Trans. JSASS Space Tech.

Japan
Vol. 7, No. ists26, pp. Pa_91-Pa_95, 2009

A Study on New Composite Thermoplastic Propellant

By Takehiro KAHARA1), Masanobu NAKAYAMA2), Hiroshi HASEGAWA3), Kazushige KATOH3), Shigehumi

MIYAZAKI4), Haruki MARUIZUMI4), Keiichi HORI5), Yasuhiro MORITA5) and Ryojiro AKIBA6)
1)
The Institute of ISTS, Hamamatsu, Japan
2)
Tokyo Institute of Technology, Tokyo, Japan
3)
NOF Corporation, Aichi, Japan
4)
IHI Aerospace, Gunma, Japan
5)
ISAS/JAXA, Kanagawa, Japan
6)
USEF, Tokyo, Japan

(Received April 25th, 2008)

Efforts have been paid to realize a new composite propellant using thermoplastics as a fuel binder and lithium as a
metallic fuel. Thermoplastics binder makes it possible the storage of solid propellant in small blocks and to provide
propellants blocks into rocket motor case at a quantity needed just before use, which enables the production facility of solid
propellant at a minimum level, thus, production cost significantly lower. Lithium has been a candidate for a metallic fuel
for the ammonium perchlorate based composite propellants owing to its capability to reduce the hydrogen chloride in the
exhaust gas, however, never been used because lithium is not stable at room conditions and complex reaction products
between oxygen, nitrogen, and water are formed at the surface of particles and even in the core. However, lithium particles
whose surface shell structure is well controlled are rather stable and can be stored in thermoplastics for a long period.
Evaluation of several organic thermoplastics whose melting temperatures are easily tractable was made from the standpoint
of combustion characteristics, and it is shown that thermoplastics propellants can cover wide range of burning rate
spectrum. Formation of well-defined surface shell of lithium particles and its kinetics are also discussed.

Key Words: Solid Propellant, Thermoplastics, Lithium, Low Cost, Low Pollution

1. Introduction storage process significantly.

Development of solid rockets with a reduced cost and This study evaluates some kinds of thermoplastic material
higher reliability has been widely discussed and is a major as a fuel binder mainly from burning rate characteristics, and
concern for solid rocket community, especially, the heavy investigated surface modification of Li particles to improve
consideration of propulsion system which occupies about its safety and stability characteristics.
forty percent of all launch cost is essential. At the present,
composite solid propellants are thermosetting 2. Experimental
hydroxyl-terminated polybutadiene (HTPB) fuel binder 2.1. Sample propellant
based, and once cured, can not change the grain shape. If the AP or potassium perchlorate (KP) were used as an
thermosetting resin can be replaced with thermoplastic resin, oxidizer, and WAX (Nippon Seiro, m.p.47 degrees
it is expected to reduce the vulnerability of the production centigrade), Polyester-polyol (PEP, UBE Industries),
management and the production facilities, which lower the Kraton-resin were used as candidates of thermoplastic
propellant cost significantly. Purchase of raw materials binder.
independently of loading schedule, continuous production Two kinds of PEP were studied, “PEP1”;mol.wt.3020 and
with a small quantity, and storability may be the major m.p.65 degrees centigrade and “PEP2”;mol.wt.3500 and
factors of favorable production economy 1). m.p.55 degrees centigrade. Both polymer has a poor
Hydrogen chloride is included in the exhaust gas of mechanical property, therefore, HTPB was mixed with them
conventional solid propellant composed of ammonium to make them elastomeric.
perchlorate (AP), Al and polymeric fuel binder at about Kraton-resin is a thermoplastic made by Kraton company
twenty percent mole fraction and causes a negative effect to (USA), and it has an excellent elasticity comparable with
the local environment. If the present metallic fuel Al is cured HTPB when mixed with the plasticizer oil, and has the
replaced with lithium, the amount of HCl can be reduced to softening point at 70 degrees centigrade. The content of the
0.4% and the specific impulse enhanced by 2.4%. Lithium is plasticizer oil depends upon the propellant composition, and
rather mild as alkali metal, however, safety consideration of in the binary system of AP and Kraton, the oil is to be mixed
Li fine particles has retarded its application to solid rocket in the resin at resin:oil=1:4(mass), in the ternary system of
propellants. However, the usage at large boosters having long Kraton-resin, AP and Al, resin:oil be 1:10(mass). The
residence time has a benefit for the complete combustion for composition of sample propellants are shown in Table 1.
large particles which reduce the hazard in the production and

Copyright© 2009 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.
Pa_91
 Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

Table 1. Composition of sample propellants. surface, probably in droplet form and evaporates into the gas
phase, AP particles, usually burn at burning surface, may be
Sample Fuel binder Oxidizer blown off from surface as they are. AP may ignite and burn
No. WAX PEP1 HTPB KP AP in the gas phase and the oxidizing gases from AP form
#1 20 80 envelope type flame surrounding Wax melt droplets, and the
#2 20 80 evaporation process being slow, the flame zone in the gas
#3 20 80 phase becomes very broad.
#4 10 10 80
500
#5 8 12 80

#1(KP/WAX)

LINEAR BURNING RATE (mm/s)

Sample Fuel binder Ox. Metal
No. PEP2 Kraton HTPB AP Al
100
#6 10 10 80
#7 8 12 80
#8 20 80
#9 15 80 15

2.2. Burning rate measurement

The linear burning rates of the sample propellants were 10
measured with a strand burner. Typically, 6mm*6mm*70mm #2(AP/WAX)
strand sample was set in the burner at the initial temperature
of 20℃ and purged with dry nitrogen gas. Pressure of the
burner was carefully controlled and the burning rates were 3
1 5 10
measured by the time interval of break wires. PRESSURE (MPa)
2.3. Surface treatment of Li particles
When Li was mixed with AP and kept at 60℃, strong Fig. 1. Burning rates of #1 & 2 propellants.
exothermic reaction was observed, thus, it is inevitable to
find a way to make Li particles safe in the mixing process
and Li-based solid propellants storable. Simplest way is to
form stable layer at the surface of Li particles to avoid the
contact with AP particles and the penetration of gaseous
species to Li molecules inside the particles.
The formation of the layers of Lithium carbonate and
Lithium nitride were studied with a reactor whose
temperature is well controlled. Carbon dioxide gas with or
without oxygen gas, and nitrogen gas were introduced at a
predetermined flow velocity to the Li sample at elevated
temperatures.

3. Results & Discussion

3.1. Burning rate characteristics Fig.2. Image of #1(KP/Wax) propellant combustion.
The linear burning rates of the WAX propellants (#.1 & 2)
are shown in Fig.1. Burning rate or KP/WAX (#1) is very Such high burning rates are not suitable for usual rocket
high; about 200mm/s(@5MPa), and AP/WAX (#2) is rather motors having internal burning type grain, however, the
slower; about 30mm/s(@5MPa). From the observation using application to the motors which deliver high thrust within
medium speed camera, approximately 1mm thickness of melt short periods with end-burning grain is feasible.
layer was found at the burning surface and luminous flame Burning rate of PEP1/AP propellant (#3) is shown in Fig.3.
spread very wide in the gas phase zone (Fig.2). Temperature Both of burning rate and pressure exponent are modest.
profile during combustion was not measured, however, heat However, the mechanical property of #3 was very poor (stiff),
feedback from gas phase flame may penetrate into the HTPB was added to this sample at two different ratios (#4 &
condensed phase profoundly due to relatively high thermal #5 propellants). Those burning rates are shown in Fig.4. Both
conductivity of Wax comparing with polymeric material for burnng rates are also modest and the dependency to the
fuel binder. Low melting temperature of Wax may lead the amount of HTPB is small.
melting process of Wax binder very fast, and this process As shown in Fig.3, burning rate of #3 propellant scatters
may dominate other processes and realize the very high significantly, however, such a scatter is suppressed with the
burning rate. Another characteristics of this burning addition of HTPB as in #4 and 5 propellants (Fig.4). This
phenomenon, very broad gas phase flame may come from scatter seems to be derived from the heterogeneity of the
evaporation process of Wax melt. Wax melt departs from combustion phenomena of #3 propellant itself.

Pa_92
 T. KAHARA et al.: A Study on New Composite Thermoplastic Propellant

combustion heterogeneity.
7 PEP2 was tested instead of PEP1 in a same manner.
6 #3 propellant Burning rate is shown in Fig.6. From the comparison of
LINEAR BURNING RATE (mm/s)

5 Figs.4 and 6, the melting point of thermoplastics has

eventually no effect on the burning rate.
4

3 10

LINEAR BURNING RATE (mm/s)

9
8
7
2
6
5

4
#6
1
1 5 10 3 #7
PRESSURE (MPa)

Fig. 3. Burning rate of #3 propellant. 2

1 5 10
10 PRESSURE (MPa)
LINEAR BURNING RATE (mm/s)

Fig. 6. Burning rates of #6 & 7 propellants.

5 Burning rates of Kraton-resin/AP propellant (#8) and

Kraton/AP/Al propellant (#9) are shown in Figs. 7 and 8.
Although, pressure exponents are relatively high, the range
of burning rates are quite adequate.

8
#4
LINEAR BURNING RATE (mm/s)

7
#5 #8 Propellant
6
5
1 4
1 5 10
PRESSURE (MPa) 3

Fig. 4. Burning rates of #4 & 5 propellants.

2

1
1 2 3 4 5 6
PRESSURE (MPa)

Fig. 7. Burning rate of #8 propellant.

10
LINEAR BURNING RATE (mm/s)

9
8 #9 Propellant
7
6

Fig.5. Image of #3 propellant combustion. 5

4
Figure 5 shows the image of combustion of #3 propellant.
Obviously, individual flame can be identified in this picture. 3
This shows that AP particles burn at burning surface and
luminous flames between the vapor of PEP are formed
following AP flames. And large amount of residual material 2
1 2 3 4 5 6
(dark color) are formed at surface melt and flow along the
PRESSURE (MPa)
side surface of the strand, which might be the cause of the
Fig. 8. Burning rate of #9 propellant.

Pa_93
 Trans. JSASS Space Tech. Japan Vol. 7, No. ists26 (2009)

Fig.9. Image of #8 propellant combustion. Fig. 10. Li3N formation.

Figure 9 shows the image of combustion of #8 propellant. As described above, Li3N formation process is
As it can be seen, this propellant has the typical features of heterogeneous and regarded as crystal growth type, and
metalless composite propellant. Streaks composed of AP nitrogen atom migration process in Li3N lattice may be the
flame and following diffusion flames are observable from the essential step in this process. Figure 11 shows the structure of
burning surface and residues from fuel binder are blown off Li3N and assumed spreading routes of nitrogen atom to a
to gas phase. Burning rate characteristics of this propellant , neighboring hole in the crystalline lattice (Route 1 and Route
therefore, may be controllable with size distribution of 2). These two routes were compared numerically in first
oxidizer particles and catalysts. principles. Figure 12 shows the activation energy of each
As described above, the mechanical property of resin itself route and it can be seen that route 2 is favorable and yet both
is excellent and comparable with HTPB, however, adhesion routes have high activate energy.
with AP particle appears to be not enough. Improvement of
the adhesion using some additives such as bonding agents for
AP/HTPB system is necessary.

3.2. Surface treatment of Li particles

Lithium carbonate is most stable in Li-based compounds,
the formation of LiCO3 layer on Li was studied at first. Only
carbon dioxide gas application has created nothing, therefore,
oxygen gas was added to carbon dioxide gas. Lots of tests
were conducted changing temperature, gas flow rate and gas
ratio, however, no formation of LiCO3 was observed.
Lithium nitride formation was studied under the
environment of pure nitrogen gas at room temperature,
however, Li did not react and keep metallic luster. A small
quantity of moisture was introduced to pure nitrogen because
moisture is known to act as a catalyst in the Li3N formation
process 2). All of Li samples were covered with white and
black spots, which shows the evidence of the formation of
lithium hydroxide along with Li3N 3) 4). Thus, it was found
that pure Li3N formation was difficult with this method.
Next attempt was to enhance the temperature 5).
Experiments were conducted at 100 degrees of C with dry
nitrogen gas, and nitrogen purge was kept for several hours.
Figure 10 is the photograph of the sample after 12hours and
shows the reaction progresses heterogeneously, which
showed the formation of homogeneous film on the surface
of Li is impossible. Consequently, We decided to give up the
formation of thin layer of Li3N and try to make the whole Li Fig. 11. Li3N structure and Route 1 and 2.
into Li3N because Li3N still has an advantage to Al in
specific impule. In this case, reaction was completed in 24
hours in the case of 6.3mmφ*10mm sample.

Pa_94
 T. KAHARA et al.: A Study on New Composite Thermoplastic Propellant

3.5 5. Summary
Three kinds of thermoplastic; WAX, polyester-polyol and
3 Route 1
Kraton-resin were studied as fuel binder of composite
ACTIVATION ENERGY (eV)

Route 2
2.5 propellant. As for the linear burning rate, wax propellant is
very high, and polyester-polyol and Kraton-resin propellants
2 are in a modest range. WAX and polyester-polyol propellants
have problems in mechanical properties, and the adhesion
1.5
between Kraton-resin and AP is to be improved.
1 From the viewpoints of low HCl emission and higher
specific impulse, the use of lithium as a metallic fuel of
0.5 composite propellant was evaluated. From the safety and
0
stability consideration, Lithium carbonate and Lithium
0 0.2 0.4 0.6 0.8 1 nitride were taken up as the candidates for the protective thin
NORMALIZED POSITION layer at the surface of Li particles, and their formation
processes were studied. It has been shown experimentally
Fig. 12. Comparison of activation energy in Route1 and 2. that Carbonization is very slow and nitrization is quite
heterogeneous.
In addition, defect formation processes were also The only way to establish safety and stability is nitrization
estimated as a function of temperature. Followings are the of the whole particles. This process was numerically studied
possible two processes, (1); shottky and (2); nitrogen release. and the mechanism and the kinetics were made clear.
Compatibility test revealed that use of large Li3N particles
null → VN + 3VLi (shottky) (1)
whose diameters are more than 500µm and at the dry
condition less than 10% humidity are necessary to keep
null → VN + 3e + 1/2N2 (nitrogen release) (2)
safety in the production and storage of the Li3N based
composite propellants. Furthermore, the establishment of the
The result is tabulated in Table 2. Probabilities are very coating technology is preferable for stabilization and safety
low, however, it can be seen that temperature has a strong of Li3N propellant.
effect and shottky path is much more probable.
References
Table 2. Comparison of probability.
Temperature (℃). Shottky1. N2 release 1) Morita, Y., Hori, K., Koreki, H. and Akiba, R.: A study on the next
generation solid Rocket, Proceedings of Symposium on Space
0 1.73E-18 1.24E-106 Transportation, ISAS/JAXA 2008, pp.22-25.
50 1.01E-15 3.02E-90 2) Yamamoto, T., Yoshikawa, S., Koizumi, M.: Formation of Li3N thin
film by nitriding the surface of lithium metal and its application to
100 1.07E-13 2.99E-78
battery, Journal of the Ceramic Society of Japan, 93 (1985),
150 3.78E-12 4.32E-69 pp.728-731.
3) McFarlane, E. F. and Tompkins, F. C.: Nitridation of Lithium, Trans,
4. Screening test for propellant production Faraday Soc, 58 (1962), pp.997-1007.
4) Deslandres, M. H.: Absorption de lazote parle lithium a froid,
Before producing solid propellant using Li3N as a metallic Comptes rendus. 121 (1895), pp.886.
fuel, compatibility of Li3N with Kraton-resin and AP was 5) Ishii, T.: A study on Lithium nitride, Journal of the National Institute
evaluated. Comparatively large particles (dia. about 500µm) for Research in Inorganic Materials, 46 (1986), pp.5-8.
and small powders whose diameters less than 100µm were
used.
At all conditions, the compatibility between Kraton-resin
and Li3N was good. However, the case with AP is complex.
With large particles, Li3N is compatible with AP. However,
with smaller particles, ignition was observed just after the
contact at the room condition with 30% in humidity, while
under the dry condition less than 10% humidity, the mixture
was stable. Consequently, production of Li3N containing
solid propellant must be under dry condition less than 10%
humidity from the safety consideration.

Pa_95