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CHAPTER-1
INTRODUCTION
CHAPTER 1
INTRODUCTION
A propellant is a low explosive material, which undergoes rapid and
predictable combustion without detonation, resulting in generation of a large
volume of hot gases. These gases are used to propel a projectile or warhead
to the target. In order to produce gas quickly, propellant must carry its own
oxygen together with suitable quantities of fuel elements like carbon,
hydrogen etc. In homogeneous propellants, fuel and oxidizer are contained
in the same molecule e.g. nitrocellulose (NC) and nitroglycerine (NG).
However, in case of heterogeneous propellants, fuel and oxidizer are
contained in separate compounds. Gun propellants are traditionally known
as homogeneous propellants. Rocket propulsion uses both heterogeneous
and homogeneous propellants.
1.1 Gun Propellants and their characteristics
Gun propellants are designed to provide large quantities of gases,
which are used to propel projectiles at high kinetic energy. The velocity of
the projectile is dependent on the rate at which the gases are produced. This
in turn is dependent on the amount of chemical energy released. The
efficiency of gun (E) is given by the equation,
E = ^ ^ (1)
Where, E = Efficiency of gun
V = Muzzle velocity of the projectile
Y = Specific heat ratio of gases
W = Weight of projectile
C = Charge mass of propellant
F = Force constant of propellant
The amount offeree exerted on the projectile by the combustion gases
is dependent on the quantity and temperature of the gases and is expressed
as,
F = nRTo = ^ ^ - -(2)
M
where, F = Force constant of propellant
n = No. of moles of gas produced from propellant (mol./g)
To= Adiabatic flame temperature (Kelvin) of combustion gases
M = Mean molar mass of combustion gases
The value of 'F' is a useful parameter for comparing the performance
of gun propellants and can be determined experimentally by burning a known
quantity of propellant inside a closed vessel bomb (CV). The main
constituents of the propellant combustion gases are H2, CO, CO2, H2O and N2.
The search is always in the direction of low molecular weight gas
products, because the sonic velocity in the combustion gases should be as
high as possible to minimize the pressure gradient between the chamber
pressure and the pressure acting on the base of an accelerating projectile.
There is always an undesirable pressure drop at the projectile base, which is
inversely proportional to the local velocity of sound in the gases. The rate of
rise of pressure in the gun is controlled by the rate of gas generation from the
burning propellant and the rate of volume increase by the moving projectile.
The rate of gas generation is described by the equation ',
m = p s r - (3)
where, m = rate of mass generation of propellant gas
p = density of solid propellant
s = surface area of the propellant
r = linear burning rate of the propellant
Thus, surface area of the propellant is one of the important parameter
to control bum rates and depends upon propellant geometry. The different
geometries used for gun propellants are ribbon, tube, multi-tube (seven,
nineteen and thirty seven perforations). These are shown in Fig. 1.1. The
relationship between the fraction of propellant burnt and the fraction of
ballistic size remaining at any moment is known as the form function. The
rate of gas generation must be controlled through control of the surface area,
because the linear burning rate and solid propellant density are physical
constants for a given type of propellant. Most important aspect of any grain
design is the surface progressivity i.e. the increase in surface area as a
function of the distance burned into the grain. A thin disc maintains almost
the same surface area throughout the burning process and its form function
of burning rate is close to zero, whereas tubular granules are less regressive
than cylindrical cords. Multitubular propellants possess negative form
function coefficient and are called to be progressive. The multi-tube granules
produce more progressive burning.
Conventional solid gun propellants continue to be investigated for
improvements. However, primary focus of the various studies has been to
reduce cost, increase performance reliability and survivability, enhanced
muzzle velocity and muzzle energy as well as system effectiveness and
efficiency.
In all guns, the performance is also constrained by the attainable
maximum pressure and acceleration that can be tolerated by the mechanical
parts and the projectile. Higher performance requires more energy and
proper programming of total energy release.
Fig 1.1 Different shapes of Gun Propellant
®1 ®1 I® iS^
Tubular Propellaut
Propellant with 7 hole geometry
Propellant with 19 hole geometry
Propellant with 37 hole geometry
Gun propellants are broadly divided into four different classes
namely, single base propellant (SBP), double base propellant (DBP), triple
base propellant (TBP) and nitramine based propellants or advanced
propellants. Single base propellants (SBP) are used in all kinds of guns from
pistol to artillery. The main ingredient of SBP is 90% of nitrocellulose (NC)
having nitrogen content from 12.2 to 13.2 %. Nitrocellulose is gelled with
solvent such as ether /alcohol mixture in 60 : 40 ratio and a plasticizer like
diethyl phthalate (DEP), dibutyl phthalate (DBP) and a stabilizer like
carbamite (symmetrical diethyl diphenyl urea) are added and propellant is
extruded into long strands and cut into required shape and size. The
operations after cutting and vacuum drying include surface moderation of
propellant in a sweetie pan, water steeping for enhancing the burning rate of
propellant and final drying, followed by glazing by graphite. Final
operations are blending and packing.
The force constant of single base propellants varies between 900 and
1000 J/g. Single base propellants have low flame temperature and hence are
less erosive. The burning rate of single base propellants can be controlled
by moderation level. They are less sensitive to impact and friction.
However, due to hygroscopicity in the propellant, irregularity in burning is a
major drawback of single base propellants. The scope of improving the force
constant of single base propellants is limited due to acute oxygen deficiency
inNC.
In order to raise the force constant of single base propellant, NC is
plasticised with nitroglycerine (NG) to form double base propellants (DBP).
DBP are manufactured by using NC-NG paste which is made by slurry
method. NG is absorbed in NC fibers in water medium and NC gets
gelatinized. This is filtered and dried. NC-NG paste is further mixed with
solvents like mixture of ether and alcohol. After addition of other ingredients
to this NC-NG paste, dough is formed and extruded into required shape.
Force constant of double base propellants varies between 950 and 1200 J/g
with comparatively higher flame temperature. These propellants are used in
pistols, mortars and tank guns. Double base propellants are highly energetic
by virtue of high flame temperature.
Due to inclusion of NG, the major disadvantage of double base gun
propellants (DBP) is the excessive erosion of the gun barrel, caused by the
higher flame temperature and the muzzle flash, which reveals gun location.
The muzzle flash is the result of fuel-air explosion of the combustion
products (carbon monoxide and hydrogen gases). In order to reduce the
muzzle flash and flame temperature of double base propellants,
nitroguanidine (NQ) to the tune of 30 - 55% is added to NC and NG matrix
to form triple base propellants (TBP). Introduction of nitroguanidine results
in lowering of flame temperature by about 300 K, without any penalty on
energy. Consequently, gun barrel erosion and muzzle flash are reduced.
TBP offers numerous advantages such as cool flashless burning, lower flame
temperature, non-hygroscopic nature, better shelf life, higher density, ease of
manufacture, safety and regularity in ballistic performance even at subzero
level temperatures. Triple base propellants are used in tank guns, large
caliber field guns and naval guns.
High energy propellants containing nitramines (RDX/HMX) are used
in tank guns, where higher energy is needed mainly for fin stabilized armour
piercing discarding sabot (FSAPDS) type projectiles.^ The main source of
energy in high energy propellants is RDX along with NC. The high energy
propellants exhibit higher energy (1100 to 1200 J/g), thereby imparting very
high muzzle velocity of the order of 1600-1800 m/s to FSAPDS type
projectile, but suffer from the drawback of causing high barrel erosion due to
high flame temperature^. However, type of propellants discussed above
suffers from the possibility of accidental initiation from fire, impact, electric
spark etc. Hence, a new class of propellants is under focus of development
for insensitive munitions (IM) with particular emphasis on low vulnerability
ammunition (LOVA). LOVA propellants contain RDX or HMX as filler, an
inert or energetic polymeric material as binder with suitable plasticizer. The
1̂ ' generation LOVA propellants were made with polyurethane binders,
which provided good mechanical strength to the propellant. Since
polyurethane binders are cross linked, they cannot be recycled. The 2"
generation of LOVA propellants utilize cellulosic binders like Cellulose
Acetate (CA), Cellulose Acetate Butyrate (CAB) etc. whereas 3'̂ '' generation
LOVA propellants use the energetic thermoplastic elastomeric binders.
These composite propellants are less vulnerable to initiation than
nitrocellulose based propellants. The important properties of different
ingredients used in propellants are given in Table -1.1 and that of different
classes of propellants are given in Table-1.2 .
Table-1.1 : Characteristics of Gun Propellant Ingredients
Ingredients
Nitrocellulose (NC)
Nitroglycerine (NG)
Nitroguanidine (NQ)
Cyclotrimethylene trinitramine (RDX)
Heat of formation
(k Cal/mol) -149.77
-83.97
-19.24
-16
Density (g/cc)
1.66
1.59
1.77
1.80
Heat of combustion (k Cal/mol)
650
364
208
501
Oxygen balance
(OB) -24.2
+3.5
-30.75
-21.6
TabIe-1.2 : Properties of Conventional Propellants
Propellant
Single base
(SBP)
Double base
(DBP)
Triple base
(TBP)
High energy
Propellant
Density (g/cc)
1.58
1.61
1.62
1.65
Flame temperature (K)
2500-3000
2600-3600
2400-3300
3200-3300
Force Constant (J/g)
940-1020
940-1180
950-1140
1150-1180
Gun propellants can also be classified based on the method of
processing Solvent, Solvent-less and Semi-solvent.
Single base propellants are processed with the mixture of ether and
alcohol, whereas double base propellants are processed with the mixture of
acetone-water or ether-alcohol. For triple base propellants, solvents used
include ether - alcohol or acetone - alcohol mixtures, depending upon the
type of NC used. The solvent processed propellants are required to be dried
upto 0.5% of volatile matter (VM) in the propellant. Otherwise, there is
possibility of deformation and change in ballistics after curing of the
propellant. The main advantage of solvent process is, the safety of
processing. In semi-solvent process, mixture of ingredients is converted into
dough form and further extruded by hot extrusion technique. Gelatinisation
is achieved by hot rolling (60-70°C). For solvent-less propellant, NC - NG
paste is rolled at high temperatures in a rolling mill and sheets of uniform
thickness are obtained. These sheets are further used for pressing into a
propellant by hot extrusion technique. Semi-solvent and solvent-less
propellants offer an advantage of regular ballistics by virtue of non-
shrinkage during processing and drying of propellants. Propellants with
higher web sizes are produced by solvent less technique.
Existing ammunitions containing conventional gun propellants are
highly vulnerable to external stimuli. This is due to the presence of nitric
esters (NC and NG), which make them sensitive to initiation by high
velocity impact of hot spall fragments and shaped charge jet. This results in
an accidental and unplanned initiation of stored ammunition in the armoured
vehicle, resulting in high casualty and loss of costly equipment'*. Hence, the
concept of Low Vulnerability Ammunition (LOVA) has been evolved.
LOVA propellants are specially designed to mitigate, projectile and
fragment impact threats, under condition of ammunition storage. To achieve
these objectives, conventional propellant ingredients such as NC, NG and
NQ are reduced or replaced by other materials such as RDX or HMX. The
most significant current trend in munition and propellant technologies is the
development of insensitive munition (IM) LOVA propellants contains fine
particle size of RDX or HMX in an inert or energetic binder along with other
additives. Polymeric binder systems have a profound effect on almost all
properties of LOVA propellants. Judicious combination of high energy
oxidizers, binders and plasticizers are employed to obtain required
insensitive high energy LOVA propellants. Replacement of existing
conventional propellants by LOVA propellants is highly advantageous in
terms of life, safety, ballistics and ageing characteristics. Thus, life cycle
cost of LOVA propellants is another major attraction^.
IMs are expected to bring about a true revolution in handling and
stocking procedures of propellants and ammunitions.
1.2 LOVA Propellant Ingredients
1.2.1 Oxidiser
RDX and HMX are widely reported to be used as an energetic
component of LOVA gun propellants to achieve low vulnerability due to
their thermal stability (decomposition > 200°C) and positive heat of
formation (+14 & +17 k Cal/mol, respectively). XM-39 and M-43
propellants developed by USA are well reported in literature^. XM-39
consists of 76% RDX, 12% cellulose acetate butyrate (CAB), 4% NC and
7.6 % plasticizer namely, acetyl triethyl citrate (ATEC) and 0.4 %
stabilizer, carbamite. For XM-43, the inert plasticizer ATEC is replaced by
an energetic plasticizer, 1:1 mixture, of bisdinitro propyl acetal (BDNPA)
and bisdinitro propyl formal (BDNPF). During 1999, a new LOVA
propellant was designed with 2,4,6,8,10,12- hexanitro-2,4,6,8,10,12-
hexaaza tetracyclo 5,5 dodecane (CL-20) as main energetic ingredient by
Mueller . The force constant achieved was 1253 J/g with flame temperature
(Tf) of 3700°C. This formulation consists of CL-20, RDX, NC and
BDNPF/A .
Schedlbauer^ studied LOVA gun propellants containing RDX, HMX
nitroguanidine (NQ), triamino guanidine nitrate (TAGN) and guanidine
nitrate (GN) with thermoplastic materials like cellulose acetate butyrate
(CAB) and energetic binders like glycidyl azide polymer (GAP). He found
that the impetus for 8% TAGN based propellant was 1164 J/g. However, the
pressure index was more than one. New energetic ingredients emerging from
research will form the basis for the next generation of solid LOVA gun
propellants, where impetus values could be more than 1400 J/g with low
flame temperature (Tf).
A number of methods have been described to improve the
performance output of gun propellants.^ Composite propellants with
energetic nitramine litce triamino guanidine nitrate (TAGN), exhibit higher
performance, when they are combined with energetic binder-plasticizer
systems. The calculations of performance parameters showed that GAP
binder exhibits excellent specific energy as compared to polybutadiene (PB)
based propellants. The main reason being better oxygen balance of GAP
(-41.15%) than PB (-53.2%) and high energy release due to azide group.
Compositions based on RDX/TAGN/GAP (43:43:14) showed specific
energy of the order of 3949 J/g with force constant 1244 J/g and flame
temperature 2952 K.
Solid oxidizers examined by Simmons^ include CL-20, RDX, 1,33-
trinitroazetidine (TNAZ), ammonium dinitramide (ADN), triamino
guanidine azide (TAGAZ), triamino guanidine nitrate (TAGN),
nitroguanidine (NQ) and diazido nitramines namely 1,7-diazido- 2,4,6-
trinitrazaheptane (DATH), l,6-diazido-2,5-dinitrazahexane (DADNH) and
1,5 -diazido 2,4-dinitrazapentane (DADZP) and a liquid oxidizer like 1,3-
diazido-2-nitrazapropane (DANP). Among these ingredients, DANP is a
liquid, while DATH, DADNH and DADZP are solids. The impetus values
reported for DANP, DADZP, DADNH, and DATH are 1761,1650, 1504J/g
and 1481 J/g, respectively. Due to higher density of CL-20, propellant
containing CL-20 will have the highest volumetric energy.
Physico-kinetical mechanisms of combustion of energetic oxidizer
CL-20 was investigated with 3,3-bis (azidomethyl)oxetane / tetrahydrofuran
(BAMO-THF), 3,3-bis (azidomethyl) oxetane / 3 - azidomethyl - 3 -
methyloxetane (BAMO-AMMO),cyclic nitramines like HMX and
polycyclic caged nitramines. HNIW (CL-20) as an oxidizer, mixed with
U
modem active binders offer many advantages like higher power, superior
performance, higher safety, environmental friendliness and possibility of
recycling. Zenin et al'* investigated combustion of these propellants using
micro thermocouple techniques. Physical parameters of burning waves of
CL-20 based mixture showed that surface burning temperature was 100° C
lower than HMX based mixtures. Heat feedback was also lesser at elevated
pressures, whereas heat release in solid phase was about twice lower than
HMX based compositions and rate of release of heat was thrice than HMX
based mixtures. Different gasification laws were established for CL-20 and
HMX based compositions. Burning rate control regions in the combustion
waves were the regions of the heat release in solid and thin low temperature
gas layer near the burning surface and wide reaction zone of chemical
reactions with distributed heat release rate in the gas phase.
An improved gun propellant with TNAZ and NC has been patented
by Manning et.al " , wherein the impetus of propellant was claimed to be 1 7
1350 J/g. Schaffers and Stein studied different LOVA compositions in 35
mm LSP (Less Sensitive Propellant ) test generally applied for the
investigation of the propellant for vulnerability against shaped charge jet .
These compositions of LOVA propellants contained FOX-7 / NC / RDX.
FOX-7 (dinitro diamine ethylene) based composition was found to be less
sensitive than RDX based composition with lower performance.
New insensitive Modular Charge System, UNIFLEX 2, is based on
the mixture of GUDN, a stable salt of dinitramide C2N4OH7 (N[N02])2 also
referred as FOX-12, with RDX and the binder system of nitrocellulose
plasticized with butyl-NENA'^. A typical composition with 60% GUDN
generates a burning rate of approximately 40 mm/s at 100 MPa with two
12
different burning rate exponents over the pressure regime. Delay time for
ignition of GUDN is also half that of RDX.
Susan peters et al''* have reported that an advanced LOVA propellant
comprising of 76% RDX with 24% cellulosic binder gave impetus of 1157
J/g and Tf of 3042 K, whereas 24% BAMO-AMMO produced impetus of
1182 J/g with Tf of 2827 K. Replacement of RDX by CL-20 in the same
formulation realized impetus of 1291 J/g and Tf of 3376 K. LOVA
propellant with 56% CL-20 and 24% BAMO-AMMO gave force constant of
1247 J/g with Tf of 3217 K .These gun propellants were processed by
solvent method.
1.2.2 Binders
A binder plays an important role in resisting conductive ignition by
hot metal particles. Propellants whose binders decompose endothermically
are considered excellent. On the other hand, exothermically decomposing
binder ignite as easily as standard nitrate ester propellants.
The V^ generation LOVA propellants were made with polyurethane
binders, which provided good mechanical strength to the propellant'^ The
2"'' generation of LOVA propellant utilize cellulosic binders like cellulose
acetate (CA), cellulose acetate butyrate (CAB) etc. The 3'̂ '' generation
LOVA use energetic thermoplastic elastomeric binders. The solvent less
process of producing a solid extruded propellant grain having a smooth
surface and predictable burn characteristics is patented by Kristofferson et
al ' . Inclusion of HMX is reported to enhance pre-cure extrudability of the
propellant grain. Ulrike has developed LOVA gun propellants with inert
binders CA, CAB and HTPB with 60-80 % nitramines, whose force
constants were comparable to conventional JA-2 propellant (1140 J/g).
These propellant formulations were evaluated for ballistics in closed vessel
13
(CV) and results were matching with theoretical predictions. These
propellants were examined for the mechanical stability by the static
compression of the propellant grains at 300 bar at +21 and -40°C and then
evaluated in closed vessel for dynamic vivacity in comparison to the original
propellant at 0.2 g/cc loading density at 2 r C . The increase in surface area of
compressed propellant was calculated by linear regression method. The
propellants were dynamically tested in 40 mm stimulator. It was
recommended that these propellants can be adopted for middle and large
caliber ammunition. Similar studies using HTPB and CAB were carried out
by Mackowick'^. The propellants with polybutadiene binder cured at 50°C
using non toxic components for curing were evaluated. The propellant had
impetus in the range of 884 to 1050 J/g, where RDX percentage varied from
71 to 83, and binder (HTPB) varied between 15 and 21%. Flame
temperature (Tf) was in the range of 2200 to 2500 K. However, the pressure
exponent (a) was more than one. The dynamic vivacity for HTPB based
propellant was greater than CAB, which in turn was more than GAP.
Compressive strength was very high for polybutadiene propellants .
Crosslinked LOVA gun propellants with HTPB as a binder, toluene-
diisocyanate (TDI) as curing agent, pentaerythritol (PET) or pyrogallol (PG)
as crosslinker have been evaluated ". PET formulations gave better results
than Pyrogallol in all respects namely force constant (1055/980 J/g), flame
temperature (2132 / 2156 K), height for 50% explosion (84/90 cm) and
friction insensitiveness (up to 36/32.4 kg).
Recent trend in development of LOVA propellants is the use of
energetic binders to obtain improved impetus level. Energetic polymers
containing azide (-N3) / nitrato (O-NO2) groups namely glycidyl azide
polymer (GAP), 3,3-bis (azidomethyl) oxetane (BAMO), 3-azidomethyl-3-
14
methyl oxetane (AMMO) and 3-nitratomethyl methyl oxetane (NIMMO)
polymers are emerging as potential binders . They are expected to
facilitate the combustion due to positive heat of formation and superior
oxygen balance than HTPB. Among these, GAP and poly BAMO based
propellants compositions have been widely evaluated. Major source of
energy of azido polymer is exothermic cleavage of azide bond (-N3)
(N=N=N), resulting in overall high energy of the propellant system. Azido
polymer extend the pressure limit of combustion to lower pressure zones and
enhance the bum rates of propellant systems.
Energetic polymers, poly bis ethoxy methyl oxetane (BEMO) and
poly bisazido methyl oxetane (BAMO) have been synthesized by varying
molecular structure to correlate the physical properties to structural changes
made^'. The effect of structure and molecular weight on melt viscosity and
morphology has been examined. BAMO-AMMO copolymer was
synthesized and used as an energetic binder in gun propellant formulations
by Wardle and co-workers . The propellant was extruded and evaluated for
compressive strength and bum rates. The oxetane block co-polymer exhibit
attractive properties in gun propellant formulations. BAMO produces the
best mechanical properties also. The energy of the oxetane block co
polymer based propellant is evident due to inherently high bum rates of
derived propellant. A comparison has been made with M-43 propellant
(RDX-76%, CAB-24%) to the propellant with 24% BAMO-AMMO and
76% RDX / CL-20 or a mixture thereof The impetus for these propellants
was found to be 1182, 1291 and 1247 J/g, respectively. The propellants were
made by solvent process. Similar studies using BAMO-AMMO are reported
by the same author^^. In these studies, the oxidizer used was CL-20. The
propellant was extruded into multiple grain sizes and configurations.
15
Mechanical properties by compressive loading at 50 cm/min gave modulus
41 MPa and maximum stress of 12.2 MPa. Safety properties of coated CL-
20 with BAMO-AMMO binder were well within the acceptable limits. The
force constant of RDX-oxetane and CL-20-oxetane based propellants was
1182 and 1297 J/g, respectively with flame temperatures 2827 and 3412°K.
The density of propellants was 1.64 and 1.77 g/cc, respectively.
Nitratopolyethers are other class of energetic polymeric binders used
for LOVA propellants^"*. The gun propellant formulations based on the
combination of non energetic binder / energetic plasticizer and energetic
binder 3-nitratomethyl 3-methyloxetane (poly NIMMO) and glycidyl nirate
(Poly Glyn) and BDNPF/A have been extruded into slotted cord shape. The
satisfactory dough rheology was achieved by adjusting the filler particle size
and shape distribution and by partially replacing HMX by nitroguanidine.
Nitroguanidine addition binds the dough together strongly. The same
compositions were used for plastic bonded explosives also. The
vulnerability of the propellants was assessed by measuring the response of
the propellants to shaped charge attack. The propellants showed LOVA
properties, when subjected to shaped charge jet attack and performance was
superior to double base and conventional LOVA propellants.
Advances in Poly NIMMO composite LOVA gun propellants are
reported by Leach and Debenham ". A programme on the alternative
technology of composite LOVA propellants was undertaken by U.K.
research agency. A range of composite LOVA propellants based on the
energetic binder poly NIMMO in combination with various fillers and
plasticizers have been examined. According to this study, factors
responsible for affecting response to shaped charge attack are propellant
composition, impetus value, shatter properties, burn rates, ignition
16
temperature and critical diameter of propellant, geometry, shape, web size
and mass confinement. Loading density also affects LOVA properties.
Ballistic data of Poly NIMMO formulations showed that the pressure index
was more than 1.2, linear burning rate coefficient (61) was in the range of
0.150 - 0.215 cm/s/MPa. Impetus obtained was between 1200 and 1300 J/g
with flame temperature from 3037 to 3340 K. Gumming ^̂ has also reported
composite gun propellants based on poly NIMMO with force constant of
1230 J/g with low vulnerability to shaped charge attack. The modified
GUDOL (NG/DEGN/NQ/RDX) formulation "̂̂ used in 155 mm Modular
Gharge System (MTLS) showed equivalence to the polymer bonded LOVA
propellants. Different tests used to assess the vulnerability of propellants
were slow cook off, bullet impact, detonation shock, cook-off in weapon,
fragment impact and shaped charge impact test. Mainly two types of
propellants, nitramine filled polymer bonded and nitrocellulose based, in
LOVA category were studied. RDX / HTPB based propellants were least
sensitive in bullet and fragment impact, no reaction was observed.
RDX/GAB/NG based propellants were, however, much more sensitive.
DREV, Ganada, has been involved in the synthesis of high molecular
weight GAP, energetic thermoplastic elastomer (ETPE) obtained by macro-
polymerization of GAP having mean molecular weight (Mn) 500, 1000 &
2000 . Ballistic evaluation of gun propellant formulations, containing
linear GAP based ETPEs has revealed that LOVA propellants with 75-88%
RDX and remaining GAP binder had ignifion temperature (Ti) of 205°G and
pressure exponent greater than one. Mechanical properties observed are
between soft and rigid structure, often stiff and brittle behaviour at low
temperatures. Stiffening was observed during ageing due to post curing
reactions.
17
1.2.3 Plasticizers
The role of plasticizer in propellant formulation is multi-functional.
In order to increase the energy of gun propellants, energetic plasticizers can
be used. Well reported LOVA propellant XM-39 containing acetyl triethyl
citrate (ATEC) as a plasticizer. Compounds such as ATEC, triacetin (TA)
and dibutyl phthalate (DBP) have been used as plasticizers for cellulosic
binders.
Evaluation of different plasticizers for LOVA propellants was carried
out by Sanghavi and co-workers^^. Plasticizers evaluated were dioctyl
phthalate (DOP), triacetin (TA), tributyl phthalate (TBP), acetyl triethyl
citrate (ATEC) and GAP. Experimental data on comparative study indicate
that amongst inert plasticizers, triacetin is superior in terms of vulnerability
aspects and mechanical properties, whereas the energetic plasticizer GAP
(low molecular weight) plays a significant role in LOVA formulation by
imparting higher impetus to propellant formulation without compromising
on vulnerability aspects. GAP behaviour was evaluated by partial
replacement of triacetin by Sanghavi et al °̂ in LOVA propellant with 75%
RDX / CA and NC. The improvement in impetus was achieved by 65 J/g
without affecting the mechanical properties but it further reduced pressure
exponent of the propellant. Propellant formulations with 78% RDX, 4% CA,
12%) NC and TA partially replaced by GAP was also studied by Pillai et
al . Most of their findings are in agreement with the earlier researchers .
They have also shown that low molecular weight GAP is a potential
plasticizer for LOVA propellants.
A US patent claimed that the LOVA propellants with RDX/HMX and
the mixture of at least two different dinitro diaza compounds as a mixed
plasticizer for the propellant produces low temperature sensitive coefficient
18
and pressure remains almost constant in tlie temperature range of -50 to
+70°C ^̂ . Other energetic plasticizers reported for LOVA propellants are bis
dinitro propyl acetal / formal (BDNPA / F), alkyl nitratoethyl nitramines
(NENA's) and l,5-diazido-3,3-nitroazapentane (DANPEf'*. Nitrate
plasticizers, trimethylo-ethane-trinitrate (TMETN), triethylene glycol
dinitrate (TEGDN), diethylene glycol dinitrate (DEGDN), butane triol
trinitrate (BTTN) have been studied in LOVA propellants with RDX.
Replacement of the inert plasticizer in LOVA formulations by energetic
plasticizer increases impetus level by 8%. Ballistic studies showed that
formulations containing TEGDN have higher pressure (1.13) exponent than
others (1.00)
1.3 Processing of LOVA propellants
LOVA propellants are processed by different techniques depending
upon the binder used in the propellant compositions. Polybutadiene based
propellants are processed through a solvent less process. The manufacture
of cured system involve addition of an oxidizer to pre-polymer (HTPB /
CTPB / CTBN), which are liquid of low viscosity. After efficient mixing in
a sigma blade incorporator, curative isocyanates (TDI, HMDI) for HTPB
and epoxy or aziridines for CTPB are added. The propellant is extruded and
final curing is carried out at elevated temperatures (75-80°C). This method
eliminates the drying step after extrusion, ensuring thereby uniform
ballistics. Details of solvent less preparation of LOVA gun propellants are
reported by Kristofferson et al ^̂ . Blending of oxidizer (HMX or fine RDX)
with polyurethane binder and cure catalyst is carried out below 50°C . A safe
process technology has been established for LOVA gun propellant with CA
/TA / RDX by Pillai et aP^. Manufacture of propellant formulation using CA
and RDX was tried by conventional solvent process using two different
19
methods. In the first method, fine RDX was first desensitized with
plasticizer coating, which in turn was incorporated with the inert binder. In
the second method, a two-stage process technology was adopted. In the first
stage, the basic composition was prepared by wet mixing process and in the
second stage the dry basic mix is incorporated along with solvent and
additives and extruded into the required size and shape.LOVA propellants
based on CAB/NC were processed by twin screw extruder by Dilleby .
The process uses a lacquer of all the ingredients except nitramines. The
lacquer is pumped as a liquid into a twin screw compounder and RDX is
added as a dry powder. The ingredients are compounded, granulated and
dried to a free flowing powder. The dry powder is fed to a twin screw
extruder with the optimum amount of solvent for extrusion. This process is
more effective and is of low cost. All the process steps including
granulation are continuous.
A LOVA propellant composed of RDX, NC and CAB was
manufactured using standard ball powder propellant technology and tailored
to match the existing for M-792, 25 mm round . Ball powder technology
involves the preparation of a lacquer which is extruded through a die head
and cut into cylinder. The cylinders are then rounded through the action of
osmotic pressure and shear forces. Once the proper shape has been
obtained, solvent is removed by distillation. In this process, critical steps are
shaping and retardation of the RDX leaching. Similar studies have been
carried out by Canterberry and Mrazek ^̂ and ballistic parameters have been
determined. The ball powder in 76/19/5 (RDX/CAB/NC)composition was
found to be extremely impact insensitive (353 kg/cm ) as against 29 kg/cm
for RDX with 4 kg drop weight tester. In bullet impact test no LOVA round
(25 mm) detonated or resulted in fire.
20
LOVA propellants with thermoplastic elastomers, which are
physically crosslinked, can be processed by solvent less process. Elastomers
are required to be heated to elevated temperatures to achieve desired fluidity.
Oxidizer and additives are then mixed with the melt. The mix is allowed to
cool down and extruded after attainment of desired consistency. These
systems offer excellent mechanical properties. High processing temperature
is the main drawback for explosives and propellants. However plasticizers
are added to bring down the process temperatures.
1.4 Ballistic properties of LOVA propellants
Ballistic behaviour of a variety of LOVA propellants has been
examined using closed vessel technique '*". The slope break behaviour
frequently observed in nitramine based propellant burning could be
eliminated by using multi modal mixtures of 2|im and 10 |im size. Bum
rates versus pressure curve exhibits linear behaviour on the conventional
log-log plot. But bum rate curve does not show any slope break. Pillai and
co-workers '** have reported the use of bimodal RDX for elimination of slope
break in P-t curve of LOVA propellants. The thermo-chemical constants of
propellants were computed by them. Results of studies on the role of fine
RDX in determining the buming rate and ballistics of LOVA gun propellants
are also discussed by these authors. Fine RDX of 4.5, 6, 13 and 32)am size
were used. This study reveals that 4.5 to 6 fim size is most suitable to get
desired buming rate behaviour. Thermodynamic properties for LOVA
propellant were calculated and compared with M-30, JA-2 and XM-39
propellants ^^. The results indicate flame temperature of 3000 K against
2671 K for XM-39. Similarly, force constant was found to be 1160 J/g for
XM-39 propellant. Thus, only limited studies have been carried out on the
ballistic behaviour of a few selected LOVA propellants.
21
1.5 Vulnerability Test
Studies on high energy LOVA compositions with respect to energetics
and sensitivity characteristics have been carried out by Choudhari etal '*̂ .
Results obtained reveal that height for 50% explosion for CAB/RDX and
CAB/GAP based compositions more than 60 cm and decomposition
temperature > 216°C, higher from the conventional propellants (160°C).
Cook and Harbersat '*'* used the laser ignition technique on LOVA
propellants for ranking of their vulnerability. LOVA propellants studied
contained CA/RDX, EC/NC/RDX, HTPB /HMX etc. Comparisons were
made with conventional triple base propellants M-30 and M-26. The test
used was the ignition delay. All LOVA propellants exhibited significantly
longer delay time compared to standard propellants and therefore need more
powerful ignition system.
Relative sensitivity of a number of LOVA propellants is reported by
Barnes etal '^^ LOVA propellants proved to be superior to M-30 propellant
with respect to impact sensitivity (32 cm/16 cm), exotherm in explosion
temperature, auto ignition temperature (192 / 167°C), explosion temperature
(336 / 254°C) hot fragment conductive ignition test (775° / 363°C) . DDT
test showed that LOVA propellants exhibit low vulnerability.
Hot fragment conductive ignition test of nitramine propellants XM-39
and M-43 in partially and highly confined status were carried out by Huang
et al ^ '̂̂ ' GO, NO-GO boundaries were predicted. The effects of initial
temperature of spall particle, and size of chamber exhaust port were
examined. Good agreement was found between calculated and experimental
ignition boundaries at 1 atm pressure. XM-39 propellant was found to be
more susceptible to ignition by the spall fragment than M-43 propellant at
low pressures ,because binder decomposition of the M-43 propellant is more
endothermic than of the XM-39 propellant.
22
1.6 Ignition/ combustion studies
Since LOVA propellants are generally formulated using inert binder
and nitramines, they possess high ignition threshold and require more heat
input for their smooth ignition. Ignition studies *^ for HTPB and CAB based
propellants reveal that the relative delay was 5.3 ms as against 1.0 ms for
NC based propellants. Delay time increases by a factor of around five. XM-
39 propellant is more susceptible to conductive ignition by spall fragment
than M-43 propellant due to difference in the binder composition. Thus, a
small change in binder composition has a pronounced effect on ignition
boundary .Vamey "*' has reported that for LOVA propellants, black powder
and boron-potassium nitrate based igniter materials were most effective than
magnesium-Teflon-Viton (MTV) based igniters. For simultaneous and
smooth ignition of LOVA propellants (CA/RDX), combustion products of
igniter play an important role. Various igniter materials namely B-KNO3-
ehylcellulose, magnesium-potassium nitrate-ethyl cellulose, magnesium-
teflon-viton (MTV) etc. were evaluated for the ignition of LOVA propellants
containing 78% RDX, by closed vessel technique.^*' The rate of rise of
pressure with respect to time (dP/dt), was used to assess the comparative
performance of various igniter materials. Results of ignition study reveal
superior performance of gun powder over other igniter compositions since
both had given nearly same ignition delay of 2-3 ms.
Regarding usage of LOVA propellants in artillery applications,
Minor '̂ has opined that a detailed investigation need to be carried out to
study LOVA propellant performance during critical ignition and the flame
spread portion of the interior ballistics cycle in order to assess their
suitability as artillery propellant. He has recommended a simulator for 155
mm Howitzer for these studies involving monitoring of flame spread
through high speed photography.
23
The ignition properties of LOVA propellant M-43 in the simulator
for 120 mm tank gun charge were investigated by Lee et al . These studies
were conducted on the ignition process for a 120 mm APFSDS projectile.
The diagnostics provide insights into the ignition process occurring during
the early phase of the interior ballistics cycle. Correlation between the
pressure data in the simulator and that of initial phase has also been
investigated. It was observed that flame front advanced to the forward part
of the chamber nearly normal to the chamber axis. Breech pressure rose
slowly indicating the difficulty in ignition of propellant and time lapsed
between temperature detectors was more as compared to single, double and
triple base propellants.
The influence of ageing on the burning rate parameters for LOVA
propellants comprising cellulosic binders and thermoplastic elastomers was
carried out at 65.5°C by Strauss et al . The combustion characteristics
were observed at different time intervals. Reduction in relative vivacity and
relative force from zero to 12 months was about 5.5% as against 1.3% of
M-30 conventional propellant. Pressure exponent for LOVA propellant
decreases from 1.16 to 0.88 as against slight upward change for M-30
propellant. Majority of LOVA propellants are inherently more chemically
stable than M-30 composition.
The combustion behaviour and thermo-chemical properties of XM-39
and M-43 revealed that activation energy for XM-39 was 4 k Cal /mol,
whereas it was 8.1 k Cal /mol for M-43 propellant '̂*. Scanning Electron
Microscope (SEM) pictures of burning surfaces of recovered propellant
samples demonstrated significant alteration of surface structure of M-43
propellant due to the use of energetic plasticizer. Increase in heat release on
the burning surface of M-43 propellant was also observed.
24
Different sensitivity characterization techniques for LOVA
propellants have been described by Kirshenbaum et al̂ .̂ The laboratory
sensitivity and thermal stability tests included impact sensitivity, DTA,
TGA, vacuum stability, hot fragment conductive ignition and deflagration to
detonation (DDT) test which can be used to select the propellant for scale
up.
LOVA composition useful for a tank gun ammunition comprising
RDX, a new cyclic nitramine, 2-nitroimino-5-nitrohexahydro-l,3,5-triazine
(NHNT), NC, TMETN, BDNPA/F has been patented by Strauss et al ^^
Low pressure exponent (a) and high linear bum rate coefficient (Pi) values
are important parameters for its multizone unicharge application. This
propellant is claimed to be less sensitive to shaped charge jet impact.
LOVA propellants with energetic plasticizers have been reported by
Urenovitch . The plasticizers used were alkyl NENA and l,3-dmitroxy-3-
nitrazapentane (DINA) and the mixtures there of The development of
LOVA gun propellants at Eureco Bofors with l,l-diamino-2,2-
dinitroethylene (FOX-7) and FOX-12 as energetic oxidizers with NC and CO
NENA plasticizers has been reported . A typical composition with 60 %
FOX-7 generates a burning rate of approximately 55 mm/s at 100 MPa with
pressure exponent of about one. FOX-7 reduces the delay time for ignition
significantly in comparison to the RDX based reference composition. The
gun performance equals that of a double base propellant but the flame
temperature as well as temperature dependence are considerably lower. The
ballistic details of some of the LOVA propellants developed by advanced
countries are given in Table 1.3.
Thus, LOVA propellants containing energetic ingredients (high
explosives) as oxidizers, energetic binders and energetic plasticizers with
suitable additives have very bright future for usage in field gun, tank guns
25
and for naval applications. These propellants provide superior ballistics with
highest order of insensitivity.
Table-1.3 : Different LOVA Propellants Developed by Advanced Countries
Country
USA
USA
USA
USA
USA
USA
USA
USA
USA
Canada
Germany
Germany
Year
1983
1983
1978
1978
1978
1976
1978
1982
1995
1990
1992
1992
Ingredients (%)
RDX-74/EC-12/NC-8/DBP-6
RDX-75/C A-16/TA-8/K2SO4-1
HMX-80/HTPB-20
RDX-80/CTBN-20
HMX-80/CTBN-15/Additives-5
HMX-75/Polymer-12/IPDI-
lO/Additives-3
HMX-80/Polymer-13/Additives-7
XM-39(RDX/76/CAB-12/NC-
4/ATEC-7.6/Carb.0.4
M-43(RDX-76/CAB-12/NC-
4/BDNPF/A-7.6)
XM-39 - ATEC replaced by GAP BuNENA (N-n-butyl-N-(2-nitroxyethyl nitramine) TEGDN DANPE (1,5-diazido-3-nitraza pentane) RDX-78/HTPB-11/Picrite-
8/Additives-3
RDX-78/HTPB-11/TAGN-
8/Additives-3
Force Constant (J/g) 1055
1000
1008
1009
1005
1000
1038
1069
1157
1159 1173
1179 1210
1164
1180
Flame Temp. K 2536
2548
2363
2387
2379
2350
2434
2671
3042
2927 2975
3054 3107
2869
2880
26
1.7 Objective of Present Study
In view of the review presented above, bringing out limitations of
earlier studies, the present study was undertaken to generate comprehensive
and exhaustive information on vulnerability and energetics of low
vulnerability gun propellants based on cellulose acetate, BAMO-THF
copolymer and GAP (high molecular weight) as binder and GAP (low
molecular weight) and BuNENA as energetic plasticizers. Plasticisers play
a dual role, as processing aid and as a desensitiser and improve mechanical
as well as ballistic properties.
1.8 Plan of Present Study :
The present thesis is divided into five chapters.
Chapter 1 : General Introduction
This gives a brief account of conventional gun propellants and concepts used
for energetic gun propellants.
LOVA propellants : An overview of R&D efforts attempted so far towards
improvements in propellant performance and reduction in vulnerability. This
also includes processing details and description of oxidizers, binders and
plasticizers used. Objective of the present study is given at the end of the
chapter.
Chapter-2 : Experimental
This chapter describes broad specification of the materials used and
methods and methodologies adopted during the present study for generation
27
of information on gun propellants witii polymeric binders like HTPB,
BAMO-THF and GAP as binders. Method of processing of propellants as
well as instrumental methods used for determination of ballistics and
vulnerability (impact, friction, ignition temperature) of the formulations are
also elaborated in this chapter. Salient aspects of closed vessel test and
determination of mechanical properties are brought out in this chapter.
Chapter-3 : Results and Discussion
This chapter gives details of ballistic results along with results of
sensitivity and mechanical properties. Results of propellant formulations
evaluated during this research programme are also included. The results
obtained for various LOVA propellants based on different energetic binders
and plasticizers are presented in this chapter including results of ballistic
performance in closed vessel at 0.1 g/cc loading density, sensitivity
characterization of propellants, their thermal characteristics and mechanical
properties.
Section-3.1 :
It contains results of the studies carried out on the effect of non-
energetic binders, cellulose acetate (CA) and hydroxyl terminated
polybutadiene (HTPB) and their mixture on LOVA propellant compositions
containing on RDX (70-80%), NC(5-15%), Carbamite (0.5%) and low
molecular weight GAP (5.5%)) as an energetic plasticizer .
28
Section-3.2 :
This section contains results of studies carried out on the effect of
BuNENA as an energetic plasticizer on LOVA propellants containing CA
and HTPB and their mixture as binders.
Section-3.3 : This section contains results of studies carried out on the
effect of energetic binder and plasticizer GAP on LOVA propellant
compositions.
Section-3.4 : This contains results of the studies carried out on the effect of
energetic binder glycidyl azide polymer(GAP-2000) on LOVA propellant
compositions containing BuNENA as an energetic plasticizer.
Section-3.5:
This section contains results of the studies carried out on the
propellant compositions based on BAMO-THF co-polymer as energetic
binder and low molecular wt. GAP as plasticizer,
Section-3.6 :
This contains results of studies carried out on the propellant
compositions containing BAMO-THF co-polymer as energetic binder and
BuNENA as an energetic plasticizer.
Chapter-4 : General Discussion
The results obtained in the present study are discussed in the light of
information reported in the literature so far.
Chapter-5 : Summary
This chapter summarizes various findings of the present study.
29
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