Burning Surface Area

A thick-walled metal tube is particularly effective at removing heat from a burning composition, and propagation of burning down a narrow column can be difficult for all but the hottest of mixtures if metal (particularly a thick metal tube) is used for the container material. On the other hand, the use of a metal wire for the center of the popular 

A mixture that bums well in a narrow tube may possibly reach an explosive velocity in a thicker column, so careful experiments should be done any time a diameter change is made. For narrow tubes, one must watch out for the possible restriction of the tube by solid reaction products, thereby preventing the escape of gaseous products. An explosion may resnlt if this occurs, especially for fast compositions. 

The burn rate of a pyrotechnic composition is determined by a large number of factors. All of these factors must be controlled in the manufacturing process to produce items that give reproducible performance from day to day and month to month. Burn rate factors include, but are not limited, to  

Selection of oxidizer, fuel, and other components Weight ratio of the components Particle sizes of the components Homogeneity of the blended composition Presence of moisture 

Consolidation pressure (of pressed materials) External pressure and temperature when used 

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As = burning surface area (sq inches), p = proplnt density (lb/cu inch). For stable operation, the mass flow rate must equal the mass discharge rate at the nozzle which is... 

Fig. 14.13 shows a typical example of a propellant grain for which the burning surface area ratio of the booster stage and sustainer stage is 2.18. The relahonship... 

Fig. 14.14 Burning surface area versus burning distance for the dual-thrust propellant grain shown in Fig. 14-13.

Fig. 14.19 shows a typical set of pressure versus time curves obtained from tests on a rocket motor. When the I/D ratio defined in Fig. 14.19 is increased, the head-end chamber pressure is increased drastically immediately after the ignition stage. These grains are seven-pointed-star-shaped neutral-burning grains (diameter D = 114 mm), and are made of an AP-Al-CMDB propellant with the composition nc(0-25), ng(0-31), Ita(0-08), ap(0-27), and ai(0 09). The ratio of the initial burning surface area (Ayg) to the nozzle throat area (Aj), = AygjAp and the ratio of the... 

The burning rate - expressed either in grams/second or millimeters/ second - will increase as the burning surface area increases. Small grains will burn faster than large grains due to... 

An advantage of the smokeless powders is their ability to be extruded during the manufacturing process. Perforated grains can be produced that simultaneously burn inwardly and outwardly such that a constant burning surface area and constant gas production are achieved. 

The nozzle discharge coefficients, Cdb and Cps, are determined by using Eq. (1.61). Using Eq. (14.29), the burning surface area of propellant 2 is obtained as... 

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