Stress Relieved Grains

Abstract

An internally ported grain having an active solids loading of from 85 to 98 percent. The configuration of the port is determined so as to provide a desired internal burning surface and relief for stored strain energy.

Description

PRIOR ART

The oldest solid propellant grain design is a simple solid cylinder which is burned from the aft end of the missile towards the forward end. This type of grain is termed an end burning grain and is frequently suitable for very small charges of propellants that burn at low temperatures. However, for larger charges or propellants having a higher burning temperature, it is necessary to insulate the wall of the motor from the flame. The coating of the insulative surface reduces the amount of propellant which can be placed in the motor, thereby reducing the volumetric loading. For the purpose of this disclosure, volumetric loading is defined as the volume of the active solids within the motor divided by the volume inside the motor casing. Active solids, of course, are those which contribute directly to the impulse of the motor.

To eliminate the need for internal insulation, internal burning propellant charges have been used. These charges have a port or ports running axially through the grain such that the entire length of the grain burns simultaneously and the flame travels from the center of the grain out towards the wall.

In such internal burning grains, the propellant serves to protect the outer case from the hot gases that evolve. Additionally, one is better able to program the burning rate and thrust to be derived from the motor through the use of various design configurations of the internal port. One of the most prevalent examples is the star center propellant charge having the internal port in a general configuration of a multipointed star. In internally ported grains two significant problems arise. First, the volumetric loading of a given motor casing is affected by the port area. In other words, 100 percent volumetric loading cannot be achieved utilizing normally internally ported grains. A typical motor only has from 70 to 80 percent volumetric loading. Second, there is a tendency of the grain to crack at stress concentration points about the port area. Any such cracking detrimentally affects the performance of the grain and often leads to a catastrophic failure of the entire motor.

An object of this invention is to eliminate grain cracks in internally ported grains.

A further object of this invention is to provide motors having very high volumetric loading.

Further objects of this invention will become apparent from the following description.

SUMMARY OF THE INVENTION

The above and other objects of this invention are accomplished by cutting the propellant grain with a thin knife blade in a configuration to expose the desired surface area while taking advantage of the relief in the grain stresses that such a controlled cut can accomplish. Thus, an internal cut in the grain controls both strain energy buildup as well as defining the initial burning surfaces. The cuts are made such that in one dimension normal to the axis of the grain, the width of the cut is negligible. The width of the cuts are so small that the volume loading of the motor is from 85 to 98 percent.

However, the slots of the instant invention must be of sufficient width to properly vent combustion gases through the throat of the motor. If the venting area is insufficient, pressure build-up within the motor can cause catastrophic failure. It can be seen that the range of widths of the slots of the instant invention is a function of, e.g., propellant combustion and throat area. However, in most propellant compositions and typical throat areas, cuts ranging in widths from 0.5 to 5 percent of the diameter of the grain are suitable.

BRIEF DESCRIPTION OF THE DRAWINGS

It is believed that the invention will be better understood from the following detailed description and drawings in which FIGS. 1 a - 1c are cross-sectional views of propellant grains provided with the internal cuts of this invention.

FIGS. 2 a - 2c are respectively side views of the grains of FIG. 1 a - 1c.

FIGS. 3a - 3c depict cutting means to provide the cuts respectively on FIGS. 1a - 1c.

FIG. 4 discloses a section of a grain cut having a mandrel disposed therein.

FIG. 5 is the same grain cut as FIG. 4; however, the propellant surrounding the cut has been cured whereby the cut is opened due to the shrinkage of the propellant.

PRINCIPLE OF THE INVENTION

The principle of this invention utilizing an extremely thin cut in a solid propellant grain uniquely combines principles of internal ballistics and grain stress relief. As will be explained, two internal ballistic phenomena are combined through this invention to produce an effect which is contrary to the effect produced by either of the phenomena separately. These two ballistic phenomena involved are (1 ) erosive burning and (2 ) flame propogation rate. In conventional solid propellant rocket motors with internal burning grains, erosive burning generally has a detrimental effect and is not desirable. When the motor is initially ignited the port cross-sectional area is the smallest. If the throat area of the nozzle of the motor is half as large as the port area of the motor, the Mach number of the exhaust gases at the nozzle end of the grain is usually high. These high velocity gases sweep the surface of the grain, eroding the softened propellant from the grain prior to its being effectively burned. The result is a highly increased effective burning rate and very high chamber pressures. In order to reduce the effect of erosive burning, the initial port area has to be increased or the initial burning surface decreased. Typically the erosive burning rate may be four times the regular burning rate of a motor for composite solid propellants.

The second internal ballistic phenomena, flame propogation rate, can be either harmful or helpful to the motor performance. If the flame propogation is used to aid motor ignition, then it can be helpful. Alternatively, when the flame propogation occurs in an unplanned grain crack, it is apparent that the uncontrolled burning of the grain in this manner would be deleterious. Flame propogation rate is very high when traveling in the direction of the combustion gases. In this direction it is on the order of a thousand inches per second. On the other hand, the flame propogation rate is relatively low, only about 200 inches per second, when traveling in the opposite direction. The motor of this invention combines the two ballistic phenomena in such a way as to permit the increase of volumetric loading to nearly 100 percent. The motor, as indicated, having a knife blade width slot cut therein, is ignited at the nozzle end and the flame propogates along the slit from the aft end forward. Due to the very small port area, the erosion on the aft end of the grain is very high but the burning surface is very low. As the flame travels to the forward end of the motor increasing the burning surface, the aft end erosion opens the port. As the port opens, the erosion decreases.

Having now explained the theory of the burning along the knife blade cut, attention should be directed to the grain stress relief provided by the cut. Two problems confronting the designers of solid propellant grains are intentional or unintentional debonding between the case and the propellant grain, and cracking of the inner bore. Past attempts to increase volumetric loading in case bonded systems resulted in severe grain stress and strain states. Ballisticians are limited in their design of port configurations because of the problem of inner bore cracking. The configurations that eliminate the possibility of cracking are seldom compatible with the demand for high volumetric loading. The configuration of the internal cut in the grain can be optimized to prevent unwanted cracking using the Griffith energy hypothesis. In the paper "The Role of Fracture Mechanics in the Design of Optimum Grain-Case Terminations" by J. S. Noel and L. D. Webb, CPIA Publication No. 119, Vol. 1, Oct. 1966 (available from the Chemical Propulsion Information Agency, John Hopkins University, Applied Physics Laboratory, 8621 Georgia Avenue, Silver Spring, Maryland), the authors discussed these energy balance concepts as applied to end releases in solid propellant grains. One can utilize the information in the paper to apply these same concepts to optimize the location of the cuts made according to this invention. In addition, the potentially destructive energy induced and stored within the grain can be reduced to a minimum by proper location of grain stress relief systems. These relief systems consist, as in the present invention, of intentional internal grain cuts which not only control strain energy buildup but also define initial burning surfaces. The grain design that results has very high volume loading with high structural integrity while possessing a port area of nearly zero. As explained, the flame propogation rate down the crack could be utilized along with erosive burning effects to ignite and open the port for an internal burning configuration. Grain stress considerations and desired port configurations will control the cross-sectional shapes of the burning surfaces.

Turning now to FIGS. 1 a - 1c, there is shown a sectional view of propellant grains having various designed internal cuts. It has been found that particular designs are suitable for both stress relief and prevention of continued crack growth. For example, turning to FIG. 1 a, there is shown an S configuration 11 cut within a propellant grain 13. As shown the grain is bonded to a case 15. Both ends of the S 17 and 19 respectively, are curved. It is generally found that curvatures at the end of such cracks serve to prevent further growth of the cut, or in other words, propogation of the crack. The exact terminus of the end 17 and 19 can be determined from calculations as disclosed in the article by Noel and Webb such that they are located in areas of minimum stress in the grain. As can be appreciated, utilizing the concepts disclosed in the article, one can predict the stresses at any given point within the grain structure. It should be apparent that it is most desirable to place the cut 11 along a line defining areas of maximum internal stress, whereby the cut can serve as a stress relief means. The terminal ends of the cut should be placed in areas of minimum stress so as to prevent further propogation thereof prior to and during burning of the grain. Fig. 1 b discloses a variation having three cuts 21, 23 and 25 emanating 120 .degree. apart and extending radially from a central point 27. Each of the cuts is terminated by a perpendicular short line such as 29 perpendicular thereto that is curved at its end inwardly toward the center point 27. As can be seen the general configuration appears as three T shaped cuts emanating from the central point 27. However as can be seen rather than a plain T, the ends of the top of the T' s are curved inwardly so as to prevent as previously indicated further extension of the cut during grain storage, and prior to firing. FIG. 1 c is a variation of the configuration of 1 b wherein three curved lines 31 emanate from a central point 33. The possible configuration of cuts is innumerable and greatly depends upon the design of the motor as well as the characteristics of the propellant charge itself. FIG. 2a illustrates the cross-sectional side view along the entire length of propellant grain 13 displaying the cut 11 partially shown.

Turning now to FIGS. 3 a - 3 c, there are shown devices used to cut the shapes respectively shown in FIGS. 1 a - 1c. The device used can be likened to a cookie cutter, in that thin sheet metal as shown in FIG. 3 a is formed to the configuration 35 of the S cut in grain shown in FIG. 1 a. Attached to the sheet metal cutter 35 is a rod 37 which would be equivalent to the length of the propellant grain. Prior to casting of the propellant, the rod and cutter is inserted into the case with the cutter being at the head end of the propellant. After curing, the rod is then pulled out of the grain from the rearward end thereof. Thus, the cutter 35 will traverse the entire grain cutting the desired configuration thereon. Alternative to casting the cutter in place and withdrawing it after the grain is cured, it is possible to first cure a propellant grain and then push the cutter through the cured grain from one end to another where both ends are exposed prior to their enclosure by bulkhead and nozzle attachments.

Turning now to FIG. 4, there is shown a portion of a cut 39 surrounding a thin metal mandrel 41. In this instance, the mandrel 41 will extend the entire length of the grain and is placed in the casting mold prior to the propellant. The drawing is enlarged and shows spacing between the edge of the cut 39 and the mandrel 41. However, as can be appreciated, in practical application the propellant material surrounding the cut is actually in contact with the mandrel material. The relationship shown in FIG. 4 is in the precured condition. However, once the propellant has been cured, there is significant shrinkage such that the cuts surrounding the mandrel are slightly open. The shrinkage is in the order of 0.6 percent for composite solid propellants. The resultant opening is shown in FIG. 5 and facilitates the easy removal of the thin metal mandrel from the entire grain leaving a slight opening or crack 43 as shown in FIG. 5.

The concept of the invention is applicable to any type of solid propellant including composite and double based ones whenever an internal port is desired. Following is an example of the predicted effect of the utilization of this invention. Simplified calculations are used, as the purpose is merely to illustrate the advantages of the invention. Visualize a solid propellant composition of carboxy terminated linear polybutadiene as a binder having solid particle ammonium perchlorate therein as an oxidizer and aluminum powder as a fuel. A typical propogation rate would be 200 inches per second. The erosive burning rate of such a composition, for example, could be 2 inches per second. Assuming a motor length of 50 inches with a single slot, the flame front would travel from the nozzle to the forward end in a travel time of approximately 0.25 second. By that time, the aft end slot width is (2 inches per sec) (.25 sec) (2) = 1.0 inches. If a single slot 5 inches wide is used, and the burning produces a wedge shaped volume, the volume would be about (5 inches)(1 inch)(25 inches)/2 = 62.5 cubic inches. If the starting volume of the slot is negligible, 62.5 cubic inches of propellant have been burned by the time the flame reaches the forward end of the grain. In prior art grains, this 62.5 cubic inches of propellant could not have been included in an internally ported grain. This additional propellant therefore represents a significant increase in total impulse over prior art motors.

Claims

We claim:

1. A stress relieved internal burning solid rocket propellant grain having an active solids content of from 85 to 98 percent by volume characterized by a series of intersecting slits located generally along a line parallel to the axis of the grain and having a width of between 0.5 and 5 percent of the diameter of the grain, at least one slit passing through the central axis of the grain and having other slits curved in cross-sectional shape located between said central axis and the outer periphery of the grain.

2. A stress relieved internally burning solid rocket propellant grain having an active solids content of from 85 to 98 percent by volume characterized by at least one slit extending lengthwise along the entire grain and located generally along a line parallel to the axis of the grain in an area of said grain having maximum internal stress and having a width of between 0.5 and 5 percent of the diameter of the grain, said slit having end portions curved in cross-section terminating in areas of the grain having minimum internal stress.