Burn rate

Different classes of solid propellants DB, CMDB, and fuel rich (FR) have been developed in order to meet the requirements of various missions in terms of specific impulse (Lsp) and wide range of burn rates with low pressure index (n). High density, low temperature sensitivity and good mechanical properties constitute other essential requirements of these propellants. The salient features of such performance parameters are  

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 exponential form of burn-rate law (known as Vielle s Law, Equation 4.5), that is, 

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. 

The pressure exponent (n) is the characteristic of a specific propellant and also depends on the pressure range. For DB propellants, it is in the range from 0.2 to 0.5 but AP-based composite propellants exhibit relatively low values (0.1 to 0.4). The mathematical derivations clearly show that the value of n can never exceed 1 and a value close to 0 (zero) is always preferred from safety considerations. 

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Tensile yield strength, 103 lb in-2 Thermal Burning rate, mm min Coefficient of linear thermal expansion, 10 °C 50-90 0.5-2.2 50-90 50-80 50-60 10-13 Self- extinguishing 40-55 46... 

Tensile yield strength, 1Q3 lb in-3 Thermal Burning rate, mm min Not Not Not Not Not Self- Self-... 

Ignition, pressure, burning rate, and thmst characteristics over temperature range... 

Experimental Determination of the Burning Rate. Experimental determinations of the burning rate are made with the closed tomb for gun propellants and the strand burner for rocket propellants. The closed bomb is essentially a heavy-wahed cylinder capable of withstanding pressures to 689 MPa (100,000 psi). It is equipped with a piezoelectric pressure gauge and the associated apparatus requited to measure the total chamber pressure, which is directly related to the force of the propellant. It also measures the rate of pressure rise as a function of pressure which can then be related to the linear burning rate of the propellant via its geometry. Other devices, such as the Dynagun and the Hi—Low bomb, have also been developed for the measurement of gun propellant performance. 

Propellants with potassium perchlorate have relatively high burning rates (1.75 cm/s at 6.9 MPa (1000 psi) and 21°C) and high burning rate exponents (0.6-0.7). 

Propellants with ammonium nitrate have very low burning rates (0.01 cm/s). 

R. C. Strittmater, E. M. Wineholt, and M. E. Holmes, The Sensitivity of Double Base Propellant Burning Rate to Initial Temperature, MR-2593, BRL, Aberdeen, Md., 1976. 

D. W. Reifler, Tinear Burning Rates of Ball Propellants Based on ClosedBomb Firings, CR 172, BRL, Aberdeen, Md., 1974. 

K. Adas, A. Method of Computing Web for Gun Propellant Grains from Closed Bomb Burning Rates, memo report 73, Naval Powder Factory, U.S. Navy, Indian Head, Md., 1954. 

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