COMBUSTION OF COMPOSITE CHARGES OF

PR.OPE L LANTS

Yu. I. Medvedev, N. P. Medvedeva,

and L. N. Revyagin

S O L I D

UDC 536.46+662.62

Consider a propellant specimen composed of laminae of the same width L of propellants 1 and 2, with thickness el and e 2 respect ively , inhibited on the la tera l surface (Fig. 1). The burning rate u 2 of solid propellant 2 is considerably g rea te r than the l inear burning rate u 1 of solid propellant 1. A cer ta in time after the ignition of Such a pack, stable burning conditions commence, during which the shape of the burn- ing surface will be formed by r idges of the slowly burning propellant 1 with an angle o~ at the apex and a fiat surface on the fas t -burning propellant .

If there is no rec ip roca l effect on the burning rate of both propellants, f rom geometr ica l cons idera- t ions we have

t t 1 S i l l ~ - -

a o

Under steady burning conditions the pack is reduced in length at a ra te u 2 which becomes the *'leading" burning ra te . If, at the f i rs t moment of ignition, the surface of the pack is flat, the mass burning ra te will be expressed as

m = L (2exu151 + e2u26:~),

where 52 and 52 are the densit ies of propellants 1 and 2. Under steady burning conditions the a rea of the burning surface of propellant 2 will r emain as before, but the burning surface of propellant 1 formed by the side faces of the p r i sms has an a rea

, 2el L S 1 = . ,

S | l l a "

The total mass burning rate in this case is expressed as

m'= L (2e161 + e262) u2.

Thus, the mass burning ra te is de termined by the initial a r ea of the end of the charge and by the burning rate of the fas t -burning inclusion and can be made quite high. It is easy to show that a s imi lar conclusion is reached also for an a rb i t r a r i ly large number of heterogeneous propellant laminae as well as for cyl in- dr ica l charges with var ied shapes of fas t -burning inclusions. It is valid in conditions in which the pro- pellants have no mutual effect on their burning rate .

In an actual case the relat ively cold combustion products f rom a slow-burning propellant are diluted with the hot ter products of a fas t -burning composit ion and vice ve r sa . Here, the l inear burning rate of the fas t -burning inclusion is somewhat slowed down, and that of the s low-burning propellant inc reases . The degree of slowing down (or increase) depends essent ia l ly on the percentage content of the slow- and fas t -burning components. In fact, if the charge consists ' ent i re ly of fas t -burn ing propellant, it will burn at the normal ra te u 2. If, however, the whole charge consists of propellant with a low burning rate , then it burns at the rate u s. In an intermediate case the "leading" l inear burning rate proves to be somewhat less

Tomsk. Trans la ted f rom Fizika Goreniya i Vzryva, Vol. 7, No. 2, pp. 308-310, Apr i l -June, 1971.

�9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission o[ the publisher. A copy o[ this article is available from the publisher for $15.00.

267

Y

Fig. 1

j.,,. { ,

40' ' . - - - - [ o 20 40 6O

Fig. 2

80 100

than the burning rate of the fast-burning com- position u~. To study this effect, experiments were set up with laminated charges in the form of a cylinder with a few rods of fas t -burning com- position symmetr ica l ly disposed along the cylinder axis. They were ignited at a tmospheric p r e s su re in a semiclosed space and under increased p r e s - sure in a bomb with a nozzle. After a certain burning t ime the specimens were extinguished and measured up to determine the burning rate of the fast-burning powder uj. As an example, Fig. 2 shows a curve relat ing the degree of slowing down

�9 T of the propellant, U2/U2, which has aburning ra te of 0.75 c m / s e c , to its percentage content in a specimen with slow-burning propellant (burning ra te 0.35 cm/sec) for two diameters of cyl indrical fast-burning in- clusions of 5 (1) and 8 (2) mm. Along the absc issa is plotted the rat io of the burning a rea of propellant 2 to the total initial a r ea of the end of the charge in percentages. It is evident that this relat ion is essent ia l ly nonlinear and the presence even of 50% of composit ion 1 re ta rds the burning rate of propellant 2 in all by 3-4% when the d iameter of its rod is 5 mm, and has pract ical ly no effect when the diameter is 8 mm. In the lat ter case a marked retardat ion of burning rate begins only with the inclusion of 20-30% of fast burning propellant. Thus for the efficient construct ion of such charges , a careful study is needed of the degree of re tarda t ion not only ve r sus the percentage fas t -burning powder content but also ve r sus its la teral l inear dimension (the diameter) . In all probability t h i sd imens ion will have its own optimum value for different pai rs of propellants and overal l s izes of the whole charge.

It is easy to show that the t ime until steady burning conditions emerge is determined by the ex- press ion

t. u 1 | 1 - - utlu~

and depends substantially on the relat ion between the burning ra tes and on tMckness and burning ra te of the slow-burning composition.

Thus, with an efficient combination of two propellants one of which has a slow and the other a high burning ra te , the total mass rate of gas evolution can be made quite high. Usually, a low burning rate c o r - responds to a low tempera ture of the combustion products; hence, with a high gas-evolut ion rate in the combustion chamber in the case under consideration, there will be a comparat ively low tempera ture . These conditions are advar~ageous for the application of such charges to solid propellant p ressure accumulators .

Such charges, which have advantages over charges with wi res , have a completely active volume and, consequently, bet ter power proper t ies .

Finally, by varying the charge components and their l inear dimensions, it is possible to achieve a gas-evolution rate known beforehand as indispensable, but in this case the conditions for stable burning must be kept in mind; these are not considered in the present ar t ic le .

268