mass and energy

equations (based upon the adapted LGCP code). As

shown in Fig. 9, the predicted or deduced results are

generally in good agreement with the X-ray image data.

Time, t=0, corresponds to energizing the igniterleads. A rapid increase in the port radius is observed in

this short time period that the chamber pressure surged

abruptly due to the onset of combustion. Thecompression of the fuel corresponding to the chamber

pressure surge from 0.5 to 1.0 second is due to the

viscoelastic nature of HTPB, which causes non-lineardeformations. Similar effects were seen in previoushybrid studies conducted with a slab motor utilizing X-

ray radiography for imaging and ultrasonic transducers

for point measurements of the regression rate. Serin, et

al.17 conducted tests and developed a correlation fordescribing a correction term, which was specific for the

test setup and material. After this filling period, a

quasi-steady pressure is seen in the rocket motor andthe agreement of the measured data is within 10% of

the predicted radius.

In-Situ Cast Fine-Wire Thermocouples

The ability to cast fine-wire thermocouples in the

solid-fuel grain allows a subsurface temperature

measurement of the fuel as the surface regresses.

Information into the surface temperature behavior forvarious fuel formulations in combination with material

property research on fuel samples allows theunderstanding of what is happening in the sub-surface

and surface regions of the fuel. Figure 10 shows a

typical temperature-time trace obtained from an

aluminized solid-fuel grain. It appears that there are

three distinct regions. First is the small plateau that is

seen before the sharp increase in temperature.

Although the exact cause of the flat region is unknown

at this time it is believed that a melt layer on the surfaceof the fuel could cause this. Approximating the linear

regression rate of the fuel as 1.4 mm/s during this time

and noting that the plateau region lasts forapproximately 60 milliseconds it can be determined that

the melt layer has a thickness of approximately 85 m.The nearly constant surface temperature of this zone

could be caused by the absorption of the energy being

fed back from the flame zone by the aluminum particles

that are at the fuel surface. A significant increase in thetemperature is not evident until about 916 K, which is

slightly below the melting temperature of aluminum

(933 K). The other two noticeable features on the

thermocouple trace are the two peaks followed by twotemperature decreases. The first peak that is seen

causes a temperature swing of approximately 75 K

while the second fluctuates a little bit more and isapproximately 100 K. The cause of these peaks is

unknown but it is speculated that the thermocouple may

be moving in the hot gases due to the flow in the shear

layer and this is causing a fluctuation in the measured

temperature. The thermocouples are self-supportingand not covered by a rigid probe.

0

500

1000

1500

2000

-1500 -1000 -500 0 500

Temperature[K

]

Distance [m]

Surface Temperature:916 K

Plateau Region

Temperature Peaks

Figure 10. A typical temperature-time trace measured

by a 25 m thermocouples cast in solid fuel grain

(Test Number XTC-05-SILBAL-13)

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Particle Analysis and Material Characterization

Characterization of physical properties of energetic

additives was performed to gain deeper understanding

of the mechanisms leading to improved performance ofsolid fuels containing these additives. Much of the

characterization of particles physical properties was

conducted during research on the LGCP motor. Furtherinformation including specific surface area, active

aluminum content, and average diameter can be found

in References 8 and 9.Figure 11 (a), (b), and (c) gives SEM photographs

of burned surfaces of HTPB fuel, HTPB with 13%

Alex fuel, and HTPB with 13% Al-325 fuel,

respectively. The Al-325 powder contains aluminum

particles sieved through a 325 mesh and yields micron-sized particles. All three images were photographed

under 250x magnification.

The recovered HTPB burned fuel surface showvery uniform structure with regular-sized voids and

protruded surfaces. The protruded materials at thesurface could represent some carbonaceous residues

and partially burned polymer. Comparing Fig. 11(b)with 11(a), there are substantial amounts of deep caves

generated on the surface of recovered HTPB fuel with

13% Alex particles. The corresponding surface

roughness for the aluminized fuel is much higher than

that of the pure HTPB fuel. The higher surfaceroughness corresponds to higher energy transfer

between the hot gas and the fuel surface. This implies a

higher regression rate of the fuel sample. This effectwas observed experimentally.

By comparing the SEM of HTPB fuel (Fig. 11(a))

with Fig. 11(c), it is quite obvious that the recoveredfuel sample surface with large sized aluminum particlesis covered by molten material which are believed to be

the Al2O3 and some aluminum. The large-sized

aluminum particles when they reach the fuel surface

will have a greater chance to melt and agglomerate.The molten material during the combustion process

could transfer greater amounts of heat to the substrate

material and cause the regression rate to enhance. Thiseffect was also observed experimentally.

Some clear differences can be noted between SEM

(b) and SEM (c). From Fig. 11(b) it is noticeable right

away that the surface is not smooth but appears to

contain caverns that go quite deep into the surface ofthe fuel. However in Fig. 11(c), for the Al-325 fuel

formulation, the recovered surface appears to be

relatively smooth in most places and displays only a

few deep holes. The texture of the surface of the fuelsis another noticeable difference between the twoformulations. An almost coral-like texture is seen onthe surface of the Alex fuel while the Al-325 fuel

surface appears smooth and molten in structure.

Further analysis to determine the composition of the

surface of these fuel samples is being conducted to

determine the extent that accumulation and

agglomeration is occurring on the surface of the fuel

during combustion.

(a) HTPB baseline fuel

(b) HTPB with 13% Alexfuel

(c) HTPB with 13% Al-325 fuel

Figure 11. SEM photographs of burned fuel surfacesAll images under 250x magnification

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CONCLUSIONS

1. Successful testing of the XTC hybrid rocket

motor has proven the concept of the real-time X-

ray radiography measurement method forcylindrical grains. Recorded X-ray images

allowed the determination of the instantaneous

port radius. Results obtained from the X-rayvideo agreed well with the calculated

instantaneous port radius from the data reduction

code.2. The diagnostic feedthroughs have demonstrated

the ability to pass 25 m fine-wire

thermocouples to be cast in-situ. The sub-surface temperature profiles were obtained and

SF7 had a surface temperature of 916 K. With

more tests, an Arrhenius plot can be constructed.

3. Pure HTPB solid fuel grains have been tested atvarying average oxidizer mass flux conditions in

the XTC motor, including low mass fluxes that

are in the same range of the LGCP motor.Results show slight differences in power law

correlations. Difference in relative roughness

could contribute to their difference in final

correlation.

4. SEM photographs of recovered fuel surfaceswere obtained for baseline HTPB fuel and

aluminized HTPB fuels. Baseline HTPB fuels

demonstrated regularly structured surface withevenly spaced voids and protrusions. Addition

of Alex powders into HTPB fuel makes the

burning surface to be highly non-uniform withdeep caves and strong protrusions. These

structures lead to greater surface roughnesswhich enhances heat transfer rates between the

hot gas and pyrolyzing fuel surface. Addition of

micron-sized aluminum particles into HTPB fuel

causes the recovered surface to be covered bythe molten Al2O3 and unburned aluminum. The

molten material also increases the heat feedback

rate to the unburned solid fuel and resulting inhigher regression rate. However the aluminum

burning is less complete for micron-sized

aluminum particles.

FUTURE WORK

Continued research with the XTC hybrid rocket

motor using the real-time X-ray radiographysystem will collect more data describing the

regression process of the fuel surface for various

solid-fuel formulations. Establish correlations

for the instantaneous regression rate based ontest operating conditions and fuel properties.

Conduct material analysis on the burned and theunburned fuel surfaces of various fuels to

determine trends that are common within a fuel

formulation.

Study the effect of various energetic additives,

high-energy oxidizers, and binders to obtain an

optimum fuel/oxidizer combination thatproduces the desired performance.

Investigate the difference between the XTC andLGCP in burning rate power-law correlations

including geometrical differences, heat loss, and

turbulent kinetic energies.

Additional instrumentation including ultrasonictransducers, resistance sensors, etc. will be

utilized to obtain data for comparison with visual

data.

Examination of the viscoelastic nature of the fuel

formulations and to develop a correction term forcylindrical grain geometry.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Mr. Carl

Gotzmer and Mrs. Nancy Johnson of the Naval Surface

Warfare Center-Indian Head division for their

sponsorship of this research project through CPBTcorporation (under contract number N00174-02-C-

0024) with a subcontract to PSU. The authors would

also like to thank Mr. Peter J. Ferrara for his help withthe fabrication of graphite nozzles and mandrel plugs

and Prof. Baoqi Zhang for his help with the fine-wire

thermocouple construction.

REFERENCES

1 Kuo, K. K., Importance and Challenges of Hybrid

Rocket Propulsion Beyond Year 2000, Invited vonKrmn Lecture in the Proceedings of the 37th Israel

Annual Conference on Aerospace Sciences, pp. II-1 to

II-31, February 26-28, 1997.2Chiaverini, M. J., Serin, N., Johnson, D., Lu, Y. C.,

Kuo, K. K., and Risha, G. A., "Regression RateBehavior of Hybrid Rocket Solid Fuels," Journal of

Propulsion and Power, Vol 16, No. 1, pp. 125-132,2000.3Strand, L. D., Ray, R. L., and Cohen, N. S., Hybrid

Rocket Combustion Study, AIAA Paper 93-2412, June

1993 and Strand, L. D., Ray, R. L., Anderson, F. A.,

and Cohen, N. S., Hybrid Rocket Fuel Combustionand Regression Rate Study, AIAA Paper 92-3302,

June 1992.4 Teague, W., Wright, A., Balkanli, D., and Hybl, L,

Effect of Energetic Fuel Additives on the Temperatureof Hybrid Rocket Combustion, AIAA 99-2138, June

1999.5Risha, G. A., Harting, G. C., Kuo, K. K., Peretz, A.,Koch, D. E., Jones, H. S., and Arves, J. P., Surface

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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.

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96, 2000.7Ivanov, G., Uniformity of Burning and Detonation of

Pyrotechnic Mixtures Based on Activated Aluminum,1995 JANNAF Propulsion Meeting CPIA Pub. 630,

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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 20039 Risha, G. A. Enhancement of Hybrid RocketCombustion Performance Using Nano-Sized Energetic

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Joint Propulsion Conference, Nashville, TN, 1992.11

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Hybrid Rocket, 4th Green Propellants International

Conference, Norolwijk, Netherlands, June 19-21, 2001.12Risha, 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 28th Joint Propulsion Conference,

Indianapolis, IN, July 7-10, 2002.13Lengelle, G., Fourest, B, Godon, J. C., and Guin, C.,

Condensed Phase Behavior and Ablation Rate of Fuelsfor Hybrid Propulsion, AIAA 93-2413,

AIAA/SAE/ASME/ASEE 29th Joint Propulsion

Conference, Monterey, CA, June 28-30, 1993.14Risha, G. A., Harting, G. C., Kuo, K. K., Peretz, A.,

Koch, D. E., Jones, H. S., and Arves, J. P., Pyrolysisand Combustion of Solid Fuels in Various Oxidizing

Environments, AIAA 98-3184,


Conference, Cleveland, OH, July13-15, 1998.15Environmental, Safety and Health, Pressure Vessel

and System Design,

http://www.llnl.gov/es_and_h/hsm/doc_18.02/doc18-02.html16Risha, G. A, Ulas, A., Boyer, E. B., Kumar, S., and

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Containing Nano-sized Energetic Powder in a HybridRocket Motor, AIAA 2001-3535,


Conference and Exhibit 8 11 July 2001 Salt Lake

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Kuo, K. K., "Pressure Correction of Ultrasonic

Regression Rate Measurements of a Hybrid Slab

Motor," AIAA Paper 99-2319, 35thAIAA/ASME/SAE/ASEE Joint Propulsion Conference,

Los Angeles, CA, 20-24 June, 1999.

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