Oxidizers, propellant ammonium perchlorate

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

There is an unmistakable tendency in PVC plastisol propellants oxidized with ammonium perchlorate for the burning rate to become more or less insensitive to pressure in the pressure interval 200-700 p.s.i.a. when... 

B. Ballistic Data and Oxidizer Size for PVC Plastisol Propellants Oxidized with Ammonium Perchlorate... 

Ammonia is generated by the reaction of diphenylguanidine and magnesium oxide with ammonium perchlorate. The cure rate of polysulfide propellant formulations can be a function, of the amount of ammonia removed during the mix cycle. Frequently, a vacuum is placed on uncured propellant to remove entrapped air. Too long a vacuum mix cycle, with polysulfide propellants, can remove excess ammonia changing the reaction condition during cure. 

Modern composite propellants are heterogeneous powders (mixtures) which use a crystallized or finely ground mineral salt as an oxidizer, often ammonium perchlorate, which constitutes between 60 and 90% of the mass of the propellant. The fuel itself is highly pyrophoric aluminum metal powder. The propellant is held together by a polymeric binder, usually polyurethane or polybutadienes. Additional compounds are sometimes included, such as a catalyst to help increase the burning rate, or other agents to make the powder easier to manufacture. The final product is a rubber-like substance with the consistency of a hard rubber eraser. 

Solid propellants are also used as rocket fuels. The mixture used in the booster rockets of the space shuttles (Figure 6.17) is a fuel containing aluminum metal powder. An oxidizer of ammonium perchlorate, NH4CIO4, is mixed with the fuel. 

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

A third type of propellant, the composite modified-double-base propellant, represents a combination of the other two types. These propellants are made from mixtures of nitroglycerine and nitrocellulose or similar materials, but with crystalline oxidizers such as ammonium perchlorate also included in the matrix. 

Horton (H9, H10) has obtained additional acoustic-admittance data for a series of composite propellants. At a given frequency, decreasing the mean oxidizer particle size increases the acoustic admittance and thereby the tendency for instability. Horton also investigated the effects on the acoustic admittance of the incorporation of traces of copper chromite, a known catalyst, for the decomposition of ammonium perchlorate, lithium fluoride (a burning-rate depressant), and changes in binder these data are difficult to analyze because of experimental errors. 

One spectacular example of the oxidizing ability of perchlorates is their use in the booster rockets of space shuttles. The solid propellant consists of aluminum powder (the fuel), ammonium perchlorate (the oxidizing agent as well as a fuel), and iron(III) oxide (the catalyst). These reactants are mixed into a liquid polymer, which sets to a solid inside the rocket shell. A variety of products can form when the mixture is ignited. One of the reactions is... 

In a review of the course and mechanism of the catalytic decomposition of ammonium perchlorate, the considerable effects of metal oxides in reducing the explosion temperature of the salt are described [1], Solymosi s previous work had shown reductions from 440° to about 270° by dichromium trioxide, to 260° by 10 mol% of cadmium oxide and to 200°C by 0.2% of zinc oxide. The effect of various concentrations of copper chromite , copper oxide, iron oxide and potassium permanganate on the catalysed combustion of the propellant salt was studied [2], Similar studies on the effects of compounds of 11 metals and potassium dichromate in particular, have been reported [3], Presence of calcium carbonate or calcium oxide has a stabilising effect on the salt, either alone or in admixture with polystyrene [4],... 

The propellant mixture in each solid rocket booster of the Space Shuttle contains ammonium perchlorate ( the oxidizer, 69.6% by weight), aluminum ( the fuel, 16% by weight), an iron oxide catalyst (0.4% by weight), a polymeric binder that... 

Most composite propellants in wide use today use ammonium perchlorate as the oxidizer component. While such propellants benefit from high specific impulse and a lower vulnerability than double-base propellants based on nitroglycerine, they have a significant environmental impact, the exhaust fumes being rich in corrosive hydrogen chloride leading to acid rain after the launch of missiles/rockets. 

Ammonium perchlorate (AP NH4CIO4) is a white, crystalline material, the crystal structure of which changes from orthorhombic to cubic at 513 K, which affects the decomposition process. AP is not hygroscopic in the atmosphere and the mass fraction of oxygen is 0.545. It is used as an oxidizer in various types of propellants and pyrolants. A rapid sublimation occurs between 670 K and 710 K at low pressures. Under slow heating, AP starts to decompose at about 470 K according to ... 

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. 

Aluminum (Al) is a silver-colored light and soft metal used as a major component of aluminum alloys, which are used to construct aircraft and vehicles, similar to Mg alloys. However, Al is known as a readily combustible metal. Thus, Al particles are used as major fuel components of pyrolants. Al particles are mixed with ammonium perchlorate particles and polymeric materials to form solid propellants and underwater explosives. The reaction between aluminum powder and iron oxide is known as a high-temperature gasless reaction and is represented by ... 

The "newest" oxidizer to appear in pyrotechnics, ammonium perchlorate has found considerable use in modern solid-fuel rocket propellants and in the fireworks industry. The space shuttle alone uses approximately two million pounds of solid fuel per launch the mixture is 70% ammonium perchlorate, 16% aluminum metal, and 14% organic polymer. 

Ammonium perchlorate (NH ClO ) This is a good oxidizer, and can be used to make excellent propellants and colored flames. However, it is a self-contained oxidizer-fuel system (much like ammonium nitrate). The mixing of NH f (fuel) and ClOa (oxidizer) occurs at the ionic level. The potential for an explosion cannot be ignored. Conclusion if this material is used, it must be treated with respect and minimum quantities of bulk powder should be prepared. 

Another substantial increase in delivered energy was obtained by incorporating substantial amounts of solid oxidizer e.g., ammonium perchlorate) and metallic fuel e.g., aluminum) in the casting powder. The resulting family of composite-modified double-base (CMDB) propellants has found widespread use in ballistic missiles and space motors. 

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