Bum rate

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base 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 oxygen release rate is directly proportional to the cross-sectional area of the candle for a specific composition and also depends on the linear bum rate. Lower fuel contents decrease the bum rate slightly, eg, ca 2 wt % iron is the lower limit for rehable room temperature operation. Low temperature starts require at least 3.5 wt % iron. Another factor is direction of flow of the evolved gas. If the hot oxygen flows over the unbumed portion of the candle, as much as 15% rate iacreases can be produced. The bum time is halved for each 3.4 MPa (500 psi) pressure rise. The highest pressure that can be produced is ca 138 MPa (20,000 psi). 

CuClO, and copper(II) perchlorate [13770-18-8] Cu(Cl04)2, form a number of complexes with ammonia, pyridine, and organic derivatives of these compounds. The copper perchlorate is an effective bum-rate accelerator for soHd propellants (39). 

Pyrotechnics is based on the estabflshed principles of thermochemistry and the more general science of thermodynamics. There has been Httle work done on the kinetics of pyrotechnic reactions, largely due to the numerous chemical and nonchemical factors that affect the bum rate of a pyrotechnic mixture. Information on the fundamentals of pyrotechnics have been pubflshed in Russian (1) and English (2—6). Thermochemical data that ate useful in determining the energy outputs anticipated from pyrotechnic mixtures are contained in general chemical handbooks and more specialized pubHcations (7-9). 

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. 

The process of designing a pyrotechnic mixture begins with the selection of oxidizer and fuel, and proceeds to incorporate additional components to achieve the exact pyrotechnic effect and bum rate desired in the end item. It is at this point that pyrotechnics takes on the dual nature of an art and science, and experience is often the only thing that can be reHed upon for the solution of a difficult problem. 

Polyhedral Boron Hydrides. These are used in neutron capture therapy of cancers (254), and as bum rate modifiers (accelerants) in gun and rocket propellant compositions. 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

Test results ( , ) for several candidate materiaIs (Table III) are reported which span the range of energetic capacity. Those values which exceed the threshold are highly susoect and have been known to result in serious fires in the past. Mix No. 1, (M49A1, Trip Flare Mixture) is a "safe" mixture that is insensitive to electrical spark, impact, and friction. It does not have a fast bum rate on the Vee Block tester and it has a low pressure-rate-of-rise. 

The results of a number of tests such as those described in Chapter 2 led to classifications for the peroxide group. These tests included the determination of the hazards of decomposition (deflagration and detonation), bum rate, fire hazard, and reactivity hazards. Five different classes were formulated, as listed in the NFPA 43B Hazard Class, from the test results. Emergency procedures have been established for these five classes. 

M55 rockets pose the greatest storage risk because they contain approximately 20 pounds of M28 propellant, a double-base propellant composed of nitroglycerine, nitrocellulose, plasticizers, a bum-rate modifier, and a stabilizer. The exact composition is given in Table 1-4. The propellant slowly decomposes exother-... 

Biomass has some advantageous chemical properties for use in current energy conversion systems. Compared to other carbon-based fuels, it has low ash content and high reactivity. Biomass combustion is a series of chemical reactions by which carbon is oxidized to carbon dioxide, and hydrogen is oxidized to water. Oxygen deficiency leads to incomplete combustion and the formation of many products of incomplete combustion. Excess air cools the system. The air requirements depend on the chemical and physical characteristics of the fuel. The combustion of the biomass relates to the fuel bum rate, the combustion products, the required excess air for complete combustion, and the fire temperatures. 

In North America the problem of moisture absorption has been addressed by developing a moisture resistant gunpowder substitute based on potassium nitrate but augmented with potassium perchlorate. The latter is said to absorb less moisture than the nitrate at a given humidity. In addition, the gunpowder substitute contains a hydrophobic binder, called ethyl cellulose, (2.22) (celluloses have a history of use in pyrotechnics) together with an organic fuel, known as phenolphthalein, (2.23) which is said to enhance the bum rate. 

Generally, bum rate increases or decreases as the pressure is raised or lowered. It is also influenced by the temperature of the propellant and increases with an increase in temperature whereas decreases with a decrease in temperature. It is therefore necessary to fix higher as well as lower temperature limits at the time of development of a propellant keeping in view the mission s requirements. The specified temperature limit for Service applications has been stipulated as -40 °C to +60 °C in India. Burn rate also depends upon the calorific value of the propellant and, other things being equal, the higher the calorific value, the higher the burn rate of the propellant. 

The burn rates of propellants are determined in a strand burner (Crawford bomb/ acoustic emission technique) at various pressures using an inert gas for pressurization. This data, when fitted in the empirical relation r = a.P" provides the pressure index n and the coefficient a. This technique is highly useful as a first approximation and is extensively used for propellant screening and quality control. The bum rates at different pressures are also determined by static testing in a ballistic evaluation motors (BEMs) and burn rates are typically scaled up from 1-5% for full scale motors. 

Potassium perchlorate (KP) KC104 2.52 46.2 High bum rates but solid parhcles are present in the exhaust... 

NENAs possess good thermal stability, readily plasticize NC and other binders, generate low molecular weight combustion products and impart favorable impact sensitivity. Butyl NENA has edge over others because it imparts better low temperature properties as well [182]. The use of NENAs as plasticizers in gun and rocket propellant formulations imparts excellent properties such as high bum rates, reduction in flame temperature and molecular mass of combustion products and high force constant or specific impulse [183]. 

Oxides of lead, basic lead stearate, lead-2-ethyl hexoate, basic lead salicylate Render the bum rate of DB rocket propellants independent of pressure over a certain pressure range [236]. Such salts are effective only with cooler propellants i.e., cal. val. below -850calgT... 

Bum-rate enhancement in HTPB-based propellants is in the order of Fe > Cr > > V > Mn > Ni > Zn. CuPc also increases pot life of the propellant mix. The use of FePc has also been reported in propellant igniter compositions [250, 251]. 

Silicon compounds having Si-H, Si-N and Si-C bonds are reported to be effective as catalysts for HTPB-based propellants. Bum rate of propellant varies as a function of the Si content in the propellant formulation [252]... 

The simplest way to modify the bum rate of composite propellants is by means of catalysts or ballistic modifiers. The solid catalysts that have been in use for many years can be classified into two groups. 

Metallic Oxides A number of metallic oxides have been studied to catalyze bum rate of composite propellants. The research of Jacobs et al. suggests that CuO is a... 

Burn-rate modifiers probably affect most of these combustion steps, that is, the endothermic and exothermic reactions and heat losses. Rastogi et al. have shown that burn rate, surface temperature, flame temperature and rate of decomposition are enhanced in case of catalyzed propellants while these are lowered in case of burn-rate retarders. This may be due to heat produced in catalytic reactions in the former case whereas bum rates are reduced on account of endothermicity of the condensed phase reactions on the propellant surface in the case of retarders. It is also reported that carbonates of copper and chromium are better catalysts... 

NANOCAT Superfine Iron Oxide(SFIO) is a novel bum-rate catalyst and performs superbly in solid rocket propellants based on ammonium perchlorate (AP). SFIO provides a higher burn rate and lower pressure exponent compared with commercial iron oxides at equal concentrations. Some characteristics of NANOCAT SFIO as a burn-rate catalyst are as follows ... 

Neidert, J.B., and Askins, R.E. (1994) Bum rate modification of solid propellants with bismuth trioxide. U S Patent 5,372,070. 

Menke, K., Maub, J.B., Brehler, K.P., Jungbluth, H., and Kalischewski, W. (1995) New ferrocenes for bum-rate modification of composite propellants. Proc. ADPA Inti Symp. on Energetic Materials Technology, Phoenix, Arizona, US, Sept. 24-27, 1995. 

Agrawal, J.P., Gore, G.M., Prasad, U.S., Bhatewara, R.G., and Tipre, K.R. (1997) Development of ferrocene derivatives as bum rate modifiers for composite propellants. HEMRL Report No. HEMRL/22/97. 

Nanoscale thermite, namely A1 (-30-90 nm) and molybdenum trioxide (formulation approx. 40-45% and 60-55% [by wt] respectively) or at O/F mass ratio as 1.4 were studied by Walter and workers for bum rates etc. At the end of the investigation, they concluded that the surface area of M0O3 affects... 

Bum rates of propellants are inversely proportional to the particle diameter of Al powder particles. This is corroborated by the increase in bum rate of propellants on incorporation of Alex in their formulations. The bum rate of solid propellant increases by about 100% on replacement of 9% Al by Alex powder (in a total of 18% Al) coupled with decrease in pressure index. 

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