Propellants. 1. Introduction. Propellants 1

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1 Propellants 1 Propellants Explosives and Pyrotechnics are separate keywords Hiltmar Schubert, Fraunhofer-Institute for Chemical Technology, Pfinztal, Federal Republic of Germany 1. Introduction History Classification and Formulation of Propellants Solid Propellants Homogeneous Propellants Composite Propellants Liquid Propellants Monopropellants Bipropellants Reactions and Properties Combustion Process Propellant Performance Mechanical Properties Chemical Stability Service Life Sensitivity and Vulnerability Uses Gun Propellants Solid Propellants Liquid Propellants Rocket Propellants Solid Propellants Liquid Propellants Propellants for Gas Generators Production Homogeneous Propellants Single-Base Propellants Double- and Multibase Propellants Composite Propellants Special Propellant Devices Test Methods Safety Precautions and Environmental Protection References Introduction Propellants are explosive agents that generate large amounts of hot gases by an exothermic reaction without the need for atmospheric oxygen. This process is generally initiated by supplying thermal energy. Burning occurs vertically to the burning surface at a rate of 10 3 to 1 m/s. Rates of several 100 m/s can also be achieved under certain conditions (deflagration). Detonation is chemically the same as deflagration but the chemical reaction is initiated by a shock wave which is propagated through the charge. The burning rate depends on the prevailing pressure and differs from the pressureindependent detonation rate ( m/s) by several orders of magnitude. Depending on the external conditions (confinement), chemical composition, and physical structure of the charge, a deflagration can become a detonation and vice versa. Propellants are used whereever reproducible, time- controlled amounts of working fluid are required to propel rockets and missiles or to launch projectiles from guns (gun powder). They are also used in gas generators that are designed to jettison loads, drive turbines and pumps, actuate mechanical devices, convey liquids, or inflate balloons. The working gases of gas generators should generally have as low a temperature as possible. Gas generators are becoming increasingly important for civil applications (e.g., airbags in automobiles). Since propellants burn without atmospheric oxygen to produce the working gas, the propellant system must contain the oxidizer required for the gas-generating reaction. The oxidizer may be available in the form of molecularly bound functional groups or in the form of discrete salts containing bound oxygen. Propellants may exist in solid or liquid form and generally consist of several components. The linear burning rate and its pressure dependence can be modified only very slightly by varying the chemical composition of the propellant. In order to satisfy practical requirements, the burning surface and/or the geometry of the propellant charge is therefore varied widely: this c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim / a22 185

2 2 Propellants means for rocket propellant as well as for gun propellant charges. Solid propellants and some liquid propellants for rockets and guns may contain the fuel and oxidizer in a single phase (monopropellants). Alternatively, the liquid propellant fuel may undergo a combustion reaction with a separately added oxidizer; the combination of fuel and oxidizer (bipropellant) constitutes the propellant. Such systems involve combustion techniques similar to those used in a conventional combustion engine. The throughput can be varied by modifying the mode of injection into the combustion chamber. The solid propellant charge and individual propellant components are subjected to mechanical stress and must have appropriate mechanical properties. With the exception of compressed powder propellants for gas generators, solid propellant charges consist of a polymeric binder matrix that may contain crystalline additives. Single- and double-base propellants comprise plasticized nitrocellulose which may be filled with a crystalline explosive agent. Composite propellants consist of a polymeric binder that contains energetic materials and oxygencontaining salts. Propellants also contain far more additives than explosives: stabilizers, burning moderators, energy suppliers, processing auxiliaries, etc. Previously, an explosive could be distinguished from a propellant on the basis of its chemical composition; nowadays, however, this is not always possible. For example, large proportions of high-performance explosives are found in propellant compositions, and the oxygen- containing salt ammonium perchlorate (NH 4 ClO 4 ), commonly used in composite rocket propellants, may be found in explosive mixtures. Compositions for rocket propellants can only be distinguished from those for guns by the specific additives. There are two reasons for this. The first is the development of formulations that are particularly insensitive to external influences such as heat, impact, shock, shaped charges, or fragments (low vulnerability ammunition, LOVA). The second is the desire to obtain maximum performance. Progressive automation of production and weapon technology within the last few decades has provided further incentives for development. Examples include the continuous production of propellants and the development of caseless ammunition, combustible casings, unit charges, and liquid propellants for guns. The use of environmentally friendly products and the possibility of recycling will also become important in the propellant sector. 2. History The first incendiary agents were the precursors of solid propellants and were used in ancient times to attack defense fortifications and ships, and also as pyrotechnics for religious purposes. Black powder-like compositions were discovered by Chinese alchemists in the eighth century. In the middle of the eleventh century such formulations were packed in bamboo tubes and used as rocket-propelled flame throwers. Black powder (a mixture of potassium nitrate, sulfur, and charcoal) probably reached Europe through India and Arabia by the thirteenth century: it was mentioned by Roger Bacon in Blackpowder mixtures were also described by Albertus Magnus before The first drawing of a cannon operated with black powder was made by Walter de Milimete in Black powder remained the only propellant for guns and rockets for many centuries until the introduction of smokeless powder (plasticized nitrocellulose). Nitrocellulose was first synthesized by Schönbein and Böttcher in In 1883 Nobel used nitroglycerine to plasticize nitrocellulose. In 1885 Vielle used an ether alcohol mixture to produce a plasticized nitrocellulose. In 1899 Abel and Dewar used acetone as a plasticizing auxiliary. The replacement of black powder by nitrocellulose containing propellants lasted up to World War I. In order to avoid the high barrel erosion produced by the high-performance powders, cold powder was developed in Germany; here, nitroglycerin was replaced by di- or triglycerine dinitrate (1935), and nitroguanidine was also added (triple-base propellant, Gudol powder 1937). Ball powder, which can easily be metered and is suitable for use in small- caliber weapons, was developed in the United States in Prior to the twentieth century the use of rockets propelled by black-powder charges was restricted almost exclusively to fireworks and dis-

3 Propellants 3 play pyrotechnics. Few reports exist concerning the military application of solid propellant rockets. The reason for this was the short range and low accuracy of the rockets. The development of compressed propellant charges consisting of smokeless powders began after World War I. The development of composite propellants consisting of a finely ground perchlorate- or nitrate salt that was incorporated in a polymeric binder, cast, and cured, began in Combustible cases for larger caliber weapons were developed in the 1960s in the United States, and caseless ammunition for small caliber weapons was developed in the 1970s in the United States and Europe. Since the 1980s internationally unit charges have been produced for howitzers to automate the charging process. Since the 1970s attempts have been made to produce charges for all types of explosives (high explosives, gun propellants, rocket propellants) that display maximum insensitivity to external influences (shock, impact, temperature, etc.) while retaining their high performance ( low vulnerability ammunition, LOVA). These trends, combined with efforts to produce environmentally compatible, possibly recyclable compositions, will continue. The use of liquid propellants for rockets and missiles achieved great importance after experiments were carried out at the beginning of the twentieth century (Goddard, Ziolkowski) and before and during World War II (Oberth, Dornberger, von Braun, Nebel). This work was continued on a very large scale after 1945 in the United States and former USSR. In contrast to solid propellants, the choice of liquid propellants is still restricted to relatively few types for military application. After World War II the use of liquid propellants for guns was investigated. It remains to be seen whether these systems have any future [23 28]. 3. Classification and Formulation of Propellants Propellants are defined as substances or mixtures of substances of an explosive nature in a solid to liquid state that burn exothermically without atmospheric oxygen to form a large volume of gas. The time course of gas formation can be reproducibly predicted from the ambient pressure and the chemical composition, shape, particle size, and density of the propellant. In order to obtain a high gas yield, the propellants must produce combustion gases that have high gas mole number, a low mean molecular mass, a high flame temperature, and a low ratio of specific heats (c p /c v ). Propellants are used as charges for guns, rockets, and gas generators. Solid or liquid propellant charges can be used for all three purposes. Pasty or gel-like systems have not achieved any importance. Propellant charges serve simply to produce working gas to launch a missile from a tube or barrel or to propel rockets. In gas generators, however, they are used for a wide variety of purposes. The generated gases are used to drive pumps, empty tanks, actuate valves, bolts and separating devices, to inflate air bags, or jettison sub-systems. In contrast to propellant charges for guns and rockets, working gases for gas generators should have a medium or low temperature ( C) Solid Propellants Solid propellants are used in guns, rockets, and gas generators. An extremely large variety of types is available with different forms and chemical compositions that depend on the intended use. Single-base propellants are based on plasticized nitrocellulose. The plasticized nitrocellulose may, however, also contain an explosive plasticizer such as nitroglycerin (double-base propellants). Nitroguanidine may also be incorporated in the double-base powder matrix (triple-base propellants) in certain artillery systems that are susceptible to high barrel erosion. In addition to nitrocellulose- containing propellant systems, a large range of heterogeneous or composite propellants is available. These propellants are formulated from a plastic fuel that is blended with oxygen- containing salts and/or energetic substances; they may also contain energy-rich plasticizers. The polymers themselves may have functional groups that increase the energy content of the propellant composition.

4 4 Propellants Finally, the principal constituents of doublebase propellants can be combined with those of composite propellants. These combinations are termed composite-modified double-base propellants (CMDB) or composite double-base propellants (CDB). The most important constituents of solid propellants and their functions are listed below: Binders (cross-linked) Thermoplastic binder Energetic binders, thermoplastic Energetic binder, cross-linked Plasticizers Energetic plasticizers Oxidizers Energetic fuels polyurethane (PUR) hydroxy-terminated polybutadiene (HTPB) carboxy-terminated polybutadiene (DTPB) cellulose acetate butyrate (CAB) nitrocellulose (NC) poly(vinyl nitrate) (PVN) polyglycidylacid (GAP) triacetin dibutyl phthalate diethyl phthalate nitroglycerine (NG) trimethylolethane trinitrate (TMETN) butanetriol trinitrate (BTTN) bis(2,2-dinitropropyl)acetal/formal (BTNA/F) dinitrotoluene (DNT) ammonium perchlorate (AP) ammonium nitrate (AN) cyclotrimethylenetrinitramine (hexogen, cyclonite, RDX) cyclotetramethylenetetranitramine (octogen, homocyclonite, HMX) nitroguanidine (NQ) triaminoguanidine nitrate (TAGN) aluminum organic lead and/or copper salts Metallic fuel Ballistic modifiers for NC propellants Ballistic modifiers for ferrocene derivatives, copper chromite composite propellants Stabilizers diphenylamine nitrodiphenylamine diethyldiphenylurea (Centralite I) methyldiphenylurea (Akardite II) Wetting agent Bonding agents Flash reducers Opacifier lecithin, polyoxyethylenesorbitantrioleate aziridine, triethanolamine potassium sulfate, potassium nitrate carbon black Details of the properties and production of the explosive components are given in [3 21], see also Explosives Homogeneous Propellants Single-base propellants consist mainly of plasticized nitrocellulose (nitrogen content e.g., %). They contain plasticizers such as dibutyl phthalate and possibly additives to improve processing, suppress muzzle flash, etc. Older types of powders are processed also with dinitrotoluene. Stabilizers (e.g., diphenylamine) are added to the propellant for good storage stability. Single-base propellants are produced with maximum web thicknesses of 12 mm to facilitate solvent removal during processing with the result that they can be used only as gun propellants or in bulk powder fillings for gas generators. Double-base propellants consist of nitrocellulose (nitrogen content % to ca %) and an explosive plasticizer such as nitroglycerine. The remaining additives are similar to those used in single-base propellants. Both propellant types contain 0.5 ± 0.3 % water. Types in which crystalline explosives such as HMX and RDX are incorporated in the double-base matrix represent a transition between double-base and composite propellants. Triple-Base Propellants. Nitroguanidine has a relatively low flame temperature but produces a large volume of gas, and is used in cold powders (triple-base propellants) to reduce the flame temperature and thus protect the barrel. In these propellants nitroglycerine is replaced by explosive plasticizers with a low heat of explosion (e.g., diethyleneglycol dinitrate or butanetriol trinitrate). Examples of gun propellants are given in Table 1 and rocket propellants in Table 2. Compositions for rockets also contain ballistic modifiers (e.g., lead salts and/or copper salts) to reduce the pressure exponent Composite Propellants The heterogeneously formulated composite propellants consist of a polymeric binder in which crystalline oxidizers (e.g., ammonium perchlorate) and/or energy suppliers (e.g., RDX or HMX) are incorporated in amounts of up to 90 wt %. The binders are generally viscoelastic cross-linked elastomers. Compositions of composite propellants for rockets and gas generators are given in Table 3, and for LOVA guns in Table 4. Many different types of plastics have been used but interest is now concentrated on castable

5 Propellants 5 Table 1. Composition (wt %) and performance of gun propellant Component or property M 1 M 8 M 30 A 502 JA 2 Nitrocellulose (nitrogen, %) (13.15) (13.25) (12.6) (13.1) (13.0) Nitroglycerine Diglycol dinitrate 21.7 Nitroguanidine 47.7 Ethylcentralite Diphenylamine Methyldiphenylurea 0.7 Dinitrotoluene 9.9 Dibutyl phthalate Diethyl phthalate 3.0 Additives Flame temperature, K Force, J/g Mean molecular mass of products M 1, A 502 = Single-base propellant, A 502 is a German 20 mm caliber propellant; M 8, JA2=Double-base propellants; M 30 = Triple-base propellant polybutadienes. Their prepolymers are terminated with carboxyl or hydroxyl groups and cross-linked with difunctional or trifunctional isocyanates in the presence of catalysts. Bonding and wetting agents and plasticizers are added to improve processing and mechanical properties. Thermoplastics (e.g., cellulose acetate butyrate) are also suitable for certain applications, in particular for gun propellants, and are compressed after incorporation of a filler [11]. The mean particle size and particle distribution of the fillers influence processing, burning behavior, mechanical properties, and shock sensitivity. Rocket propellants contain aluminum powder to increase the flame temperature, although this has the disadvantage that a large amount of smoke is produced due to formation of aluminum oxide (primary smoke). Ballistic modifiers and additives for reducing pressure oscillation are added to improve burning behavior. Composite propellants based on ammonium perchlorate release hydrogen chloride on burning, which produces intense smoke in the presence of atmospheric moisture (secondary smoke). Ammonium perchlorate is replaced by chlorine-free oxidizers (e.g., ammonium nitrate) to reduce or avoid the formation of smoke and corrosive combustion gases. Energetic substances (e.g., RDX or HMX), explosive plasticizers (e.g., nitroglycerine, BTNA/F), and energy-rich binders (e.g., GAP) are used to improve performance and/or avoid or reduce the ammonium perchlorate content. If nitrocellulose or poly(vinyl nitrate) is also added to the nonexplosive binder, composite-modified double-base propellants are obtained (CMDB, C in Table 3). Table 2. Compositions (wt %) and performance of double-base rocket propellants Component or property Extruded Cast Nitrocellulose (nitrogen, %) (12.75) (12.6) Nitrogylcerine Dibutyl phthalate 3.91 Dimethylformamide 1.2 Triacetin 9.1 Methyldiphenylurea 1.50 Diethylphenylurea Lead salts Additives Flame temperature, K Specific impulse (70: 1) equilibrium, Ns/kg Mean molecular mass of products c p/c v Composite propellants are particularly suitable for rockets up to very large diameters, but have also been used as insensitive LOVA gun propellants (Table 4). Corresponding developments can also be observed in the field of rocket propellants (insensitive rocket propellants, IRP). In such less- or low-sensitive propellants, nitrate esters are avoided wherever possible; instead nitramines (e.g., RDX or HMX of small grain size, d < 10 µm) and/or nitroguanidine (spherical or high bulk density) or triaminoguanidine nitrate, are used as energy compounds [29 32].

6 6 Propellants Nitro compounds (e.g., BTNA/F) are used as explosive plasticizers. Thermoplastics (e.g., CAB) are used as binders for gun propellants. Filled viscoelastic cross-linked polymers that have been extruded and then cured may also be used as an alternative. Performance may be improved by employing energetic binders such as GAP. Insensitive rocket propellants have similar compositions, both ammonium nitrate and ammonium perchlorate are used. The disadvantages of ammonium nitrate are its high hygroscopicity and the transformation of its crystal structure at C, which is accompanied by a change in density. This transformation can be displaced to higher temperatures by additives like diammine complexes of copper zinc, or nickel [33]. Propellants for larger gas generators are generally of the composite type. For small charges also single- or double-base propellants are used. Composite types have a low binder content and contain crystalline fillers (e.g., nitroguanidine, triaminoguanidine nitrate) that, on combustion, produce a large amount of gas with a relatively low flame temperature. Endothermically decomposing compounds (e.g., oxalates) are added to reduce the flame temperature. Passenger restraint systems for vehicles (e.g., air bags) generally contain sodium azide as a gas-generating charge which minimizes the concentration of carbon monoxide and carbon dioxide formed after the reaction Monopropellants Monopropellants consist of substances that already contain the oxidizer necessary for combustion (nitro compounds, nitrate esters), or of a mixture of fuel and oxidizer. These monopropellants must not react in the applied temperature range ( 40 Cupto+50 C) and must be stable on storage. Monopropellants have litte importance for rocket propulsion systems, apart from auxiliary systems (e.g., correction of satellite position). Examples of monopropellants are % hydrogen peroxide (which eventually decomposes, however, despite the addition of stabilizers) and hydrazine and its derivatives. Both compounds are injected into the combustion chamber over decomposition catalysts. Monopropellants consisting of a mixture of components were initially used in addition to hydrogen peroxide to propel torpedoes. Examples of such mixtures are hydrazine, hydrazine nitrate, and water and compositions containing hydroxylammonium nitrate (HAN). These mixtures are known by the names Otto Fuel, Duplex, Solex, and NOS. Monopropellants were also used as propellants for guns; mixtures of nitromethane and isopropyl nitrate or methanol, as well as mixtures of hydroxylammonium nitrate and alkyl-substituted ammonium nitrate and water are of particular interest [35]. The following composition is typical: 3.2. Liquid Propellants [3], [15] LP % hydroxylammonium nitrate (HAN) 20.0 % triethanolammonium nitrate (TEAN) 16.8 % water Liquid propellants contain the fuel and oxidizer as a single substance or mixture of substances (monopropellants), or in two separate phases (bipropellants). In contrast to solid propellants, where the solid for combustion is located in the combustion chamber, the liquid propellant is injected into the combustion chamber from a storage tank. The type of combustion and the propellant throughput is principally determined by metering. The first rockets and guns based on liquid propellants employed single substances or simple mixtures of substances. Only later did the compositions become more complex through the use of additives, which are used to modify the combustion process [34] Bipropellants Bipropellants consist of an oxidizer and a fuel that are injected into the combustion chamber in two separate phases. Both the oxidizer and fuel may consist of more than one component. Conventional systems for rocket propulsion consist of hydrogen peroxide, concentrated nitric acid, nitrogen dioxide, or liquid oxygen as oxidizer, and hydrocarbons (e.g., rocket propellant, RP 1; jet propellant, JP-5), alcohols, amines, hydrazines, and alkyl derivatives as fuel. Liquid fluorine or a mixture of fluorine and oxygen (FLOX) and liquid hydrogen are also used in

7 Propellants 7 Table 3. Composition (wt %) and performance of composite rocket propellants and gas generators Component A B C D E or property AP RDX 52 NQ 50 TAGN 75 Aluminum PUR and additives HTPB and additives GAP and additives 8 NG and additives 24.5 Triacetin 15 BTNA/F 12 Flame temperature, K Specific impulse (70: 1), Ns/kg Mean molecular mass of products c p/c v Density, g/cm For explanation of abbreviations, see 3.1 A) Composite propellant with secondary smoke; B) Composite propellant with primary and secondary smoke; C) Composite modified double-base propellant with reduced smoke; D) Insensitive composite propellant; E) Gas-generator composition Table 4. Composition (wt %) and performance of LOVA gun propellants Component A B C or property RDX NC 4.0 NQ 26 TAN 20 HTPB and additives 15 GAP and additives 13 CAB and additives 12.0 Acetyltriethyl citrate 7.6 Diethyldiphenylurea 0.4 Flame temperature, K Force, J/g Mean molecular mass of products For explanation of abbreviations, see 3.1 A) Propellant with thermoplastic binder; B) Propellant with cross-linked binder; C) Propellant with cross-linked energetic binder. aerospace applications to improve performance. Some propellant combinations are hypergolic (i.e., the fuel and oxidizer ignite spontaneously on contact). Examples are concentrated nitric acid and hydrazine or alkyl derivatives. This hypergolic behavior is exploited in guns, where the propellant combination ignites immediately when injected into the combustion chamber; an example of such a propellant is concentrated nitric acid and monomethyl hydrazine (MMH) or triethanolamine. 4. Reactions and Properties Propellants have to satisfy various criteria and must also have specific properties, depend-ing on their use. When choosing and developing a propellant its properties must therefore be matched against one another to achieve the best solution. This requires interdisciplinary cooperation between the system developer and propellant manufacturer. Factors that have to be observed when choosing a propellant are listed below:

8 8 Propellants Ignition conditions Burning rate (depending on pressure and temperature) Performance Internal ballistic behavior (combustion instability and pressure irregularities, pressure time curve) Density and bulk density Chemical stability and compatibility Long storage behavior service life Mechanical properties (depending on temperature and stress rate) Deflagration to detonation behavior Sensitivity and vulnerability Manufacturing characteristics (availability, cost, hazards etc.) Rocket exhaust smoke Gun smoke and flash 4.1. Combustion Process [12], [36 38] The burning behavior of a solid propellant depends on its composition, pressure, propellant temperature, and on the characteristics of the flow field. The propellant burns in parallel layers vertically to the burning surface, provided that the combustion gases can escape unimpeded. The mass flow rate depends on the amount of heat produced during combustion and its feedback to the burning surface. Ignition is effected by heating the propellant surface above the decomposition temperature with hot gases, hot particles, or lasers. Many combustion models exist for different compositions and pressure ranges. In practice the following equations are used: For propellants in guns ( p > 200 kpa) r=a+bp and for rockets ( p < 30 kpa) r=cp n where r = linear burning rate p = pressure a, b, c = temperature-dependent constants n = pressure exponent The gaseous combustion zone above the burning surface constitutes the heat source for propellant decomposition. A rise in pressure compresses the combustion zone, thus increasing the heat feedback to the surface and producing a higher burning rate. A higher propellant temperature also increases the burning rate because less heat is then required to heat up the surface. At higher temperatures, higher pressures are therefore obtained in the gun or rocket combustion chamber. Burning mechanisms differ according to the composition of the propellant. With homogeneous (single- and double-base) propellants the nitrate ester decomposes near the propellant surface (fizz zone) at ca. 600 K (Fig. 1). The primary products react in the nonluminous dark zone at ca K, which disappears at higher pressures. Further reaction up to thermodynamic equilibrium then occurs in the overlying flame zone ( K). Figure 1. Diagram of the flame structure of a double-base propellant [36] In the pressure range ca kpa, the use of burning modifiers consisting of lead and/or copper salts of aliphatic and aromatic acids in NC- containing propellants may result in a pressure-independent burning rate (plateau n =0)or a reduction in the rate due to a pressure rise (mesa effect, n becomes negative) (Fig. 2). In general the pressure exponent in rockets should not exceed n = 0.6. Nitramines such as RDX or HMX melt before or during decomposition, depending on the heating rate. The decomposition products then

9 Propellants 9 react like the other combustion products in the zones described above [39] Propellant Performance The performance of a propellant is determined by its composition, its reaction products, and the difference between the heat of formation of the reactants before and after the reaction. The most important thermodynamic features of the combustion products are the flame temperature, gas volume, heat capacity, ratio of specific heats (c p /c v ), and, where appropriate, the covolume of the gases at high pressures. Figure 2. Burning behavior in solid, double-base propellants a) Propellant without catalyst; b) Plateau propellant; c) Mesa propellantpropellant temperatures are indicated Oxygen- containing salts, polymeric binders, and metal powders (aluminum) exhibit different combustion mechanisms, which can also change with pressure and temperature [40], [41]. The oxygen-rich gases of the decomposing oxygencontaining salt and the pyrolysis products of the organic binder produce a diffusion flame. The grain size of the ammonium perchlorate greatly influences the burning rate: the smaller the grain size the higher the burning rate. Burning modifiers (e.g., copper chromite) also increase the burning rate (Fig. 3). The grain size of ammonium nitrate has far less influence; these propellants have a relatively low burning rate, which can be increased by adding similar burning modifiers like ammonium dichromate. Oxidation of the metal powder occurs a long way from the burning surface and has only a slight effect on the burning rate. The small metal particles often agglomerate to form larger aggregates. In liquid monopropellants vaporization and decomposition of the liquid occur on the droplet surface and depend on the composition and heating rate. Combustion follows in the gaseous phase. With bipropellants a diffusion flame is formed between the oxidizer and fuel. Figure 3. Burning rate range for polybutadiene propellants [16] a) AP propellants with burning rate modifiers like metal oxides; b) AP propellant with different particle size; c) Propellant with mixed AP and AN oxidizers Since the early 1970s computer programs have been developed to calculate the performance of gun and rocket propellants, they generally have an accuracy of 1 2%[42], [43 45]. Differences between the results of the programs are generally attributed to different thermodynamic input data.

10 10 Propellants The specific energy or force f (in m tkg 1 or J/g) is derived from the equation of state for gases ( f = pv = nrt ) and is used as the performance parameter for gun propellants, where p = pressure V = volume n = number of moles of gas R = universal gas constant T = explosion temperature Since propellant powders in a gun at virtually constant volume and varying pressure ( 600 kpa) burn at temperatures up to 4000 K, the calculation requires an equation of state that takes into account the behavior of the gases at high densities (virial equation) [43 45]. The computer programs also provide information on the heat of explosion, explosion temperature, average molecular mass of the explosion products, total mole number, composition of the explosion gases, specific heat ratio (c p /c v ), and covolumes. Examples of performance data of gun propellants are given in Tables 1 and 4. The most important performance parameter of rocket propellants is the specific impulse I s : I s= Ft w inns/kg where F = average thrust t = burning time w = weight of propellant As the following equation shows, the specific impulse depends on the flame temperature in the chamber T c, the average molecular mass of the flame gases m, the specific heat ratio k = c p /c v, and the pressure expansion ratio p c /p e [ ] I s= 2 g RTc m k 1 p(k 1)/k e Ns/kg k 1 p c where g is the acceleration due to gravity, p c is the chamber pressure and p e is the pressure at the nozzle end. In contrast to gun propellants, rocket propellants generally burn at a uniform pressure. The specific impulse is often given at an expansion ratio of 70: 1, a distinction is made as to whether the combustion gases at the nozzle end are in equilibrium I s, equil or in a frozen state I s, froz. As well as the specific impulse the following information can be obtained from the computer programs: Chamber temperature (adiabatic flame temperature) Temperature of exit gas (equilibrium or frozen) Composition of the burnt gases Average molecular mass of the burnt gases Total mole number of the burnt gases Ratio of specific heats c p /c v Since, in practice, the gas jet does not expand fully due to the restricted nozzle dimensions, a difference of up to several percent is found between the theoretical and experimentally determined specific impulse. Aluminum oxide occurs as condensed particles in the combustion jet and these particles often do not reach the gas flow rate. The difference between theory and practice is thus even greater in aluminum- containing propellants. The combustion gases only reach equilibrium at the nozzle end in rockets with very large diameters. Examples of rocket-propellant performance data are shown in Tables 2 and 3. Experiments in standard combustion chambers are carried out on a kilogram scale to determine the performance, burning behavior, and chamber pressure at different Klemmung (ratio of the burning surface area of the propellant charge to the nozzle throat area). Since the volume of the loading chamber of a gun and the volume of a rocket combustion chamber are restricted, a charge with a propellant of a high density has advantages as regards performance. The velocity of a rocket V b on completion of burning is also influenced by the ratio of the rocket mass at the start and completion of burning. For vertical liftoff the following equation applies: V b = V g ln ma m e gt where V g = velocity of the combustion gases m a = rocket mass at start m e = rocket mass at completion of burning g = acceleration due to gravity t = burning time

11 Propellants 11 The densities of single- and double-base propellants are g/cm 3. Composite propellants may have densities as high as 1.76g/cm 3, particularly if they contain aluminum powder Mechanical Properties Propellant charges are subjected to mechanical stresses that depend on their shape, size and the form in which they are used. Since propellants mainly consist of polymers, their mechanical properties also depend on the temperature and the stress rate. According to the superposition principle, the low-temperature behavior of the embrittlement, tensile strength, and elongation is nearly the same as that of propellants at room temperature but at high stress rates [46 48]. Gun Propellants. In guns the propellant powders are subjected to the force of the pressure wave in the powder bed. In extreme cases they are thrown against the base of the projectile during ignition. This leads to the danger of embrittlement, especially at low temperature with double- and triple-base propellants. Embrittlement results in an uncontrolled rise in pressure in the lower temperature range. Rocket Propellants. The mechanical properties of rocket propellants are of greater importance. The thermoplastic double-base propellants generally have a high tensile strength and a relatively low elongation at break. They are therefore used as cartridge case propellant charges. The temperature range is limited due to embrittlement in the low temperature range (ca. 30 C) and softening above +70 C. Viscoelastic composite propellants generally have a high elongation at break and a relatively low tensile strength. Composite propellants are therefore almost always used as casebonded propellants, the large expansion capacity of the propellant absorbs the chamber movement as a result of temperature cycling. The tensile strength needs only be sufficiently large to ensure stability of the propellant under load. The larger the diameter and length of the propellant charge, the greater must be its elongation at break. During the service life of a solid propellant the mechanical properties change in varying degrees. With composite propellants the elongation at break can deteriorate resulting in embrittlement caused by postcuring effects. Softening can also occur as a result of depolymerization, and leads to deterioration of the tensile strength [48]. The mechanical properties are also greatly affected by adhesion between the filler and binder. Moisture, frequent temperature variations, and vibration can cause dewetting of the filler from the binder, resulting in a decrease of the mechanical properties. In nitrocellulose the mean molecular weight decreases during aging, which likewise negatively affects the mechanical properties [49]. Surfaces of rocket propellants that are not meant to burn for internal ballistic reasons are provided with firmly adhering insulation layers made of polymers that are compatible with the propellant (e.g., ethylcellulose for double-base propellants and PVC or HTPB for composite propellants). The insulation layers may also contain inert fillers and plasticizers. The mechanical properties of the propellant and/or insulation can also change due to mutual migration of their constituents, and may result in malfunctions Chemical Stability Nitrate esters decompose to form nitrogen oxides which provide the decomposition of ester groups (autocatalysis). This process is exothermic and can lead to self-ignition of the propellant. Stabilizers (e.g., substituted diphenylamines, ureas, or urethanes) are added to inhibit this reaction. Examples include diphenylamine, nitrodiphenylamine, and diphenylureas (Centralite, Akardite) [50 52]. The stabilizers trap the released nitrogen oxides to form polynitro compounds. The final stability of such a propellant is measured by the residual capacities of the stabilizers to trap nitrogen oxides. The process is temperature dependent and obeys the Arrhenius equation. Information of the service life of a propellant at normal temperature can thus be obtained from stability testing at elevated temperature. Besides NO x also N 2 O, CO, CO 2 and H 2 O may be formed during the aging process by re-

12 12 Propellants action of NO x with the remaining reactants. In large propellant charges these gases lead to crack formation. Especially CO 2 may also accumulate underneath the insulation and detach it from the propellant surface. The conventional composite propellants containing ammonium perchlorate are extremely stable. Chemical reactions involving the binder affect the mechanical properties. Nitramines and plasticizers containing nitro groups are also more stable than nitrate esters. Hot-storage tests are used to check the chemical stability: samples of propellants are stored at C and the weight loss or release of gasis measured. Propellants that come into contact with other materials (insulation materials, liners, metal parts, etc.) are subjected to compatibility testing, i.e. the propellant is investigated to determine whether it reacts with the other material at elevated temperature (release of gas) Service Life The service life of a propellant ends when the reproducibility of performance and function can no longer be guaranteed within the prescribed limits. The duration of the service life depends on the chemical composition, size of the propellant charge, type of use, conditions of storage and use (storage temperature, temperature fluctuations, moisture, mechanical stress, etc.), and the prescribed operational conditions. The service life of gun propellants may be assumed to be about 30 years or even higher under good storage conditions and with very stable powders. Rocket propellants generally have a shorter service life, in some cases only 12 years [53] Sensitivity and Vulnerability Sensitivity. As in the case of high explosives, propellants are also screened to investigate their handling safety. Various methods and equipment are used to test the sensitivity with respect to impact, friction, electrostatic discharge, heat, and flame. Standard national test regulations exist for the certification of propellants and obtaining permission for transportation [54 57]. In 1987 the United Nations Committee of Experts on the Transport of Dangerous Goods issued recommendations for tests and criteria and has classified dangerous goods in nine classes [58]. Class 1 includes five explosive divisions 1.1) Substances and articles which have a mass explosion hazard 1.2) Substances and articles which have a projection hazard but not a mass explosion hazard 1.3) Substances and articles which have a minor blast hazard or a minor projection hazard or both, but not a mass explosion hazard 1.4) Substances and articles which present no significant hazard 1.5) Very insensitive substances which have a mass explosion hazard but are so insensitive that there is very little probability of initiation or of transition from burning to detonation under normal conditions of transport. As a minimum requirement they must not explode in the external fire test. Handling and transport regulations are governed by the classification according to these five subdivisions. In some countries these subdivisions also have consequences for munition storage regulations. The United Nations scheme provides for six test series which can also be applied to liquid propellants (monopropellants). The following parameters are tested: 1) Detonative shock under confinement 2) Intensive heat (fast cook-off) 3) Impact and friction 4) Thermal stability 5) Ignition 6) Free-fall impact of a test unit 7) Shock from a detonator cap 8) Transition from deflagration to detonation 9) Exposure of substance in packing to an external fire With an additional Test Series 7, articles of Division 1.5 and extremely insensitive detonating substances (EIDS) are identified and specified precisely. For example, for EIDS the following are additionally specified: 1) Threshold for propagation of steady-state detonation conditions = Critical Diameter (confined and unconfined)

13 Propellants 13 2) Detonative shock under confinement = EIDS GAP Test 3) Relative behavior of high explosive loads in projectiles and fired against a target = Susan Test 4) Mechanical behavior of a compact high explosive under a large impact (velocity of pieces = 150 m/s) = Friability Test 5) Response of an EIDS to kinetic energy associated with impact and penetration = Bullet Impact Test 6) Reaction to a gradually increasing thermal environment (3 K/h) = Slow Cook-Off Test Vulnerability. The vulnerability of munition depends on the behavior of its explosive components with respect to external influences. These may vary in nature and intensity according to the use of the munition and the expected military threat. The inherent vulnerability is a combination of 1) The susceptibility of munition to initiate an unintentional reaction (i.e., its sensitivity) 2) The violence of such a reaction (i.e., its explosiveness) caused by accident (e.g., shock, impact), environmental stress factors (e.g., aerodynamic heating, electrostatic charging), and enemy action (e.g.,fragmentation shock). The vulnerability of munition can be improved by modifying munition construction, implementing appropriate mechanical measures, and using insensitive energetic materials such as insensitive high explosive (IHE), insensitive gun propellants (LOVA), and insensitive rocket propellants (IRP). These insensitive energetic materials should react reliably on demand and fulfill performance requirements. Regardless of whether such materials are confined in munition or not, undesirable external influences caused by heat, shock, fragmentation projectiles, impact, friction, and electrostatic discharge should not lead to violent reactions that produce hazardous fragments or blast waves. In order to check the vulnerability of gun and rocket propellant charges, the tests of the abovementioned Test Series 7 (Division 1.5) are used to the specific threat. 5. Uses 5.1. Gun Propellants Solid Propellants The Firing Process. In the firing process the propellant charge is ignited by a pyrotechnic device. Solid propellants for guns are also known as gun powder. Ignition is effected by hot gases, and is often assisted by the emission of hot particles. Lasers may be also used in future. The objective is to initiate ignition simultaneously over the whole surface of the propellant charge. Ignition lasts between < 1 ms and several 10 ms depending on the caliber size. The propellant charge then reacts to form hot gases, and the generated gas pressure ejects the projectile with increasing velocity from the barrel. Only % of the chemical energy is converted into kinetic energy to propel the projectile; % is converted into kinetic energy of the gases or remains as heat in the gases; a further % heats the barrel, case, and projectile. The pressure falls due to the energy losses of the gases and the increase in volume as a result of movement of the projectile. Thus, the gas pressure usually reaches a maximum value as a result of combustion of the propellant charge, and then falls. The propulsion process generally lasts < 10 ms, and accelerations of more than 10 5 g are produced. Muzzle velocities may reach 2200 m/s with highperformance artillery, at maximum gas pressures of more than 700 kpa [59], [60]. The firing process involves not only thermodynamic and gas dynamic processes, but also the reaction kinetics of the decomposition products of the propellant. The firing process can be simulated with internal ballistic computer models. The gas pressure curve produced on burning the powder can be influenced by the following parameters: 1) Linear burning rate r of the propellant. 2) Treatment of the powder grains. The surface of the powder grain is modified so that it burns more quickly or more slowly than the underlying layers; treatment with plasticizers (e.g., camphor, dibutyl phthalate) produces an initially slower burning rate (progressive behavior). Incorporation of salts

14 14 Propellants (potassium nitrate) into the powder grain and subsequent dissolution yield a porous powder structure with a large, initial rise in pressure (manoeuver powder). 3) Loading density of the propellant = Propellantweight Combustionvolume g/cm3 The loading density is in general between 0.1 (mortars) and 1.1 g/cm 3. 4) Total free propellant surface area. 5) Shape of the propellant particle. The quotient of the powder area at time z (A z ) and the initial area A a is termed the form function f = A z /A a. The dynamic vivacity is defined as they are formed: long tubes (perforated or unperforated), strips, sticks (slotted or unslotted), short- cut perforated tubes, short- cut multiperforated tubes, flakes, and ball powder [62], [63], [64]. Types of Ammunition. Since the early 1980s caseless ammunition has been developed for small- caliber weapons. The advantage of this type of ammunition is a short, compact cartridge in which an empty case no longer has to be ejected after firing. It consists of a porous powder body which partially or completely encloses the projectile (Fig. 4). L= ṗ wherepm= maximumpressure. p m p Gun Barrel Erosion. Chemical and physical interactions between the high-velocity, hot combustion gases and the wall of the barrel result in barrel wear due to erosion. These phenomena occur particularly in high-performance weapons and powders with high combustion temperatures. Barrel erosion and wear can be reduced by using additives (e.g., titanium dioxide or talcum). To avoid erosion the combustion temperature can be lowered by adding nitroguanidine and replacing nitroglycerine by less energetic explosive plasticizers (cold powders) [59]. Muzzle Flash. Only substoichiometric amounts of oxygen are available to react with the propellant charge powders. Combustible gases (CO, CH 4, etc.) therefore leave the muzzle during firing, which can mix with the air and ignite (muzzle flash). This can be suppressed by adding appropriate compounds (e.g., K 2 SO 4 or KNO 3 ) to the propellant [61]. Charge Technology. The propellant charge can be combined with the projectile to form a munition unit (cartridge munition) which is packed in the loading chamber. Alternatively, the propellant charge is introduced separately into the weapon in powder bags in some cases enclosed in a metal cartridge. Gun propellants may be classified according to the shapes into which Figure 4. Caseless ammunition (4.73 mm X 33, Dynamit Nobel) for the G11 rifle Ignition is initiated by a combustible cap. The powder charge often contains a small priming charge that comminutes the propellant charge into small elements during ignition. The propellant charge body is also provided with a combustible protective layer to prevent ignition outside the weapon. Unintentional ignition of caseless ammunition on contact with the hot walls of the loading chamber after previous firing (cook off) may be a problem. Solid propellant compositions having a high thermal stability are therefore used. An example of such a high ignition temperature propellant (HITP) is a finely crystalline, plastic-bonded nitramine (e.g., HMX) [65]. The desire to save material and avoid having to eject powder cartridges has led to the development of combustible cases for ammunition for large- caliber weapons. The case consists of a mechanically and thermally stable material like

15 Propellants 15 condensed cellulose fibers or a polymeric material which burns together with the powder charge under the high pressure existing in the weapon without forming a residue. The outer wall of the case also has a protective layer to prevent ignition outside the weapon. In the case of ammunition that is charged separately, the cartridge contains several partial charges for the different firing zones. Modular charges have been developed to reduce the number of partial charges. The ultimate aim is to use only one universal partial charge which can be snapped together to build up the complete charge and provide the required range. The unicharge is packed in a rigid combustible cartridge case which contains its own priming charge. The tubular unicharges are closed at both ends to protect the igniter and propellant from the environment. To ensure that the flame spreads along the full length of the unicharge, barriers are ruptured when the charges are combined Liquid Propellants Spurred on by the rapid development of rocket technology using liquid propellants, experiments began soon after the end of World War II to see whether liquid propellants could also be used for guns. This concept promised considerable advantages: 1) Improvement of specific performance 2) Optimization of the pressure time curve 3) Avoidance of a cartridge 4) Less space required for the ammunition and thus better space utilization 5) Adjustment of the firing range by controlled metering of the liquids 6) Increase in the firing rate (cadence) 7) Lower propellant costs This concept also opens up potential solutions for automating many aspects of artillery technology. Disadvantages were also anticipated, for example a complicated conveyance and metering system, corrosion and handling problems, and an internal ballistics behavior that was largely unknown. After 45 years of international development, it was found that such systems are feasible [66]. Two systems exist: 1) Bulk-loaded propellant guns (BLPG) 2) Regenerative liquid propellant guns (RLPG) Figure 5. Schematic of a regenerative test fixture for a liquid-propellant gun chamber [59] a) Propellant fill port; b) Ignition port; c) Injection orifices; d) Propellant reservoir; e) Barrel; f ) Combustion chamber; g) Chamber; h) Regenerative piston; i) Seals; j) Breech plug These systems can be operated with monopropellants (see Section 3.2.1), or bipropellants (see Section 3.2.2). In BLPG using monopropellants, the loading chamber behind the projectile is filled with the liquid propellant (charging density g/cm 3 ). Ignition is produced with a pyrotechnic device or submerged electric spark. Although the concept appears extremely simple, the internal ballistics are extremely complex and difficult to control. For example the energy and geometry of the igniter as well as the geometry of the chamber greatly influence the reaction of the liquid. The use of non-hypergolic bipropellants in BLPG has not been investigated in detail since it is even more difficult to control. With RLPG the combustion process can be controlled more easily. The liquid is injected into the chamber as a result of gas pressure. A model of a monopropellant regenerative test fixture is shown in Figure 5 [59]. The regenerative system can also be used for bipropellants, both liquids are injected separately into the chamber. Such a system is more complex but offers better performance. The advantages of a hypergolic combination (e.g., nitric acid triethylamine) can be exploited. Gun systems with liquid propellants are unsuitable for small- caliber weapons. They are becoming increasingly important for machine guns (> ca. 30 mm caliber), but especially for cannons, howitzers and mortars. The regenerative