Formulation of Propellants

The physicochemical properties of explosives are fundamentally equivalent to those of propellants. Explosives are also made of energetic materials such as nitropolymers and composite materials composed of crystalline particles and polymeric materials. TNT, RDX, and HMX are typical energetic crystalline materials used as explosives. Furthermore, when ammonium nitrate (AN) particles are mixed with an oil, an energetic explosive named ANFO (ammonium nitrate fuel oil) is formed. AN with water is also an explosive, named slurry explosive, used in industrial and civil engineering. A difference between the materials used as explosives and propellants is not readily evident. Propellants can be detonated when they are subjected to excess heat energy or mechanical shock. Explosives can be deflagrated steadily without a detonation wave when they are gently heated without mechanical shock. 

Molecules in which fuel and oxidizer components are chemically bonded within the same structure are suitably predisposed for the formulation of energetic materials. Nitropolymers are composed of O-NO2 groups and a hydrocarbon structure. The bond breakage of O-NO2 produces gaseous NO2, which acts as an oxidizer fragment, and the remaining hydrocarbon structure acts as a fuel fragment. NC is a typical nitropolymer used as a major component of propellants. The propellants composed of NC are termed nitropolymer propellants . 

Crystalline particles that produce gaseous oxidizer fragments are used as oxidizer components and hydrocarbon polymers that produce gaseous fuel fragments are used as fuel components. Mixtures of these crystalline particles and hydrocarbon polymers form energetic materials that are termed composite propellants . The oxidizer and fuel components produced at the burning surface of each component mix together to form a stoichiometrically balanced reactive gas in the gas phase. 

The polymeric hydrocarbon also acts as a binder of the particles, holding them together so as to formulate a propellant grain. Ammonium perchlorate (AP) is a typical crystalline oxidizer and hydroxy-terminated polybutadiene (HTPB) is a typical polymeric fuel. When AP and HTPB are decomposed thermally on the propellant surface, oxidizer and fuel gases are produced, which diffuse into each other and react to produce high-temperature combustion gases. 

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Double-base propellants may be formulated to include fuels such as aluminum metal, oxidizers such as ammonium perchlorate (AP), or energetic materials such as the nitramines RDX or HMX. The resulting compositions are termed composite-modified double-base propellants. A further modification, the addition of a polymer that is curable with a low molecular weight curing agent, allows formulation of propellants with much improved mechanical properties over the temperature range of usage. Such propellants are termed elastomer-modified composite double-base (EMCDB) propellants. 

N. S. Cohen and D. A. Flanigan, Effects of Propellants Formulation on Bum Rate—Temperature Sensitivity, CPIA Pubhcation 390, Vol. 3, CPIA, Johns Hopkins University, Laurel, Md., 1984. 

The prediction of burning-rate characteristics, on the other hand, has not been possible. This has caused rocket designers to adopt a trial-and-error approach to the development of specific propellants to meet specific mission requirements. In an effort to reduce the large development effort required for each new propulsion system, considerable basic research effort has been directed toward the definition and quantitative characterization of propellant combustion mechanisms. The ultimate objective of this effort is to provide methods for predicting the burning-rate characteristics of particular propellant formulations. 

The vapor pressure and density of propellants are employed to assist in formulation. The vapor pressure dictates the force of emission of the droplets from the metering valve of the inhaler. The force of emission is... 

Formulation of dry powders for inhalation must rely on a very short list of excipients to fulfill the customary roles of diluent, stabilizer, solubilizer, processing aid, and property modifier (e.g., flow enhancer). In the United States, only a few materials are approved for use in inhalation products, and of those (e.g., propellants, surfactants) many are of little help in dry powder formulation. 

The formulation of small organic molecules in most cases uses established processes and only a limited number of excipients (mainly lactose or a small number of propellants). In contrast,... 

Double-base propellants containing azide polymers are termed nitro-azide polymer propellants. DEP used as a plasticizer of double-base propellants is replaced with azide polymers in order to increase energy density. The compatibility of GAP prepolymer with NG serves to suitably desensitize the mechanical sensitivity of NG and gives superior mechanical properties in the formulation of rocket propellant grains. 

Azide polymers such as GAP and BAMO are also used to formulate AP composite propellants in order to give improved specific impulses compared with those of the above-mentioned AP-HTPB propellants. Since azide polymers are energetic materials that burn by themselves, the use of azide polymers as binders of AP particles, with or without aluminum particles, increases the specific impulse compared to those of AP-HTPB propellants. As shown in Fig. 4.15, the maximum of 260 s is obtained at (AP) = 0.80 and is approximately 12 % higher than that of an AP-HTPB propellant because the maximum loading density of AP particles is obtained at about (AP) = 0.86 in the formulation of AP composite propellants. Since the molecular mass of the combustion products. Mg, remains relatively unchanged in the region above (AP) = 0.8, decreases rapidly as (AP) increases. 

Nitroglycol (NGC) has similar physical and chemical properties to those of NG. However, its vapor pressure is too high to permit its use as a major component of propellants and explosives. NGG is used as an additive to reduce the freezing temperature of NG and to formulate explosives. However, the shock-sensitivity of the resulting NG-based explosives is much higher than that of other types of explosives. 

Hydrocarbon polymers (HCP) are used not only as fuel components but also as binders of crystalline oxidizers and metal powders in the formulation of pyrolants, similar to composite propellants and plastic-bonded explosives. There are many types of HCP, the physicochemical properties of which are dependent on their molecular structures. The viscosity, molecular mass, and functionality of the poly-... 

Thus, AP is a valuable oxidizer for formulating smokeless propellants or smokeless gas generators. However, since the combustion products of AP composite propellants contain a relatively high concentration of hydrogen chloride (HCI), white smoke is generated when they are expelled from an exhaust nozzle into a humid atmosphere. When the HCI molecules diffuse into the air and collide with H2O molecules therein, an acid mist is formed which gives rise to visible white smoke. Typical examples are AP composite propellants used in rocket motors. Based on experimental observations, white smoke is formed when the relative humidity exceeds about 40 %. Thus, AP composite propellants without any metal particles are termed reduced-smoke propellants. On the other hand, a white smoke trail is always seen from the exhaust of a rocket projectile assisted by an aluminized AP composite propellant under any atmospheric conditions. Thus, aluminized AP composite propellants are termed smoke propellants. 

Another type of substance, which is less sensitive than primaries, is the class called propellants. They are explosive but formulated with different intention than HE s. Their function is to burn in a controllable fashion and, ideally, not to detonate under any circumstance. An important property of propellants is their sensitivity to ignition, which can be determined by subjecting them to sparks or flames... 

The specimen shall consist of approx lOg of the proplnt with minimum atmospheric exposure in order to reduce loss of volatiles or adsorption of water. This procedure has been used successfully on the following single-base proplnts using original size M-l SP, M-l MP, Ben ite, IMR CBI and on crushed M-6. It should be applicable to other single-base formulations provided the specimen is cut to convenient size as specified in Method 509.3 entitled Preparation of Propellant Samples, described in MIL-STD-286B (1 Dec 1967). This... 

The resin used to manufacture plastisol propellants must be dispersion grade. The resin particles should be spherical (18) (or nearly so), preferably a maximum diameter of about 30p or less (14, 17), free from porosity (14), and have a clean surface (15). This will permit the formation of a smooth, creamy plastisol when mixed with approximately an equal weight of the usual plasticizers for the polymer in question. Further, the plastisol of the resin and plasticizer must be capable of being heavily loaded with oxidizer and other fine solids to permit the formulation of a useful propellant composition. 

Probably in no other propellant binder system will one find the number of excellent examinations of propellant failure criteria as have been carried out on the urethane propellants. This can be partly attributed to dewetting of the oxidizer that occurs when many of these formulations are strained, the high ultimate elongations that have been obtained on many of them, and because of the large number of binder ingredient permutations that can be made. The development of propellant science owes much to the many investigations performed on these materials. 

The solids content of propellants ranges between 70 and 90%. The higher limit is dictated by processability. Propellants with less than 70% solids are rare since the specific impulse of such combinations is unduly low. Ammonium perchlorate is the preferred oxidizer and takes the largest percentage in the composition. For special formulations ammonium nitrate or other oxidizing agents may be encountered. Aluminum powder is the most frequently used additive to boost specific impulse and may be found in quantities up to 25%. 

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