4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
Micro and Nano-sized Metal Additives for Solid Fuels in
Hybrid Rocket Lab-scale Burner
Christian Paravan, Andrea Sossi, Alice Reina, Giovanni Massini, Miriam Manzoni, Emiliano Duranti,
Giorgio Rambaldi, Andrea Adami, Eleonora Seletti, and Luigi T. DeLuca
Space Propulsion Laboratory (SPLab), Politecnico di Milano, Aerospace Engineering Department, 34, via LaMasa,
20156 Milan, MI, Italy
Abstract
Low regression rate limits application of hybrid rocket engines. In this paper, ballistics of HTPB-based
fuels loaded with two different metal additives was investigated. Nano-sized Aluminum (ALEX™,
100nm, uncoated) and micrometric Magnesium-Boron (MgB, ~5µm) were tested in a lab-scale burner.
Under an oxidizer mass flux of 350 kg/(m2s) MgB produced a higher regression rate increase than
nano-sized Aluminum (+53% vs. +45% with respect to Baseline). In spite of nano-Aluminum high
performance at high oxidizer mass fluxes, overall MgB ballistics and relatively ease of manufacturing
enable to consider this novel additive as an interesting candidate for hybrid systems performance
enhancement.
Nomenclature
aD
ar
D
D0
Latin Symbols
: multiplier factor in Eq. (1), mm / s n
: multiplier factor in Eq. (5), mm / s n
: diameter, mm
: nominal initial diameter, mm
D
: ignition time, s
∆
ρf
Greek Symbols
: Difference
: solid fuel density, g/cm3
σ
: standard deviation
r
Di
: space-averaged diameter, mm
: space-averaged diameter at strand
ignition, mm
: sampled space-averaged diameter, mm
Dh,i
: sampled horizontal diameter, mm
Dv,i
: sampled vertical diameter, mm
Gox
: oxidizer mass flux, kg/(m2s)
< Gox >
D
Dign
t ign
Abbreviations
2D
: two-dimensional
ALEX
BET
: ALuminum EXploded
: Brunauer Emmet Teller
CCPs
:
: time-averaged oxidizer mass flux in
Eq. (4), kg/(m2s)
DOA
: DiOctyl-Adipate
m& f
: fuel mass flow rate, g/s
EEW
: Electric Explosion of Wire
m& ox
: oxidizer mass flow rate, g/s
fps
: frames per second
nD
: time exponent in Eq. (1)
GOX
: Gaseous OXygen
nr
: time exponent in Eq. (5)
HRE
pc
: chamber pressure, bar
O/F
rf
: Oxidizer to Fuel ratio
: regression rate, mm/s
IPDI
: Hybrid Rocket Engine
Hydroxyl-Terminated
:
Polybutadiene
: IsoPhorone Di-Isocyanate
LRE
: Liquid Rocket Engine
< rf >
: time-averaged regression rate, mm/s
MgB
: Magnesium-Boron
R2
t
t0
: correlation index
: time, s
: reference time in Eq. (1), s
nAl
nlpm
SRM
: nano-sized Aluminum
: normal-liter per minute
: Solid Rocket Motor
Tign
: ignition temperature, K
TOT
: Thickness Over Time
HTPB
Condensed Combustion
Products
Copyright  2011 by C. Paravan et al. Published by the EUCASS association with permission.
PP: HYBRID ORPHEE
1. Introduction
Thermo-chemical propulsion is the leading technology for access to space and in-space propulsion. SRMs and LREs
are mature technologies servicing in several applications. Nevertheless, the former lacks of operation flexibility, and
produces highly pollutant exhausts, whereas the latter, though high performing, is characterized by complicated
design and higher costs [1][2][3]. HREs offer high theoretical performance, flexibility and reliability while enabling
possible costs savings [1][4]. Therefore, hybrid systems could satisfy new requirements from access-to-space market
(as improvement of present launch systems performance or private access to space needs) and in-space navigation.
The main cons of HREs are the low regression rate of the solid fuel and the poor combustion efficiency. These
drawbacks hinders the possible advantages coming from the exploitation of hybrid systems, and are related to the
diffusive nature of HRE flame [1][2][3].
1.1 Literature Survey
In a HRE oxidizer and fuel are stored in different phases. In the most common configuration, the former is fluid
(liquid or gaseous) while the latter is solid. Fluid oxidizer flows toward solid fuel grain generating a boundary layer.
After ignition of the system, gasified fuel diffuses into oxidizer flow. Therefore, when O/F reaches flammability
limits for the considered mixture, a flame is established within the boundary layer. Fundamentals of this phenomenon
were investigated by Marxman et al. [5][6][7]. In the proposed model regression rate was identified as a strong
function of total mass flux (resulting from Gox and fuel mass flow from the regressing surface): r f ∝ G 0.8 [5].
Under the investigated conditions coupling between convective and radiative heat-transfer was identified but
proposed model was not accurate enough to consider a pressure-dependence of regression rate for pc lower than 10
bar reducing rf as pressure was decreased. Marxman and co-workers addressed this effect to chemical kinetics
dominated regression rate. Further investigation on hybrid systems combustion was conduced by Smoot and Price
[8]. A dependence of regression rate from Gox as well as from pressure for pc below 12 bar was identified in this
work. The former resulted in a power-law relationship reflecting the one proposed by Marxman and co-workers,
while heterogeneous surface reactions were identified as possible source for pressure dependence. A recent
discussion about results on pressure influence on rf in hybrid systems is reported in [9].
In past studies, regression rate was usually evaluated by spatially and temporally averaging procedures based on
TOT [9][10]. In recent studies conduced at Penn State University, instantaneous regression rate measurement leads
to r f ∝ G 0.6 for HTPB/GOX tests [10].
In order to overcome the intrinsic low regression rate limitations affecting HRE, several techniques have been
proposed [1][3][9][14][15][16]. In present work, particular attention is given to HTPB-based solid fuel formulations
containing metal additives and Paraffin-based formulations producing entrainment of melted fuel droplets. The latter
was originally proposed by Karabeyoglu et al. [17][18]. Though high performing in terms of regression rate
enhancement, entrainment-producing fuels such as solid Paraffin exhibit poor mechanical properties that are
incompatible with large scale applications [19]. Metal additives used for solid fuel loading include micro-metric and
nano-sized powders of metals or metal hydrides [20][21][22][23][24]. With micro-sized particles, regression rate
increase comes mainly from enhanced radiative heat-transfer from flame zone to fuel grain surface [20]. Nano-sized
particles, due to elevate reactivity coming from the high specific surface (~10-50 m2/g vs. ~1 m2/g of micro-metric
additives) could release energy closer to the regressing surface and thus resulting highly effective in rf enhancement
[20]. Nevertheless, nano-particles require proper dispersion technique contrasting cold cohesion/clustering in order to
be effective in ballistic performance enhancement [21][22][23][25].
SPLab of Politecnico di Milano has developed several test rigs and diagnostic techniques to investigate hybrid
combustion [26][27][28][29][30][31][32]. In particular a time-resolved technique for regression rate has been
developed [31]. Aim of SPLab experimental activity is to identify a solid fuel formulation with high enough
regression rate as well as a suitable set of properties for industrial, large scale applications (performance, safety, and
costs).
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MICRO AND NANO-SIZED METAL ADDITIVES IN HYBRID ROCKET LAB-SCALE BURNER
2. Tested Fuel Formulations and Ingredients
Baseline formulation was HTPB-R45 plasticized by DOA, cured with IPDI (with a curing level -NCO/-OH = 1.04)
with Dibutyltin Diacetate as curing catalyst. Manufacturing process was carried out in a controlled atmosphere vessel
with constant temperature of 60°C to improve miscibility and manufacturing [25][30].
In addition to Baseline, two different HTPB-based fuel formulations were tested: one contains 2% Carbon and 10%
nAl, the other one 2.8% MgB90 (20% Mg). The latter is an innovative metal additive where Boron with 90-92%
purity (B90) is bonded to Magnesium (mass fraction of 20% with respect to additive total mass). MgB-powder
specific surface results ~3.5 m2/g (see Figure 1). Boron offers the highest volumetric heat of oxidation in Oxygen
between metal/metalloid fillers, but is characterized by hard-ignition and poor combustion efficiency [20]. MgB
powder is designed with the aim of lessening Boron cons by MACH I (King of Prussia, Pennsylvania, USA). Nanosized Aluminum, produced by EEW at APT LLC (Advanced Powder Technology LLC, Tomsk, Russia) has a
nominal size of 100 nm, with a BET surface area of 12 m2/g and is uncoated [33][34].
Table 1: Ignition temperature of metal additives considered in this study (quiescent atmosphere, controlled pressure
[35]). Note MgB mean Tign decrease with respect to Boron (B90) for increasing Mg content.
B90
MgB90 (15% Mg)
MgB90 (20% Mg)
nAl (ALEX™, 100 nm, uncoated)
Oxidizer/pc
Tign
Air/1bar
Air/1bar
Air/1bar
Air/1bar
1004 ± 31
949 ± 40
927 ± 57
865 ± 15
Figure 1: Particle size distribution of MgB90 (20% Mg) as evaluated at SPLab by Malvern Mastersizer 2000. Mass
weighted mean size of particles is 5.2 µm (S. Dossi, 2011).
Solid Paraffin-based fuel is manufactured according to procedure presented in [28][29]. Nano-sized energetic
additives require special manufacturing technique with proper treatment to prevent cold cohesion/clustering of
powder particles [20[21][23][25][30]. These treatments were performed on nAl-doped HTPB-based fuel. In the
present study no nAl-loaded wax formulation was investigated. Paraffin-based fuel loaded with MgB is considered to
evaluate ballistics of metallized wax fuel.
3. Experimental Setup and Data Reduction Technique
Experimental investigation was conduced by 2D-radial micro-burner [26][30][31][32]. This facility enables burning
of cylindrical-shape samples with single central port perforation (initial nominal diameter of 4 mm) with quasisteady chamber pressure and controllable oxidizer mass flow. Combustion test is visualized and recorded by digital
camera (working between 25 fps and 1000 fps depending on operating conditions). An optical time-resolved
technique is used to analyze solid fuel formulation ballistics [32]. Time-resolved technique is based on diameter
sampling in time. During test strand head-end is visible and Dh,i , Dv,i can be measured by recorded combustion
images. Starting from the latter measured parameters Di can be defined. Sequence of sampled space-averaged
diameters, a discrete information in time, is then interpolated to achieve a continuous evolution of D (t ) , see Eq. (1).
3
PP: HYBRID ORPHEE
Diameter evolution in time is valid starting from t ign . This parameter is ad-hoc defined for each test treated by
SPLab time-resolved procedure as the one maximizing data fitting of Eq. (1). A comparison between ad-hoc defined
t ign and estimated ignition delay evaluated under the hypothesis of exclusive convective ignition is reported in Table
2. Solid fuel ballistics can be completely defined starting from Eq. (1) [31].
D (t ) − D0 = a D ⋅ (t − t0 ) n , t ≥ tign
(1)
D
Results achieved by time-resolved technique are checked by comparison with TOT data in order to gain information
on data consistency between different reduction techniques, see Eqs. (2)-(4).
( )
r f t ign =
(
< r f t final
(
< Gox t final
)
)
? n
D (t ign ) − D0
1
a D n D (t ign − t 0 ) n −1 = D ⋅
2
2
t ign − t 0
(2)
D
1
>=
t final − tign
1
>=
t final − tign
t final
t final
?
t ign
?
∫ Gox (t )dt = π
t ign
1 D (t final ) − D (tign )
t final − tign
∫ r f (t ) ⋅ dt = 2 ⋅
4
(3)
m& ox
⋅
{ [D (t
ign ) +
(4)
] }2
D (t final ) 2
Table 2: Convective ignition delay (tign, convective, [36][37]) and ad-hoc evaluated tign. Note general trend agreement for
increasing pc. High difference for 7 bar tests due to irregular primer charge ignition.
Gox ( Dign )
tign, convective
tign
Baseline (HTPB) – 7 bar
Baseline (HTPB) – 10 bar
Baseline (HTPB) – 13 bar
Baseline (HTPB) – 16 bar
296.2
370.4
353.2
365.3
0.031
0.021
0.022
0.021
0.245
0.034
0.059
0.046
Equation (1) is defined for each performed test. In order to summarize results achieved under the same operating
conditions for a given fuel, various D (t ) are collapsed into an ensemble curve. The latter is defined by a power-law
interpolation of time-trends identified by application of Eq. (1) to the performed tests. In order to compare achieved
results with literature data, a power-law approximation of analytical rf vs. Gox is proposed for each test and for
ensemble curves, see Eq. (5).
r f (Gox ) = ar ⋅ Gox (t ) n , t ≥ tign
r
(5)
4. Results and Discussions
In this section results of ballistic investigation are reported starting from data achieved for Baseline.
Ballistics of all tested formulations was investigated under GOX, with oxidizer volumetric flow rate of 210 nlpm,
while chamber pressure was varied in the range from 7 to 16 bar for Baseline, with metallized formulations tested at
10 or 13 bar. Figure 2, Figure 3, Table 3 and Table 4 report results for Baseline tested with pc of 13 bar in order to
illustrate time-resolved technique operating steps. Figure 4 and Table 5 shows data for baseline formulation under
the four tested chamber pressures. Note data at pc of 10 bar are presented twice to underline effect of one of the five
performed tests. In fact, in Test No. 04 checks [see Eqs. (2)-(4)] are satisfied but rf is faintly dependent from Gox.
Data reported in Table 5 exhibit low value of nr and data fitting index and due to possible solid fuel fragmentation
phenomena, while behavior of regression rate at high oxidizer mass fluxes (see Figure 3).
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C. Paravan et al.
MICRO AND NANO-SIZED METAL ADDITIVES IN HYBRID ROCKET LAB-SCALE BURNER
Diameter Change in Time, D-D0, mm
7.0
6.0
5.0
4.0
Test No. 01
Test No. 02
3.0
Test No. 03
2.0
Test No. 04
Test No. 05
1.0
Ensemble
0.0
0
1
2
3
4
5
6
7
Time, t , s
Figure 2: Baseline (HTPB) instantaneous diameter change in time. Diameter sampling frequency: 2-5Hz at sample
ignition, 1-3Hz from 1 s after ignition (depending on operating condition, visualization quality and video frame rate).
2.0
Test No. 01
1.8
Test No. 02
Regression Rate, rf , mm/s
1.6
Test No. 03
1.4
Test No. 04
1.2
Test No. 05
1.0
Ensemble
0.8
Ensemble (Power-law Approx)
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
350
400
2
Oxidizer Mass Flux, G ox , kg/(m s)
Figure 3: Baseline (HTPB) burning under GOX (instantaneous data). Note ensemble analytical data
trend with respect to power-law approximation (concavity and behavior at high oxidizer mass flux).
Table 3: Baseline (HTPB) parameters for Eq. (1) and Eq. (6). Low data fittings for approximation of rf
vs. Gox due to general poor agreement between power-law approximation and analytical data (see
Figure 3).
Test No. 01
Test No. 02
Test No. 03
Test No. 04
Test No. 05
Ensemble
aD
1.741 ± 0.012
2.060 ± 0.016
1.716 ± 0.016
1.779 ± 0.033
1.963 ± 0.011
1.836 ± 0.003
nD
0.719 ± 0.007
0.669 ± 0.007
0.714 ± 0.008
0.739 ± 0.014
0.649 ± 0.006
0.704 ± 0.001
R2, Eq.(1)
0.999
0.999
0.997
0.995
0.999
0.986
ar
0.030 ± 0.001
0.030 ± 0.001
0.038 ± 0.001
0.059 ± 0.001
0.018 ± 0.001
0.042 ± 0.001
nr
0.585 ± 0.006
0.615 ± 0.005
0.534 ± 0.005
0.468 ± 0.005
0.688 ± 0.005
0.531 ± 0.002
R2, Eq. (6)
0.943
0.955
0.900
0.906
0.970
0.906
5
PP: HYBRID ORPHEE
Table 4: data consistency checks Baseline (HTPB) burning under GOX, see Eqs. (2)-(4). Differences up to 12% can
be reached due to solid fuel grain fragmentation phenomena, and peculiar behavior of hybrid system.
Eq. (3) a
Eq. (4) a
Eq. (2) a
Test No. 01
0.1%
-1.6%
3.3%
Test No. 02
0.4%
-0.7%
2.8%
Test No. 03
-0.7%
3.7%
1.8%
Test No. 04
2.2%
5.0%
2.9%
Test No. 05
0.7%
-1.8%
1.8%
a
Percentages evaluated with respect to TOT values
Table 5: rf vs. Gox for instantaneous ensemble curves for Baseline (HTPB) burning under GOX. Note poor values of
data fitting, and low value of nr.
nr
R2
ar
7 bar
0.081 ± 0.001
0.393 ± 0.001
0.970
10 bar
0.041 ± 0.001
0.512 ± 0.002
0.912
13 bar
0.042 ± 0.001
0.531 ± 0.005
0.945
16 bar
0.060 ± .0001
0.468 ± 0.005
0.935
2.0
Regression Rate, rf , mm/s
Baseline (HTPB), 7 bar
1.8
Baseline (HTPB), 10 bar
1.6
Baseline (HTPB), 13 bar
Baseline (HTPB), 16 bar
1.4
Baseline (HTPB), 10 bar *
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
350
400
2
Oxidizer Mass Flux, G ox , kg/(m s)
Figure 4: Ensembles of instantaneous regression rate versus oxidizer mass flux for Baseline (HTPB) burning under
GOX. Pressure exhibits no significant influence on regression rate.
Ballistics of Baseline formulation under the investigated condition can be summarized by a power-law
approximating function were both Gox and pc effects are considered.
r f (Gox , pc ) = (0.0732 ± 0.0002) ⋅ Gox (t ) (0.430 ± 0.001) ⋅ pc ( −0.039 ± 0.001) , R 2 = 0.917
(6)
In spite of the low data fitting, Eq. (6) evidences the apparent independency of regression rate from chamber
pressure. Under the investigated conditions, dependence of Baseline ballistics from oxidizer mass flux is below the
0.8 value identified by diffusion-limited theories considering convective heat transfer as the exclusive/leading
mechanism for fuel regression phenomena. For HTPB/GOX this holds for all investigated operating conditions as
witnessed by previously presented results, see Table 5 and Eq. (6). The observed trend can be related to
fragmentation of solid fuel grain. Under the investigated conditions, toward the end of the combustion process fuel
fragments are detached by the fuel grain (see Figure 5 and Figure 6). Fragmentation enhances regression rate with
respect to vaporization-driven phenomenon: decreasing Gox, rf results higher than predicted by theoretical models
based on heat transfer between flame zone and regressing surface due to sliver detachment. Further investigation on
this phenomenon are required, nevertheless if data concerning diameter evolution in time for Gox < 100kg/(m2s) are
6
C. Paravan et al.
MICRO AND NANO-SIZED METAL ADDITIVES IN HYBRID ROCKET LAB-SCALE BURNER
removed from analysis, nr ~0.6 is achieved for HTPB/GOX under the investigated conditions. This cut of available
data does not change the general trend for analytical data vs. power-law approximation reported in Figure 4.
2mm
2mm
tign + 2.638 s
tign + 2.642 s
tign + 2.646 s
Figure 5: Baseline (HTPB) burning under GOX with pc of 16 bar. CCPs detaching from regressing surface. Solid fuel
fragmentation is clearly visible toward the end of the combustion and appears more intense as pressure increases.
tign + 2.769 s
tign + 2.771 s
tign + 2.772 s
tign + 2.761s
Figure 6: Baseline (HTPB) burning under GOX, chamber pressure of 13 bar, close visualization test. CCPs detaching
from regressing surface (compare this ~ 0.20 mm fragment with ~ 1 mm CCPs in Figure 5).
Pressure effect was investigated also for Paraffin wax formulation burned in GOX with pc of 13 and 16 bar. Due to
entrainment of melted fuel droplets, this fuel formulation exhibits regression rates that are 3 times higher than those
of HTPB under similar operating conditions. Regression rate versus Gox power-law approximation resulted quite
close to available literature data (see Table 6) [17][18]. Test conduced at chamber pressure of 13 bar are characterized
by high data fitting of power-law approximation of rf vs. Gox. This is related to the low Gox induced by relatively high
diameter measured at ignition. Poor mechanical properties of Paraffin fuel causes elevated diameter consumption at
ignition, thus combustion starts from relatively low oxidizer mass flux (with respect to nominal value). As a
consequence, analytical data trend does not exhibit definite shift from power-law trend. Equation (7) enables to
evaluate chamber pressure effect on regression rate for Paraffin-based fuel that results quite high under investigated
conditions. Even though more tests are required to properly investigate the phenomenon, it is possible that, under the
investigated conditions, change in chamber pressure alters entrainment phenomenon.
Table 6: rf vs. Gox for instantaneous ensemble curves for solid Paraffin wax burning under GOX. As for HTPB-based
formulation, data fitting results quite poor due to ballistic behavior at high Gox.
ar
nr
R2
13 bar
0.074 ± 0.001
0.634 ± 0.002
0.977
16 bar
0.064 ± .0001
0.635 ± 0.003
0.956
r f (Gox , pc ) = (0.448 ± 0.009) ⋅ Gox (t ) ( 0.635± 0.002) ⋅ pc ( −0.703± 0.007) , R 2 = 0.966
(7)
Ballistic analysis of Baseline formulation and non-loaded solid Paraffin wax enabled data-collection for relative
grading of metallized formulations.
Metallized HTPB-based fuel formulations differ from additive-particles size and mass fractions and were tested
under the same oxidizer volumetric flow rate of 210 nlpm but with different pc. HTPB loaded with 2% Carbon and
7
PP: HYBRID ORPHEE
10% nAl was tested under a chamber pressure of 10 bar while fuel formulation loaded with 2.8% MgB90 (20% Mg)
was tested at pc of 13 bar. Relative ballistic grading for the two fuel formulation is therefore defined with respect to
proper Baseline data. Additives mass fractions have been tailored so that MgB-loaded fuel contains half of the moles
of metal (Boron) of the nAl-loaded fuel.
3.5
Baseline (HTPB), Instaneous
Regression Rate, rf , mm/s
3.0
Baseline (HTPB), Time-Averaged
HTPB w 2%C and 10% nAl, Instantaneous
2.5
HTPB w 2%C and 10% nAl, Time-Averaged
Baseline (HTPB) (Power-law Approx)
2.0
HTPB w 2% C and 10% nAl (Power-law Approx)
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
350
400
2
Oxidizer Mass Flux, G ox , kg/(m s)
Figure 7: Ballistics of HTPB loaded with 2% C and 10% nAl burning under GOX. Relevant regression rate increases
are observed at high oxidizer mass flux (+70% with respect to Baseline at 380 kg/(m2s)).
Ballistics of HTPB loaded with nAl is reported in Figure 7. Four tests were performed providing the shown ensemble
curve. The latter is characterized by an ad-hoc tign of 0.006 s. This result comes from SPLab mathematical procedure
for time-resolved regression rate. Nevertheless, comparing this datum with corresponding Baseline ad-hoc tign value
under the same operating conditions (see Table 2) it is possible to note that addition of nAl reduces calculated
ignition delay. This finding is in agreement with possible considerations on nAl high reactivity possibly shortening
ignition delay of the fuel (see Table 1). nAl-doped fuel formulation reaches high regression rate increases,
nevertheless also sensitivity to Gox is elevated. As a consequence, regression rate enhancement of 70% with respect
to baseline at 380 kg/(m2s) reduces to +58% at 370 kg/(m2s) and is almost negligible at oxidizer fluxes below 150
kg/(m2s).
Under the investigated conditions fuel formulation loaded with MgB exhibits regression rate increases with respect
to baseline of nearly 45% over the overall tested Gox interval, see Figure 8. Ballistics dependence from Oxidizer mass
flux is weaker for MgB-loaded fuel than for HTPB doped with nAl as can be seen comparing Figure 7 with Figure 8.
HTPB loaded with MgB is characterized by an ad-hoc tign of ensemble curve of 0.038 s. This value is higher than the
one characterizing fuel loaded with nAl, though it results lower than that of Baseline (see Table 2). Considering
higher ignition temperature of MgB with respect to nAl (see Table 1), this result can be still related to the ballistics of
the fuel formulation. This performance are achieved by a fuel formulation where metal content is reduced in terms of
both mass and molar fractions with respect to nAl-loaded fuel. This could mean that a further addition of MgB could
enhance ballistic properties of tested fuel. On the other hand, when considering system performance, this could
enable lower performance losses related to lower mass fraction of CCPs originated by metal combustion. MgB
regression rate enhancement can be due to the dual nature of MgB powder. Magnesium present in the compound can
ease additive ignition also at reduced temperature/heat fluxes due to heterogeneous combustion of this metal.
Due to the relative ease of manufacturing and reduced dispersion problem of a micro-metric additive with respect to
a nano-sized one, MgB was tested also as energetic filler in a Paraffin-based fuel. At high oxidizer mass fluxes MgBdoped wax exhibits a significant regression rate increase with respect to both Baseline and non-loaded Paraffin (see
Figure 9). Starting from an oxidizer mass flux of 150 kg/(m2s) regression rate of loaded and non-loaded wax are
equivalent, while at Gox of 100 kg/(m2s) pure wax formulation exhibits a regression rate increase with respect to
baseline of +182%, while MgB-loaded Paraffin reaches a +149%.
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MICRO AND NANO-SIZED METAL ADDITIVES IN HYBRID ROCKET LAB-SCALE BURNER
3.5
Baseline (HTPB), Instantaneous
Regression Rate, rf , mm/s
3.0
Baseline (HTPB), Time-averaged
HTPB w 2.8% MgB90 (20% Mg), Instantaneous
2.5
HTPB w 2.8% MgB90 (20% Mg), Time-Averaged
Baseline (HTPB) (Power-law Approx)
2.0
HTPB w 2.8% MgB90 (20% Mg) (Power-law Approx)
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
350
400
2
Oxidizer Mass Flux, G ox , kg/(m s)
Figure 8: Ballistics of HTPB loaded with 2.8% MgB (20% Mg) under GOX. Note MgB regression rate increase over
the whole investigated Gox range.
10.0
Baseline (HTPB), Instantaneous
Baseline (HTPB), Time-averaged
Regression Rate, rf , mm/s
8.0
Paraffin Wax, Instantaneous
Paraffin Wax, Time-Averaged
6.0
Paraffin Wax w 2.8% MgB90 (20% Mg)
Instantaneous
Paraffin Wax w 2.8% MgB90 (20% Mg)
Time-Averaged
Paraffin Wax (Power-law Approx)
4.0
Paraffin Wax w 2.8% MgB90 (20%Mg)
(Power-law Approx)
2.0
0.0
0
50
100
150
200
250
300
350
400
2
Oxidizer Mass Flux, G ox , kg/(m s)
Figure 9: Ballistics of Solid Paraffin-based fuels burning under GOX. Under a Gox of 250 kg/(m2s), regression rate
increase for MgB-loaded Paraffin wax with respect to Baseline is + 312%. Non-doped Paraffin reaches + 255%.
An overview of achieved result is given in Table 7, Table 8 and in Table 9. In Table 8 percentage variations of rf
with respect to corresponding Baseline ballistics and their standard deviation, σ∆, are presented [34]. The higher σ∆
(with respect to ∆) the higher the formulation sensitivity to Gox variation. HTPB loaded with 10% nAl exhibits high
regression rate increases at higher oxidizer mass fluxes, but as Gox is decreased so is ∆, thus yielding high σ∆. MgBdoped formulation is characterized by almost constant regression rate enhancement with respect to baseline and is
therefore characterized by small σ∆. Similar considerations are valid for Paraffin-based fuels. With data collected in
Table 9 is possible to evaluate effects of solid fuel density, considering mass burning rate as a parameter.
Table 7: Power-law approximation of instantaneous rf vs. Gox for metal-loaded fuels burning under GOX. Note poor
data fitting for nAl-loaded fuel due to high sensitivity to Gox.
ar
nr
R2
a
HTPB w 2% C and 10% nAl
0.021 ± 0.003
0.662 ± 0.003
0.914
HTPB w 2.8% MgB90 (20% Mg)b
0.055 ± 0.004
0.542 ± 0.002
0.947
Paraffin Wax w 2.8% MgB90 (20%Mg)b
0.011 ± 0.001
1.015 ± 0.005
0.959
a
Tested under pc of 10 bar, b Tested under pc of 13 bar.
9
PP: HYBRID ORPHEE
Table 8: Percentage variations of rf with respect to Baseline ballistics and its standard deviation σ∆. Note high σ∆ for
nAl-loaded formulation when compared to averaged regression rate increase and with MgB data.
Instantaneous Gox
Averaged
100
150
200
250
275
300
330
350
370
HTPB
w C, nAl a
-2.5
4.3
11.0
18.6
23.0
28.4
36.9
45.0
57.9
24.7
19.6
HTPB
w MgB90 (20%Mg) b
35.8
38.6
41.2
44.0
45.7
47.5
50.4
53.0
/
44.5
5.8
182
205
228
255
/
/
/
/
/
217
31.3
149
196
248
312
354
407
498
/
/
309
122
Paraffin Wax b
Paraffin Wax
w MgB90 (20%Mg) b
a
%
∆
σ∆
Tested under pc of 10 bar, b Tested under pc of 13 bar.
Table 9: Percentage variations of fuel mass burning rate with respect to Baseline and increase standard deviation σ∆.
MgB fuel exhibits significant increases over the whole investigated Gox range.
Instantaneous Gox
Averaged
100
150
200
250
275
300
330
350
370
HTPB
w C, nAl a
5.7
13.0
20.3
28.5
33.4
39.2
48.4
57.2
71.2
30.7
17.5
HTPB
w MgB90 (20%Mg) b
38.2
41.0
43.7
46.6
48.2
50.1
53.0
55.7
/
47.1
6.0
176
198
221
247
/
/
/
/
/
211
30.5
149
196
248
312
354
407
498
/
/
307
122
Paraffin Wax b
Paraffin Wax
w MgB90 (20%Mg) b
a
%
∆
σ∆
Tested under pc of 10 bar, b Tested under pc of 13 bar.
5. Conclusions and Future Developments
HTPB-based and Paraffin-based fuels were tested by a 2D-radial lab-scale burner under GOX. Data were reduced by
a time-resolved technique for regression rate. Cured HTPB was taken as Baseline for relative ballistic grading.
Baseline exhibited relative independence of regression rate on chamber pressure under the investigated conditions.
Unloaded Paraffin tested under similar operating conditions with chamber pressure of 13 and 16 bar revealed a
higher sensitivity to pc.
Metal-loaded formulations exhibited significant regression rate increases. With respect to Baseline, MgB-loaded
HTPB exhibited a mass burning rate increase of 50% at 300 kg/(m2s) and a low sensitivity to Gox. nAl-doped HTPB
is characterized by significant mass burning rate increases at high oxidizer mass fluxes but sensitivity to oxidizer
mass flux decreases this performance enhancement as combustion proceeds. Paraffin loaded with MgB enables
further regression and mass burning rate enhancement with respect to non-loaded wax and Baseline formulation.
Starting from achieved results, the possible tailoring of performance of hybrid fuel formulation combining nAl
regression and mass burning rate enhancement at high Gox with MgB overall performance appear as a promising step
in HRE development. In this respect, performance enhancement of coated-nAl powders should be considered [34].
10
C. Paravan et al.
MICRO AND NANO-SIZED METAL ADDITIVES IN HYBRID ROCKET LAB-SCALE BURNER
Acknowledgements
This work has partially been supported by:
• CNES (Commande No. 4700024752/DLA090 and Commande No. 4700028003/DLA094);
• European Framework Program No. 7 (Theme 9: Space, program ORPHEE - Operative Research Program
on Hybrid Engine in Europe, ORPHEE – WP 320 under Grant Agreement No. 218830).
The authors wish they could acknowledge Advanced Powder Technology LLC (Tomsk, Russia) and Mach I (King of
Prussia, Pennsylvania, USA) for the supplied materials and the precious collaboration.
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