AIAA 2010-7031
46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit
25 - 28 July 2010, Nashville, TN
Effect of Paraffin-LDPE Blended Fuel in
Hybrid Rocket Motor
Soojong Kim1, Jungpyo Lee2, Heejang Moon3, Honggye Sung4, JinkonKim5
Korea Aerospace University, Goyang, Gyeonggi, 412-791, South Korea
and
Jungtae Cho6
Hanwha Corporation, Yeosu, Jeollanam, 550-190, South Korea
A Novel blended solid fuel which mixes paraffin wax of alkane and LDPE of alkene is
invented and tested in a slab motor and hybrid rocket motor to visualize droplet
entrainment and to analyze combustion characteristics. The mechanical strength of blended
fuel was investigated increasing the LDPE wt%. Overall regression rate of PR95PE05 is
found to be 3.9 factors higher compared to that of HDPE. Improved combustion efficiency
was achieved with respect to pure paraffin fuel where performance gain was comparable to
that of SP-1a fuel of Stanford University. Analysis of the spectrum of the chamber pressure
revealed no critical instability for the range of this study. The PR95PE05 blended fuel can be
regarded comparatively effective for the hybrid rocket fuel in terms of mechanical strength,
combustion performance, and combustion instability.
Nomenclature
Af
Ai
Apf
Api
At
*
cexp
=
=
=
=
=
=
*
ctheo
Dpf
Dpi
Go
= theoretical characteristic velocity
= final grain port diameter
= initial grain port diameter
Hf
final chamber cross sectional area
initial chamber cross sectional area
final grain port area
initial grain port area
exhaust nozzle throat area
experimental characteristic velocity
= space-time averaged oxidizer mass flux
= final slab fuel grain height
Hi
Lf
m& o
Pc
r&
tb
Wf
∆mf
η c*
ρf
=
=
=
=
=
=
=
=
initial slab fuel grain height
fuel grain length
time averaged oxidizer mass flow rate
chamber pressure
space-time averaged fuel regression rate
burning time
fuel grain width
mass difference between initial and final
of fuel grain
= efficiency of characteristic velocity
= fuel grain density
I. Introduction
T
he low fuel regression rate is considered to be a main disadvantage in using the hybrid rocket for commercial
development. Recent research at Stanford University has led to the identification of a class of paraffin-based
fuels whose regression rates are 3~4 times higher than that of conventional polymeric fuels indicating its great
1
Graduate Student, Department of Aerospace and Mechanical Engineering, Student Member AIAA.
Graduate Student, Department of Aerospace and Mechanical Engineering, Student Member AIAA.
3
Professor, School of Aerospace and Mechanical Engineering, Member AIAA.
4
Professor, School of Aerospace and Mechanical Engineering, Member AIAA.
5
Professor, School of Aerospace and Mechanical Engineering, Member AIAA.
6
Research Engineer, Yeosu Plant Research and Development Department.
1
American Institute of Aeronautics and Astronautics
2
Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
potential application1. However, manufacturing of large fuel grain is difficult because pure paraffin fuels have poor
mechanical characteristics and lower combustion efficiency compared to the usual polymeric fuel2. In many
researches1-5, use of heterogeneous materials are presented where metal particle and carbon black powder were
added to pure paraffin wax in order to improve the mechanical strength, combustion efficiency and fuel
density. However, adding such metal particle and carbon black particle to paraffin wax has always been a problem
since it leads to combustion instability, chamber pressure sensitivity and settling of added material for mixing and
casting process.
In this respect, this paper suggests an effective solid fuel by blending pure paraffin wax and LDPE(low density
polyethylene). Paraffin-LDPE blended fuels may have many advantages compared to typical metalized paraffinbased fuels. The fact that both paraffin wax(alkane) and LDPE(alkene) are series of homologous materials, blended
fuel can be considered as uniform material, and thus, settling of added material during manufacturing process,
occurrence of combustion instability and of chamber pressure sensitivity provoked by the added metallic material
can be avoided. Furthermore, paraffin-LDPE blended fuels can improve the mechanical strength and combustion
efficiency of pure paraffin fuels since the solidity of blended fuel is higher than the paraffin itself.
In this study, paraffin-LDPE blended fuels having two kind of mixture ratio were manufactured and basic
experimental investigations were performed to analyze the applicability of paraffin-LDPE blended fuel for hybrid
rocket motors. The regression rate, the characteristic velocity and the chamber pressure spectrum data of blended
paraffin fuels are studied in conjunction with pure paraffin fuel, pure HDPE fuel, pure LDPE fuel and pure HTPB
fuel results.
II. Experimental Setup
Figure 1 shows the schematic of experimental setup for propulsion performance using the lab-scale hybrid rocket
motor. The experimental setup is mainly composed of oxidizer feed system, ignition system, DAQ system and
hybrid rocket motor. The oxidizer mass flow rates are controlled by pressure regulator and orifices with some kind
of diameter and are measured by turbine flow meter. All experimental process are automatically controlled by using
PLC(Program Logic Controller). For this function and data acquisition, NI(National Instruments) DAQ board and
LabVIEW software are used.
Figure 2 illustrates the lab-scale rocket motor. The rocket motor is composed of injector, fuel grain, exhaust
nozzle and pre and post combustion chamber. Oxidizer injectors are used two types of shower-head and single-port.
Single-port type injector is only used in comparison of combustion instability with the injector configuration. Water
cooled copper nozzle is used against erosion in lab-scale rocket motor and pressure transducers are mounted in pre
and post combustion chamber for static pressure measurement. Loadcell is also used for thrust measurement. Three
different kind of liquefying fuel and three different kind of non-liquefying fuel are used for comparison of
propulsion performance with respect to the solid fuel type. Gaseous oxygen is used as oxidizer in all experiments.
Figure 1. Schematic of experimental setup
Figure 2. Schematic of hybrid rocket motor
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The pure paraffin wax, PR100(fully refined soft paraffin wax, melting point = 61 ℃, Mw = 394 g/mol, density =
916 kg/m3 at 25 ℃, carbon number = C28) supplied by Nippon Seiro Co., Ltd. and pellet type LDPE(5301 model,
melting point = 110 ℃, Mw = 3.0 x 105 g/mol, density = 921 kg/m3, MFI = 0.3 g/10 min at 200 ℃ and 2.16 kg)
supplied by Hanwha Chemical Co. Ltd. were used in this investigation. These materials are commercial products.
Two blended fuel are manufactured with different mixture ratio. PR95PE05 blended fuel has 95 wt% pure paraffin
wax and 5 wt% LDPE whereas PR90PE10 blended fuel has 90 wt% pure paraffin wax and 10 wt% LDPE. Pure
LDPE(low density polyethylene) and HDPE(high density polyethylene) are made by machining polyethylene rod of
density 926 and 950 kg/m3, respectively. Pure HTPB is a solid fuel made by combining hydroxyl-terminated
homopolymer of butadiene and isocyanate cross linking agent. In case of pure paraffin wax(PR100) and blended
fuels(PR95PE05, PR90PE10), all fuel grains were melted in the melting pot and cast into pressure vessel to obtain
the required geometrical grain configuration. Detail specifications of fuel grain configuration and experimental
conditions for propulsion performance are shown in Table 1.
Table 1. Specification of experimental condition using hybrid rocket motor
Oxidizer
Gaseous oxygen
Solid fuel type
PR100
3
Fuel density(kg/m )
Burning time(s)
Oxidizer mass
flow rate(g/s)
Averaged oxidizer
mass flux(kg/m2-s)
Grain outer
diameter(mm)
Grain length(mm)
Initial port
diameter(mm)
L/D ratio
PR95PE05 PR90PE10
HDPE
LDPE
HTPB
SP-1a
910
5
911
5
912
5
950
10
926
10
928
5〜10
808〜911
6〜10
18〜127
19〜124
18〜135
14〜135
36〜54
12〜54
1560〜5550
33〜195
40〜286
42〜327
36〜475
132〜257
55〜130
110〜370
70
70
70
70
50
50
191
200
200
200
200
200
200
775〜1148
20
20
20
20
10〜15
10〜15
75〜154
10
10
10
10
13〜20
13〜20
5〜13
Figure 3. Slab motor for non-reactive flow
visualization
Figure 4. Slab motor for reactive flow
visualization
A slab hybrid motor was manufactured to visualize the behavior of liquid layer motion and fuel droplet
entrainment under non-reacting and reacting condition for all fuel considered in this study. The schematics of slab
hybrid motor for non-reacting and reacting are shown in Fig. 3 and Fig. 4, respectively. The slab motor for non3
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reacting flow is composed of radial injector, slab fuel, calming chamber, upper spacer, pre and post combustion
chamber and quartz window for visualization. In the non-reacting flow experiments, nitrogen is heated by gas torch
and it is injected into the slab motor to examine the dynamic behaviors of liquid layer and fuel droplet on the fuel
surface. For the reacting flow experiments same components are used except that water cooled copper nozzle is
installed and upper spacer is removed. In the reacting flow experiments, instead of nitrogen the oxygen is supplied
into the slab motor in order to observe the combustion phenomena and flame structures. These visualized images
under non-reacting and reacting condition were captured by 400 fps using SVSi(Southern Vision Systems Inc.)
Memview high speed camera.
III. Results and Discussion
A. Morphology comparison of liquefying solid fuels
Image scanning using Scanning Electron Microscope(SEM) was performed to observe the morphology of
blended fuel which is novel for hybrid solid fuel where SEM observation was performed for PR100, PR95PE05,
PR90PE10 and PR50PE50. The samples for SEM were cracked at room temperature and fractured sample surfaces
were observed. SEM analyses were carried out using a Hitachi S-4700 microscope with an acceleration voltage of
5.0 keV. The surfaces of the samples were coated with Pt and Rh by an electro-deposition method to impart
electrical conduction before analyzing the SEM images.
The SEM results of pure paraffin and LDPE/Paraffin wax blends are shown in Fig. 5. These images clearly show
the differences in the morphology of paraffin-based blends with respect to the LDPE mixture ratio. Figure 1a)
represents SEM image of PR50PE50 fuel which has relatively high LDPE mixture ratio. This image clearly
demonstrates phase separation of LDPE and paraffin wax. Krupa et al.6 explained that this behavior is due to the
a) PR50PE50 : non-uniform mixture
b) PR100 : uniform mixture
c) PR95PE05 : uniform mixture
d) PR90PE10 : uniform mixture
Figure 5. Morphology of liquefying blended fuels
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differences of molecular weight and structure between LDPE and paraffin wax in their recent publication. On the
other hand, from Fig. 5c) and 5d) whose LDPE ratio is low, LDPE is well dispersed within paraffin wax in the three
dimensional net structure and shows a fairly uniform surface without paraffin wax separation. This behavior
indicates that LDPE can disperse better in paraffin-based blends for low LDPE concentration. Thus, one can notice
that PR95PE05 and PR90PE10 blended fuels having low LDPE mixture ratio under 10wt% are uniform mixtures.
B. Mechanical strength comparison of liquefying solid fuels
Tensile test and compression test were performed in order to evaluate the mechanical strength with respect to the
LDPE wt%. Figure 6a) and 6b) show the result of tensile and compression test, respectively. As can be seen in these
figures, tensile and compression strength were increased as the LDPE wt% increases in paraffin-based fuels.
In the mechanical strength measurement of tensile and compression test, the tensile and compression strength of
PR95PE05 fuel having LDPE 5 wt% were increased by 24.8%, 34.0% respectively compared to those of pure
paraffin. The tensile and compression strength of PR90PE10 fuel were increased by 42.4%, 42.2% compared to pure
paraffin, respectively. These results show that the possibility of slump under storage and under operational condition
in large sized grain can be decreased by adding LDPE in paraffin.
a) Tensile strength of paraffin based fuels
b) Compression strength of paraffin based fuels
Figure 6. Mechanical strength to the mixture ratio of paraffin-based fuels
Table 2. Tensile strength and compression strength of liquefying paraffin-based fuels
Fuel type
Tensile strength
(MPa)
Rate of increase
(%)
Compression strength
(MPa)
Rate of increase
(%)
PR100
PR95PE05
PR90PE10
1.57
1.96
2.23
24.8
42.4
2.80
3.76
3.98
34.0
42.2
C. Visualization of non-reacting and reacting flow in slab motor
The non-reacting flow visualization was performed in order to understand the dynamic behavior of liquid layer
wavelet and fuel droplet, and to find dominant factor for the combustion performance improvement. This visual
information was supplied for the combustion modeling.
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Figure 7. Liquid layer wavelet and fuel droplet of liquefying solid fuel
Figure 7 is the image obtained during the non-reacting hot gas injection experiment. This image shows scattering
droplet of fuel in the chamber and the moving wavelet on the melt layer. Through the non-reacting visualization
experiment, one can observe that liquid layer wavelets on the fuel surface have periodicity and that droplet
scattering is generated from the liquid layer throughout the whole fuel surface. Additionally, significant amount of
liquefied fuel flowing at the liquid layer is directed toward the post-chamber and this phenomenon can cause the low
combustion efficiency in the liquefying hybrid fuels.
a) Flame shape of the paraffin wax(PR100)
b) Flame shape of the blended fuel(PR90PE10)
c) Flame shape of the non-liquefying fuel(HDPE)
Figure 8. Flame shape of the liquefying and non- liquefying solid fuels
Figure 8 shows different flame shapes depending on fuel types in the reacting region. High speed camera used in
this study indicates the bright region as white color. Therefore one can discriminate the reacting and non-reacting
region. In the reacting visualization experiment, the flame for the non-liquefying HDPE was located near the fuel
surface and droplet scattering was not observed. On the other hand, flames for pure paraffin fuel and blended fuel,
which are considered to be liquefying fuels, were located farther from the fuel surface compared to the HDPE. This
discrepancy of the flame distance from the fuel surface is due to the difference of fuel droplet entrainment rate
which decreases as the LDPE wt% increases.
As can be seen in Fig. 8a) and 8b), reacting droplets of the PR90PE10 blended fuel were observed more
frequently than those of the PR100 pure paraffin fuel. This result shows that unburned fuel droplets amount of
PR100 are relatively more frequent than those of PR90PE10, inasmuch as the regression rate of PR100 fuel is higher
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than that of PR90PE10 fuel. Thus, one can expect that combustion efficiency of the blended fuel such as PR95PE05
and PR90PE10 is higher than that of paraffin wax. From these results, it is evident that the fuel droplet entrainment
rate and flame distance can be controlled by the LDPE addition rate. Thus, LDPE addition rate can be a main factor
which affects the regression rate and the combustion efficiency.
Also, the flow of liquid fuel into the post-chamber which was shown in non-reactive experiment was not
observed in PR90PE10 fuel. On the other hand, the flow of liquid fuel into the post-chamber was observed in PR100
fuel. Although heat flux from the combustion flame increases the vaporization on the surface of liquid layer the flow
of liquid fuel into post-chamber still exist in PR100 fuel. In this condition, post-chamber inner volume can be an
important geometric parameter related to combustion efficiency.
D. Regression rate comparison of non-liquefying polymeric fuels and liquefying paraffin-based fuels
The regression rate and oxidizer mass flux are calculated by two kind of method with respect to the fuel
configuration. The calculated overall regression rate is a space-time(fuel length-burning time) averaged value. In
case of slab type fuel, the overall regression rate can be calculated by using the following equations:
(
)
Δm f = A f − Ai L f ρ f , A f = Ai + ( H i − H f )W f , r&s =
(
)
Δm f = Apf − Api L f ρ f , Apf =
π
4
D 2pf , r&p =
Hi − H f
D pf − D pi
2tb
tb
(for slab fuel grain)
(for cylindrical fuel grain)
(1)
(2)
In this study, we have used the classical averaged regression rate formula of Eq. (3) which represents the overall
regression rate where the burning fuel mass is measured before and after the firing test.
r& = aGon
(3)
Where, the oxidizer mass flux is defined as follow:
Go, s =
Go, p =
(
2m& o
)
2 Ai + H i + H f W f
16m& o
π ( D pi + D pf
)
2
(for slab fuel grain)
(4)
(for cylindrical fuel grain)
(5)
Figure 9 compares measured regression rate of HDPE, PR100 and PR90PE10 fuel in slab motor. The surface
tension and melt viscosity of three kinds of fuels are shown in Table 37.
Table 3. Surface tension and melt viscosity of paraffin-based fuels[7]
Fuel type
Surface tension(N/m)
Melt viscosity(Pa-s)
PR100
PR95PE05
PR90PE10
0.0228
0.0233
0.0237
0.0013
0.0060
0.0212
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Figure 9. Regression rate to the averaged oxidizer mass flux in the slab hybrid motor
In low mass flux region(G<4), regression rate of PR100 which is liquefying fuel is highly increased compared to
the PR90PE10 and HDPE fuel. On the other hand, though PR90PE10 fuel is a liquefying fuel like PR100 fuel,
regression rate of PR90PE10 is similar to that of HDPE. This results show that the fuel droplets from the liquid layer
are generated a lot during combustion of the PR100 fuel which have low surface tension and low melt viscosity,
while the fuel droplets are not generated during combustion of the PR90PE10 fuel due to the high surface tension
and high melt viscosity. Also, the difference of regression rate between HDPE and PR90PE10 is not large because
the thermochemical properties of both fuels which are homologous materials are very similar each other.
On the other hand, in relatively high mass flux(G>14), regression rate of PR100 and PR90PE10 which are
liquefying fuels is highly increased compared to the HDPE fuel since the fuel droplet is generated. In comparison
between PR100 and PR90PE10, the regression rate of PR100 fuel is higher than that of PR90PE10. The
thermochemical properties of all kind of fuel(PR100, PR90PE10, HDPE) in this visualization experiment are very
similar since they are homologous materials. Therefore, fuel mass flow rate by vaporization which is affected to the
Figure 10. Regression rate to the averaged oxidizer mass flux
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thermochemical property is also similar. Thus, this difference of regression rate is due to the difference of fuel mass
flow rate by droplet entrainment. The latter is affected by the physical properties such as surface tension and melt
viscosity.
Figure 10 compares measured overall regression rate of all test cases. One can notice that the overall regression
rates of all liquefying paraffin-based fuel(PR100, PR95PE05, PR90PE10) are larger than that of non-liquefying
polymeric fuel(HDPE, LDPE, HTPB). Also, overall regression rate of liquefying fuels is decreased as LDPE wt%
increase.
Table 4. Comparison of regression rate correlation with the fuel type
Fuel Type
a
n
PR100
SP-1a
PR95PE05
PR90PE10
HTPB
HDPE/LDPE
0.410
0.117
0.234
0.120
0.072
0.026
0.37
0.62
0.39
0.49
0.50
0.58
Rate of increase
relative to HDPE
6.4
5.7
3.9
3.0
2.0
1
These results can also be seen in table 4 which shows the comparison results of the regression rate correlations.
Overall regression rate of PR95PE05 fuel and PR90PE10 fuel is increased by 3.9 and 3.0 factors respectively when
compared to that of HDPE and overall regression rate of PR100 fuel is highly increased by 6.4 factors when
compared to that of HDPE. It is believed that the relatively small increase of regression rate of PR95PE05 and
PR90PE10 with respect to PR100 is due to the decrease of entrainment regression rate since the entrainment
regression rate is usually decreased as the viscosity and surface tension increase. Thus, viscosity and surface tension
of blended fuels is larger than those of PR100 and it is regarded that this property difference leads to the overall
regression rate difference. As can be seen in Fig. 10, results of HTPB and HDPE found by our study are quite close
to the results of other researcher8, 9. This small discrepancy of our study offers better reliability for the “rate of
increase relative to HDPE” of our Table 4.
Figure 11. Chamber pressure, oxidizer mass flow rate and thrust trace of PR100 fuel
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Figure 11 compares trace of chamber pressure and thrust during burning time under same oxidizer mass flow
rate condition. Generally, experimental parameters in combustion test of hybrid rocket motor uses time averaged
values. In case of non-liquefying fuel with low regression rate, variation of parameters such as pressure, thrust is
small since the port volume change during combustion is not large. Therefore, difference of time averaged value
depending on the burning time is small. On the other hand, in case of liquefying fuel with high regression rate,
variation of parameters is large and so is the error. This result implies that comparison study of experimental
parameter has to be performed under same burning time condition when high regression rate fuel is used as solid
fuel.
E. Combustion efficiency comparison of non-liquefying polymeric fuels and liquefying paraffin-based fuels
Characteristic velocity and combustion efficiency based on characteristic velocity were used to evaluate the
performance of various fuels. Experimental characteristic velocity and efficiency of characteristic velocity can be
calculated by using the following equations:
*
cexp
=
At
∫
tb
0
∫
tb
0
Pc dt
(6)
m& o dt + Δm f
*
*
ηc* = cexp
/ ctheo
(7)
Theoretical characteristic velocity was calculated by CEA10 code. Figure 12 and Fig. 13 compare the
characteristic velocity and characteristic velocity efficiency with respect to the O/F ratio, respectively. One can
remark that most measured characteristic velocity reside in the range 60-95% of characteristic velocity efficiency
where theoretical characteristic velocity computed by CEA are normalized to the same chamber pressure(200 psi) to
allow comparisons. Difference between stoichiometric O/F ratio(O/Fstoic) and maximum characteristic velocity
allowed O/F ratio(O/Fmax) with respect to the fuel type is not large. The O/Fstoic ratio is 3.4 whereas the O/Fmax ratio
is 2.1. This figure demonstrates that the characteristic velocity of blended fuel(PR95PE05, PR90PE10) is higher
than that of paraffin wax fuel(PR100). Comparison of between PR95PE05 and PR90PE10 shows that the
characteristic velocity of PR95PE05 is slightly higher than that of PR90PE10. These can be explained by the
chamber pressure to the propellant mass flow rate shown in Fig. 14.
Figure 12. Characteristic velocity vs. O/F
Figure 13. Characteristic velocity efficiency vs. O/F
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Figure 14. Chamber pressure vs. propellant mass flow rate
Comparisons between blended fuels with PR100 fuel clearly show that although difference of the propellant
mass flow rate is small, difference of the chamber pressure is relatively large. One can suspect the incomplete
combustion effect of pure paraffin wax fuel on characteristic velocity. Although a large amount of fuel mass due to
the fuel droplet entrainment can be generated during combustion, entrained fuel droplets may not be completely
burned in combustion chamber. This implies that the required chamber pressure increase with PR100 fuel may not
be sufficient compared to the chamber pressure increase with blended fuel.
Also, one can notice that the chamber pressure and propellant mass flow rate of HDPE fuel are in the similar
range compared to those of blended fuel. However, measured O/F ratio of HDPE fuel is in fuel lean side not like
blended fuel which operates nearby the fuel rich side. In case of blended fuel, range of O/F ratio is more approached
towards the stoichiometric O/F ratio of pure paraffin wax(C28H58). It can be seen from Fig. 12 and Fig. 13 that the
motor efficiency improves with increasing O/F ratio.
Additionally, one can observe that the characteristic velocity of blended fuel do not show large difference
compared to that of SP-1a fuel. Generally, combustion efficiency in hybrid rocket motor is increased with increasing
motor scale(L*), increasing mass flux and increasing O/F ratio. Considering that L* of hybrid motor and mass flux
level using SP-1a fuel are larger than those of this study, the above fact clearly demonstrates that the LDPE can be
an effective blending material with paraffin wax for improved efficiency.
Therefore, one can conclude that the PR95PE05 blended fuel is the most effective hybrid rocket fuel from results
of regression rate, characteristic velocity and characteristic velocity efficiency.
F. Combustion instability comparison with the fuel types and injector types
Combustion instabilities can introduce strong mechanical vibration on the rocket structure or can damage
payload and control system. They even can lead to the destruction of rocket structure. Fortunately, unlike solid
rocket motor and liquid rocket engine, critical combustion instability such as the structural failure of the motor case
is not occurred in hybrid rocket motor. The typical hybrid combustion mechanism is not capable of producing large
amplitude of pressure oscillation, limiting the possibility of catastrophic consequences11. The chamber pressure
oscillations in hybrid motor are occurred mainly at low frequency region. However, chamber pressure oscillation of
low frequency has to be also examined carefully in order to prevent the unpredictable regression rate in hybrid
rocket motor.
Thus, combustion instability characteristics of blended fuel were analyzed and compared to pure paraffin wax
fuel and polymeric fuel in this study.
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Figure 15a), Fig. 16a), Fig. 17a) and Fig. 18a) show the trace of chamber pressure during burning time in
experiment using HDPE, PR95PE05, PR90PE10 and PR100 fuel. The stability of combustion is generally validated
according to the level and randomness of pressure oscillations. Although, the oxidizer supply pressure is stable, in
liquefying fuels, relatively large chamber pressure oscillation is clearly observed during the burning time compared
to the HDPE fuel. It is regarded that this instability is mainly associated with oscillations in fuel production which is
generated by droplet form. On the other hand, in the chamber pressure of HDPE any remarkable oscillation is not
observed in Fig. 15a).
a) Pressure time trace
b) Pressure amplitude spectrum
c) Pressure spectrogram
Figure 15. Chamber pressure analysis of HDPE non-liquefying fuel using shower-head injector
a) Pressure time trace
b) Pressure amplitude spectrum
c) Pressure spectrogram
Figure 16. Chamber pressure analysis of PR90PE10 liquefying fuel using shower-head injector
a) Pressure time trace
b) Pressure amplitude spectrum
c) Pressure spectrogram
Figure 17. Chamber pressure analysis of PR95PE05 liquefying fuel using shower-head injector
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a) Pressure time trace
b) Pressure amplitude spectrum
c) Pressure spectrogram
Figure 18. Chamber pressure analysis of PR100 liquefying fuel using shower-head injector
a) Pressure time trace
b) Pressure amplitude spectrum
c) Pressure spectrogram
Figure 19. Chamber pressure analysis of PR100 liquefying fuel using single-port injector
Also, as can be seen in these figures, though the oxidizer supply pressure is fixed, the chamber pressure of all
hybrid fuels goes down as the burning duration increase. In case of HDPE, chamber pressure is relatively constant
compared to the liquefying fuels, while in case of liquefying fuels, the chamber pressure drop during burning time
increase as the LDPE wt% decreases. This can be explained from results of fuel regression rate. The inner volume of
fuel grain port which plays as a combustion chamber is forced to increase as the fuel is burned in typical hybrid
motor. According to these characteristics of hybrid motor, the chamber pressure decrease rate is proportional to the
fuel regression rate. Thus, the difference of chamber pressure gradient is due to the difference of the fuel regression
rate.
Figure 15b), Fig. 16b), Fig. 17b) and Fig. 18b) show the result of chamber pressure amplitude spectrum. The
chamber pressure is measured with a fast response Kistler pressure transducer to allow spectral analysis of the
pressure time trace. The amplitude spectrum of the pressure oscillations contains two distinct peaks at around 50Hz
and 450Hz in HDPE fuel. The most dominant peak is that around 50 Hz, which is the intrinsic HLF(hybrid low
frequency) mode caused by a coupling between the delay in the boundary layer formation and the thermal lag in the
solid fuel12. The peaks at around 450Hz is associated with the chamber bulk mode. The amplitude spectrum of the
pressure oscillations also contains two distinct peaks at around 50Hz and 450-500Hz in Liquefying fuel. The peaks
at around 50Hz and 450-500Hz are regarded as the HLF mode and as the chamber bulk mode, respectively. These
chamber pressure spectrums can be explained by spectrograms which consider the time variation.
Figure 15c), Fig. 16c), Fig. 17c) and Fig. 18c) are chamber pressure spectrograms which show the frequency
mode and the amplitude with the burning time. In Fig. 15c), the peak at around 450Hz of HDPE fuel keeps its initial
frequency during combustion. On the other hand, the peak at around 450-500Hz of all liquefying fuel migrates
toward low frequency region during combustion. Generally, pressure oscillation can be easily amplified when the
natural frequency match to some other frequency in the fixed chamber configuration. However, the chamber
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pressure oscillation of liquefying fuel which has fast regression rate do not allow enough time for the amplification
of chamber pressure oscillation.
Thus, though the combustion of liquefying fuels generating fuel droplets may be unstable, the critical
combustion instability can hardly occur in liquefying fuel. This is because the rapid change of inner chamber
volume due to fast regression rate limits the amplification of chamber pressure oscillation.
a) Shower-head injector
b) Exhaust plume
Figure 20. Schematic of injector and exhaust plume using shower-head injector
a) Single-port injector
b) Exhaust plume
Figure 21. Schematic of injectors and exhaust plumes using single-port injector
Characteristics of the combustion instability were also examined for different type of oxidizer injector
configuration. Figure 20a) and Fig. 21a) show the schematic of shower-head injector and single-port injector,
respectively. As can be seen in Fig. 21a), this study use the extreme case of equal port with injector diameters where
the flow field is a cavity flow instead of a backward-facing step flow3. In this case, most of injected flow is directed
into the fuel grain port passing through the center of the pre-chamber. On the other hand, in case of shower-head
injector, injected oxidizer flow is stagnated in the pre-chamber then after pass through the fuel grain port. Although
the experiment is performed under same oxidizer mass flux condition, the shear force exerted on the fuel surface can
have large difference depending on injector type. These differences of shear force on the fuel surface leads to the
difference of fuel droplet entrainment.
As shown in Fig. 19, chamber pressure oscillation violently occurs during combustion using single-port axial
injector. The high oscillation is due to the large increase of fuel droplet production induced by the shear force of
single-port axial injector. Generally, single-port axial injector type is known to be an effective injector for
combustion instability since the flame holding is easily established in pre-chamber13. However, results in Fig.19
clearly show that the single-port injector may give the cause for combustion instability. Further evidence can be seen
by the oscillation and diffusion angle of the plume through the comparison of Fig. 20b) and Fig. 21b). This implies
that the shower-head type (more exactly diffuser type shower-head injector) injector is more effective than the
single-port axial injector in terms of combustion instability.
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American Institute of Aeronautics and Astronautics
IV. Conclusion
In this study, paraffin-LDPE blended fuels were manufactured and fundamental and basic experimental
investigation were performed to analyze the applicability of paraffin-LDPE blended fuel for hybrid rocket motors.
The main findings of this paper are:
1. The PR95PE05 and PR90PE10 blended fuels with LDPE mixture ratio under 10 wt% have been demonstrated
to have a uniform mixtures.
2. As the LDPE wt% is increased, the tensile and compression strength of PR95PE05 are increased by 24.8%,
34.0%, whereas the tensile and compression strength of PR90PE10 are increased by 42.4%, 42.2% compared
to the pure paraffin, respectively.
3. In the case of pure paraffin, the liquid layer wavelets on the fuel surface have periodicity and droplet
scattering is generated from the liquid layer throughout the whole fuel surface. Significant amount of liquefied
fuel flowing at the liquid layer is directed toward the post-chamber and this phenomenon can cause the low
combustion efficiency in the liquefying hybrid fuels. The flame for the non-liquefying HDPE was located near
the fuel surface and droplet scattering was not observed. On the other hand, flames for pure paraffin fuel and
blended fuel were located farther from the fuel surface compared to the HDPE.
4. Overall regression rate of PR95PE05 and PR90PE10 is increased by 3.9 and 3.0 factors respectively and
overall regression rate of PR100 is highly increased by 6.4 factors when compared to that of HDPE. This
difference of regression rate is due to the difference of fuel mass flow rate by droplet entrainment. The latter is
affected by the physical properties such as surface tension and melt viscosity
5. The characteristic velocity of blended fuel(PR95PE05, PR90PE10) is higher than that of paraffin wax
fuel(PR100) and, the characteristic velocity did not show large difference compared to that of SP-1a fuel.
6. In case of the liquefying fuel, relatively large chamber pressure oscillation is observed during the burning time
compare to the HDPE. The variation of chamber pressure during burning time increased as the LDPE wt%
decreased. The amplitude spectrum of the pressure oscillations contains two distinct peaks at around 50Hz and
450-500Hz in HDPE and in liquefying fuel. The peak at around 450Hz of HDPE keeps its initial frequency
during combustion while the peak at around 450-500Hz of all liquefying fuel migrates toward low frequency
region during combustion.
7. The critical combustion instability can hardly occur in liquefying fuel. This is because the rapid change of
inner chamber volume due to fast regression rate limits the amplification of chamber pressure oscillation.
8. The chamber pressure oscillation violently occurs during combustion using single-port axial injector compared
to the shower-head injector. This implies that the shower-head type injector is more effective than the singleport axial injector in terms of combustion instability.
9. Finally, one can conclude that the PR95PE05 blended fuel is the most effective hybrid rocket fuel in terms of
mechanical strength, combustion performance, and combustion instability.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of
Korea(NRF) funded by the Ministry of Education, Science and Technology (No. R0A-2007-000-10034-0)
References
1
Karabeyoglu, M. A., Cantwell, B. J. and Altman D., “Development and Testing of Paraffin-Based Hybrid Rocket Fuels,”
AIAA-2001-4503, 37th AIAA/ASME/SAE /ASEE Joint Propulsion Conference and Exhibit, Salt Lake City, Utah, July 2001.
2
Kilic, S., Karabeyoglu, A., Stevens, J., and Cantwell, B., “Modeling the Slump Characteristics of the Hydrocarbon-Based
Hybrid Rocket Fuels,” AIAA-2003-4461, 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville,
Alabama, July 2003.
3
DeZilwa, S., Zilliac, G., Karabeyoglu, A., and Reinath, M., “Combustion Oscillations in High Regression Rate Hybrid
Rockets,” AIAA-2003-4465, 39th AIAA/ASME /SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, Alabama,
July 2003.
15
American Institute of Aeronautics and Astronautics
4
Evans, B., Favorito, N. A., and Kuo, K. K., “Study of Solid Fuel Burning-Rate Enhancement Behavior in an X-ray
Translucent Hybrid Rocket Motor,” 41th AIAA/ASME /SAE/ASEE Joint Propulsion Conference and Exhibit, Tucson, Arizona,
July 2005.
5
Karabeyoglu, A., Zilliac, G., Cantwell, J., DeZilwa, S., and Castellucci, P., “Scale-Up Tests of High Regression Rate
Paraffin-Based Hybrid Rocket Fuels,” Journal of Propulsion and Power, Vol. 20, No. 6, 2004, pp. 1037–1045.
6
I. Krupa, G. Mikova, and A.S. Luyt, "Phase change materials based on low-density," polyethylene/paraffin wax blends,"
European Polymer Journal 43 (2007) 4695-4705
7
Soojong, K. , “A Study on Combustion Characteristics of Hybrid Rocket using Liquefying Solid Fuel-Gaseous Oxygen,”
Ph.D. Dissertation, The Korea Aerospace University, August 2010(in press).
8
Sutton, G. P., Rocket Propulsion Elements: An Introduction to the Engineering of Rockets, 7th Edition, John Wiley and
Sons, Inc., 2000.
9
Karabeyoglu, M. A. and Zilliac, G., "Hybrid Rocket Fuel Regression Rate Data and Modeling", 42th AIAA/ASME/SAE/
ASEE Joint Propulsion Conference & Exhibit, AIAA 2006-4674, Sacramento, CA, 2006.
10
Gordon, S., and McBride, B.J. 1976. "Computer program for calculation of complex chemical equilibrium compositions,
rocket performance, incident and reflected shocks, and Chapman-Jouguet detonations", NASA SP-273, Interim Revision, March
11
Chiaverini, M. J.and Kuo, K. K. "Fundamentals of Hybrid Rocket Combustion and Propulsion," 7th Edition, AIAA, 2007.
12
Karabeyoglu, M. A., De Zilwa, S., Cantwell, B., and Zilliac, G., “ "Modeling of Hybrid Rocket Low Frequency
Instabilities,”" Journal of Propulsion and Power, Vol. 21, No. 6, 2005, pp. 1107-1116.
13
T. A. Boardman, D. H. Brinton, R. L. Carpenter and T. F. Zoladz, "An Experimental Investigation of Pressure Oscillations
and Their Suppression in Subscale Hybrid Rocket Motors", 31th AIAA/ASME/SAE/ ASEE Joint Propulsion Conference &
Exhibit, AIAA 95-2689, San Diego, CA, 1995.
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