D. P. MISHRA 6.1 INTRODUCTION
6.4 SOLID PROPELLANTS
Any solid propellant usually consists of fuel, oxidizer, and additives. Fuel and oxidizer are principal ingredients. Additives are used in very low per- centage to enhance the burning rate, control fabrication process, minimize temperature sensitivity, to ensure chemical/physical stability during stor- age, to increase mechanical properties, and so on. Generally, solid pro- pellants are designed for specific applications, namely, sounding rocket, missile, launch vehicle, gas generator, and so on. The desirable properties of solid propellants are enumerated as follows [2,5]:
1. Solid propellant must have higher heating value to have higher com- bustion temperature, leading to high characteristic velocity C*.
2. Propellant combustion products should have low molecular weight to have high exhaust velocity Ve, leading to high specific impulse Isp. 3. It should have a high density such that large amount of chemical energy
can be stored in the smallest volume and thus can have a compact design.
4. Easy to ignite even under low pressure condition. But it must not ignite due to shock or pressure pulses.
5. Its constituents should be easy to handle.
6. Constituents of propellants must be locally available and cost-effective.
7. Processing of propellant should be simple and reproducible in nature.
8. Properties of propellant grain must not be physically and chemically unstable during storage and transport.
9. Propellant grain must be inexpensive to manufacture and easy to handle during operation.
10. Solid propellant grain must not react with atmospheric air/moisture.
11. Propellant grain must have high mechanical strength.
12. It must be smoke-free and nontoxic in nature.
13. It must be less prone to explosion hazard.
As discussed, solid propellants fall mainly into two categories:
(1) homogeneous and (2) heterogeneous propellants, depending upon the physical entities of fuel and oxidizer in the propellant. Various types of homogeneous and heterogeneous solid propellants are discussed in the following.
6.4.1 Homogeneous Solid Propellants
In homogeneous propellants (also known as colloidal propellants), the oxidizing and reducing groups are present in the same molecule. They are thermoplastic in nature and are generally smokeless. The homoge- neous propellants are mainly classified as single base and double base (see Figure 6.1). Single-base propellants, which contain only NC, are mainly used as gun propellants. NC [(C6H7O2–(ONO2)3]n in gelatinized form constitutes the main ingredient in these propellants. NC is basically cel- lulose nitrate, which is produced by nitrating the cellulose materials [(C6H7O2(OH)3)n] using a nitrating agent (e.g., nitric acid) as per the fol- lowing chemical reaction:
[C6H7O2(OH)3]n + xHNO3→ (C6H7O2)n(OH)3n − x(ONO2)x + xH2O (6.1)
The cellulose material mainly consists of a linear chain of carbon, hydrogen, and oxygen. During this process of nitration, certain portion of hydroxyl radical (OH) in the cellulose is replaced with nitrate radical ONO2. Note that the energy content of NG is dependent on the extent of nitration and kind of cellulose materials. In earlier days, these were also known as flash paper, flash cotton and guncotton, and flash string, being flammable in nature. By nitrating cellulose, the extent of nitrogen content in NC can be enhanced to a maximum of 14.4% by mass. But in the case of propellant application, nitrogen content in NC varies generally from 11.0% to 13.3%
by mass. Note that the heat of formation of NC decreases with increase in nitrogen content [3]. During autocatalytic reaction, NC decomposes into aldehydes (HCHO and CH3CHO) and NO2 gas due to breaking of weak- est bond O–NO2. This decomposing reaction can be considered to be first order between 363 and 448 K.
Double-base propellants are made by plasticizing NC with an energetic compound like NG. NG is formed from glycerin/polyol (alcohol contain- ing multiple hydroxyl group; e.g., propane triol; C3H5(OH)3) by replacing OH with nitrate radical ONO2. The chemical structure of NG is shown in the following:
H2C ONO2 ONO2 ONO2 H2C
HC (6.2)
Note that it is an aliphatic with a straight chain structure that has a rela- tively low molecular mass of 227.1 kg/kmol as compared to NC. Note that it is slightly oxidizer-rich. It remains in liquid state at room temperature but solidifies when temperature goes below 286. NG at 418 K undergoes autocatalytic reaction and decomposes into aldehydes and NO2 gas due to breaking of weakest bond O–NO2. It has an activation energy of 109 kJ/
kmol. It can undergo self-ignition at 491 K at certain critical concentra- tion of NO2. Note that both NG and NC contain both fuel and oxidizer together in their molecular structure and can be used as single-base pro- pellants in the rocket engine. However, both are commonly used together in the rocket engine as double-base propellant. Generally, NC in the form of fine nodules or stripes are mixed in a sigma blade mixer along with the NG liquid and other ingredients to form a stiff dough at temperature around 60°C. Subsequently, this dough can be extruded through a suitable
die of an extrusion press to form desired shaped propellant grain. Besides this, double-base propellant grain can be produced using casting method.
Generally, extruded propellant grain has higher mechanical strength as compared to casted propellant grain. Depending upon the process of man- ufacture, DB propellants are classified as extruded double base (EDB) and cast double base (CDB). These propellants have burning rates in the range of 5–20 mm/s and have a heat of combustion around 3800–5000 kJ/kg.
Specific impulse of the order of 230 s can be obtained from unmodified DB propellants. This double-base propellant is also known as colloidal/mixed propellant. Its trade name is cordite.
Let us consider a typical JPN ballistite double-base propellant with its constituents along with its respective functions as shown in Table 6.1. Note that ballistite is basically a smokeless double-base propellant made of two main explosives, namely, nitroglycerine (NG) and nitrocellulose (NC). It was first devised and patented by Alfred Nobel in 1888. The diethyl phthal- ate (C12H14O4) is used as a nonexplosive plasticizer to improve mechanical properties of the propellant grain. The ethyl centralite (C17H20N2O) acts as a stabilizer to counteract the autocatalytic decomposer of major con- stituents. The potassium sulfate (K2SO4) ensures smooth burning at low temperature to avoid combustion instabilities. The carbon is added to the transparent propellant to avert transmission of radiant energy, which may cause self-ignition around internal parts of propellant grain. The candelilla wax is used in this propellant as a lubricant for extrusion die that facilitates extraction process for maintaining accurate shape of grain. Besides this, metal powders in some cases are added to enhance higher performance of
TABLE 6.1 Typical DB Propellant JPN (Ballistic)
Material %Weight Purpose
Nitrocellulose (NC) (13.25%N) 51.22 Polymer (fuel)
Nitroglycerine (NG) 43 Explosive plasticizer (oxidizer) Diethyl phthalate (C12H14O4) 3.25 Nonexplosive plasticizer Ethyl centralite (C17H20N2O) 1.0 Stabilizer
Potassium sulfate (K2SO4) 1.25 Flash suppressing agent
Carbon black 0.2 Opacifier
Candelilla wax 0.08 Lubricant for extrusion process Sources: Barrere, M. et al., Rocket Propulsion, Elsevier Publishing Company,
New York, 1960; Kubota, N., Propellants and Explosives, Thermochemical Aspects of Combustion, Wiley-VCH, Weinheim, Germany, 2002.
propellant. In certain cases, NG in double-base propellants is substituted with other constituents such as di/tri-nitrodiethyleneglycol.
A third variety of homogeneous propellant is called triple-base propel- lant. This contains an organic compound known as nitroguanidine (NQ:
(NH2)2CNNO2) in addition to NC and NG. The advantage of triple base propellants is that they have significantly more energy than single-base propellants while their combustion temperatures still lie below 3000°C.
6.4.2 Heterogeneous Propellants
A heterogeneous propellant is one in which solid crystalline oxidizer (e.g., ammonium perchlorate NH4ClO4) and organic fuel, and metallic fuel pow- ders are held together in a plastic (rubber) matrix. Generally, an organic plastic binder (fuel) surrounds the fine crystalline oxidizer particle. Note that this organic fuel acts as the main fuel component. Its main function is to produce heat while undergoing overall exothermic chemical reaction.
Generally, this fuel is known as the binder because it binds the metal pow- ders, solid crystalline and other ingredients in the propellant grain. As this kind of propellant is heterogeneous in nature, it is also termed as composite propellants (CP). Let us consider a typical composite propellant containing between 70% AP, 17% Al powder, and 12% elastomeric binder. Note that plasticizers, stabilizers, curing agent, bonding agents, burn rate catalysts, combustion instability suppressant, and antioxidants are used in small per- centage (%) in CP. Typical ingredients of CPs are given in Table 6.1. Certain combinations of these ingredients can be provided in the propellant for opti- mized performance for either rocket engine or explosive applications. The selection of ingredients will be dependent on both optimized characteristics with desired mechanical properties of propellant grain. Note that CPs have a wide range of burning rates (7–20 mm/s) and densities (1700–1800 kg/m3).
The composite propellants can have higher specific impulse in the range of 250–300 s as compared to double-base propellants. The commonly used fuel (binder) and oxidizers in solid propellants are discussed briefly, as follows.
6.4.2.1 Solid Fuel (Binder)
Polymers, plastics, rubber, PVC are some of the binders commonly used for CP, as shown in Table 6.2. The mechanical properties of CP are depen- dent on the type of binder and its constituents. For selecting proper fuel, the following properties, namely, high heat of combustion, high combus- tion temperature, mechanical properties such as strength and elastic- ity, thermal properties, smooth burning characteristics, and good aging
characteristics are to be considered. It is preferable to have the binder in liquid state at ambient temperature and pressure, because it helps in the mixing of oxidizer and solid fuel. This mixing process is followed by a cur- ing process, during which binder and fuel mixture turns into solid CP.
Generally, organic prepolymers or low-molecular-weight polymers containing fuel elements like carbon, hydrogen, and oxygen are used as binders in composite propellant. The binders should have workable viscos- ity, high heat of formation, low glass transition temperature, good stabil- ity, and compatibility with other ingredients of the formulation. Binders can be generally classified as thermoplastics and thermosetting. Some of the thermoplastics such as asphalt binders, polyvinyl chloride (PVC), and polystyrene are linear polymers. These were used in earlier propellant for- mulations. Unfortunately, the propellants made of these binders exhibit poor mechanical properties. In contrast, thermosetting binders are chemi- cally cross-linked during the curing process. It is transformed into a tough and insoluble solid matrix that can withstand large thermal and mechan- ical stresses, even in large solid booster rocket engines. Some examples of thermosetting binders are polysulfide, polybutadienes with different functional groups such as carboxyl terminated polybutadiene (CTPB) and hydroxyl terminated polybutadiene (HTPB), polybutadiene–acrylic-acid (PBAA), and polybutadiene–acrylic acid–acrylonitrile (PBAN). Note that polybutadiene is basically a linear chain structure polymer. This polybuta- diene consists of alkadienes with four carbon atoms and two double bonds which can be chemically represented as
—(CH2═ CH — CH ═ CH2)n— (6.3)
TABLE 6.2 List of Typical Ingredients Used for Heterogeneous Propellants
Fuel (Binder) Oxidizer Plasticizer
PU: Polyurethane PVC: Polyvinyl chloride PBAN: Poly Butyl Acrylo
Nitrate PS: Polysulfide HTPB: Hydroxyl
terminated polybutadiene CTPB: Carboxyl
terminated polybutadiene Metal fuel: Aluminum,
Magnesium, Beryllium, Boron
AP: Ammonium perchlorate
AN: Ammonium nitrate KP: Potassium perchlorate NP: Nitronium perchlorate ADN: Ammonium
dinitramide
RDX: Cyclotrimethylene trinitramine
HMX: Cyclotetramethylene tetranitramine
DOP: Dioctyle phthalate DOA: Dioctyl adipate IDP: Isodecyl pelargonete Curing agent
TDI:
Toluene-2,4-Di-Isocyanate MAPO: Tris(1-(2-methyl)
Aziridinyl)phosphine oxide IPDI: Iso-phorone
di-isocyanate
where n is the number of butadiene groups. Note that when the polybuta- diene chain attaches to poly–acrylo–nitrile group and acrylic acid groups, then polybutadiene–acrylic acid–acrylonitrile (PBAN) is formed. Its typi- cal structure is given in the following:
(CH2 CH2)n CH)x COOH
CH)y (CH2 (CH2
CN CH CH
(6.4) In order to enhance the mechanical properties of binder, randomness of the cross-linking is decreased by relocating the carboxyl group at the end of the polybutadiene chain ends shown as follows:
CH CH (CH2 CH CH CH2)nl CH)xl CH2)n2
COOH (CH2 (CH2
(6.5) This binder is known as the carboxyl terminated polybutadiene (CTPB).
When the carboxyl group at the end of the polybutadiene chain is replaced by hydroxyl (OH) radical, it is termed as hydroxyl terminated polybuta- diene (HTPB). Its performance gets improved as compared to the PBAN and CTPB as OH is more reactive as compared to COOH. Hence among all polymers, HTPB is currently considered superior because of its out- standing processing characteristics and better mechanical properties.
Some of the properties of HTPB made by ISRO are as follows: molecular weight: 2300–2900 kg/mol, hydroxyl value: 40–50 mg KOH/g, functional- ity: 1.8–2.5, viscosity: 40–65 cps at 30°C, specific gravity: 0.90–0.92, and Tg: −80°C. Recently, a high energetic binder like glycidyle azide polymer (GAP) is being developed. GAP provides higher performance as it contains more number of hydrogen atoms. Besides these polymers, metal powders, namely, aluminum, magnesium, boron, and beryllium, are used as fuel in CP to enhance energy content of propellant. These metal powders must be sufficiently fine (particle size must be 0.1–10 μm) so that complete com- bustion can be ensured. Aluminum powder being lighter is preferred over other metals. But metal oxides are formed during combustion, which have higher negative heat of formation. In recent times, metal hydrides are being considered over metal powders as more amount of hydrogen is released during combustion of metal hydrides. Note that the use of the metal pow- ders decreases the mechanical properties of the propellant. This calls for higher proportion of binder to maintain same level of mechanical strength.
6.4.2.2 Solid Oxidizer
In CP, oxidizer plays a very important role in releasing heat during oxi- dization process and maintaining the grain’s structural stability. In some cases, it accounts for more than 70% by weight of the total propellant weight. Ideally, the oxidizer should have high oxygen content and should be compatible with the binder and other ingredients in the propellant formulation. Some of the commonly used oxidizers are nitrates and per- chlorates of ammonium, potassium, and so on, as shown in Table 6.3.
For selecting a proper oxidizer, the following properties, namely, high heat of formation, high amount of available oxygen, high density, slightly hygroscopic, and smooth burning characteristics are to be considered.
The most commonly used solid oxidizers are ammonium perchlorate (AP: NH4ClO4), ammonium nitrate (AN: NH4NO3), nitronium per- chlorate (NP: NO2ClO4), potassium perchlorate (KP: KClO4), potassium nitrate (KN: KNO3), and so on. Among all these oxidizers, ammonium perchlorate (AP: NH4ClO4) is preferred as it has lower negative heat of formation and dissociates easily. Besides, this it is not very hygroscopic in nature and is compatible with most commonly used binders. Some of the common oxidizers and compounds that are currently pursued as promising oxidizers/ingredients in future high energy formulations are given in Table 6.4.
Although AP is a versatile and widely used oxidizer, the chlorinated combustion products formed during combustion of AP-based composi- tion is a serious environmental concern. On the other hand, ammonium nitrate (AN) is an eco-friendly oxidizer. However, it is less energetic and is used mostly as a gas-generating propellant as its phase transition occurs around 32°C with a significant volume change. Hence, it is not used as a rocket propellant oxidizer in the rocket engine. Of course, it can be a promising substitute for AP only when used along with energetic binders like glycidyl azide polymer (GAP). High explosives made of ring structure nitramine compound, namely, research and development explosive (RDX (C3H6N3(NO2)3): cyclotrimethylenetrinitramine). High melting explosive (HMX (C4H8N4(NO2)4): cyclotetramethylenetetranitramine), as shown in Table 6.4, are used as high energetic smokeless propellants. In order to improve energy of the propellant, not more than 10%–20% of RDX/HMX is recommended as nitramines have less oxidizing power than AP. Besides, this addition of nitramines enhances the hazardousness of the propellant due to the presence of explosive materials.
TABLE 6.3Some Oxidizers for Propellant Compositions OxidizerChemical SymbolMolecular WeightDensity (kg/m3)Oxygen Balance (%)Heat of Formation (kJ/mol) Ammonium perchlorate (AP)NH4ClO4117.51.9534−296 Ammonium nitrate (AN)NH4NO3801.7220−369 RDXC3H6N3(NO2)32221.82−21.670 HMX2961.91−21.684 Ammonium dinitramide (ADN)C4H8N4(NO2)4 NH4N(NO2)21241.8125.8−150 HNIW (CL-20)C6H6 N12O124382.04−10.9372 Hydrazinium nitroformate (HNF)N2H5C(NO2)3−1831.8313−72
6.4.2.3 Composite Modified Double-Base Propellant
In order to enhance energy and density levels of DB propellants, AP crystal can be added which also improves specific impulse of the rocket engine.
Generally, oxidizers like AP and metallic fuels like aluminum are added to the fuel-rich DB propellants that reduce its fuel-richness and improve its performance. This kind of propellant is commonly known as composite modified double base (CMDB). The specific impulse Isp of DB propellant can be enhanced by almost 60% with addition of CP, which is comparable to most of the common composite propellant formulations. These CMDB propellants are preferred in missile propulsion and upper stage of rocket engine as they are stronger as compared to the CP. The performance of DB propellant can be further enhanced by the addition of HMX/RDX in place of AP. In order to ease the casting of DB propellant, elastomeric binders are used, which are known as modified double-base propellants (EMCDB).
Other ingredients such as inert plasticizers, stabilizers, darkening agents, burn rate modifiers, flash suppressors and platonizing agents are being added to these modified propellants.
6.4.2.4 Advanced Propellants
Several kinds of new propellants are being devised across the globe. Some of the advances and future directions in chemical propellants are discussed briefly as this subject is beyond the scope of this book.
Several new compounds that can act as burn rate catalysts, oxidizers, binders, and energetic ingredients or nanomaterials in rocket propellant formulations have been developed in recent times. Efforts have been made to develop minimum smoke, clean burning, high energy, and insensitive armaments for various propulsive devices. More energetic binders and oxi- dizers are being developed by using energetic groups, namely, azido (N3−), nitramino (–NHNO2–), nitro (NO2−), and nitrato (−ONO2–). The addition
TABLE 6.4 Comparison between Solid and Liquid Propellants
No. Solid Propellants Liquid Propellants
1. Low specific impulse High specific impulse
2. Easy to store, handle, and transport Difficult to store, handle, and transport 3. Simple to design and develop
solid-propellant rocket engine Complex to design and develop liquid-propellant rocket engine
4. More economical Less economical
5. Difficult to test and calibrate
solid-propellant rocket engine Difficult to test and calibrate liquid- propellant rocket engine
of these groups enhances the overall energy of the formulation, while increasing the overall oxygen balance.
Some of these energetic binders are glycidyl azide polymer (GAP), polyglycidyl nitrate (poly GLYN), nitrated polybutadiene (NHTPB), poly-nitratomethyl-1-3-methyl oxetane (poly NIMMO), poly 3,3-bis(azidomethyl)oxetane (BAMO), N–N bonded binders, and strained ring hydrocarbon binders. Among these energetic binders, GAP is consid- ered to be the most prominent and effective in enhancing the performance of solid-propellant rocket engine, because it has a higher burn rate with positive heat of formation. Besides this, it has high density and insensi- tivity. As compared to conventional binders, GAP provides a better com- promise between energetic performance and vulnerability of the energetic materials. But its main disadvantage is that it has higher glass transition temperature compared to HTPB.
Advanced oxidizers should have high density, high enthalpy of forma- tion, high oxygen balance, and environmental compatibility. Ammonium dinitramide (ADN) is considered to be a promising oxidizer as it has higher heat of formation as compared to AP and chlorine-free combustion products. When it is mixed with energetic binders like GAP, it enhances specific impulse even at a lower solid loading of 80%. But it is not pre- ferred as it has poor thermal stability and relatively high cost of produc- tion. Another advanced oxidizer is hexanitrohexaazaisowurzitane HNIW (CL20). It is one of the most powerful and dense single-component explo- sives. Although it is explosive by nature, it can be used in rocket propel- lant formulations in place of HMX. Hydrazinium nitroformate (HNF) is basically the salt of nitroform and hydrazine that is considered to be a promising oxidizer. This oxidizer with new energetic binders can have higher burn rates and specific impulse values as compared to conventional propellant. Many other energetic explosives with caged structure, namely, hydrazinium mono and diperchlorates, hydroxyl amine perchlorates, and difluramino compounds are being explored in order to enhance the perfor- mance of solid propellant. Interested readers can refer to advanced books on propellants and explosives [3].
In order to enhance burning rate of AP-based composite, ultrafine ammonium perchlorate and butacene catalysts are being used [6], which enhance burning rate of propellant even up to 100 mm/s, of course, in a very large range of pressure. Note that butacene is an HTPB prepolymer with attached ferrocene groups. In recent times, nanomaterials are being used to improve the performance of solid-propellant rockets as they alter