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Neutral

Small

^Neutral if ends are unrestricted, otherwise progressive.

ftHas up to 4 or sometimes 8% sliver mass and thus a gradual thrust termination. NA: not applicable or not available.

^Neutral if ends are unrestricted, otherwise progressive.

ftHas up to 4 or sometimes 8% sliver mass and thus a gradual thrust termination. NA: not applicable or not available.

extended by alterations. The movement of the center of gravity influences the flight stability of the vehicle. Relative values of this CG shift are also shown in Table 11-4. Most solid propellant manufacturers have specific approaches and sophisticated computer programs for analyzing and optimizing grain geometry alternatives and permitting burn surface and cavity volume analysis. See Refs. 11-15 and 11-16 and Chapters 8 and 9 of Ref. 11-1.

The end burning grain (burning like a cigarette) is unique; it burns solely in the axial direction and maximizes the amount of propellant that can be placed in a given cylindrical motor case. In larger motors (over 0.6 m diameter) these end burners show a progressive thrust curve. Figure 11-18 shows that the burning surface soon forms a conical shape, causing a rise in pressure and thrust. Although the phenomenon is not fully understood, two factors contribute to higher burning rate near the bondline: chemical migration of the burning rate catalyst into and towards the bondline, and local high propellant stresses and strains at the bond surface, creating local cracks (Ref. 11-17).

Rockets used in air-launched or certain surface-launched missile applications, weather rockets, certain antiaircraft or antimissile rockets, and other tactical applications actually benefit by reducing the thrust with burn time. A high thrust is desired to apply initial acceleration, but, as propellant is consumed and the vehicle mass is reduced, a decrease in thrust is desirable; this limits the maximum acceleration on the rocket-propelled vehicle or its sensitive payload, often reduces the drag losses, and usually permits a more effective flight path. Therefore, there is a benefit to vehicle mass, flight performance, and cost in having a higher initial thrust during the boost phase of the flight, followed by a lower thrust (often 10 to 30% of boost thrust) during the sustaining phase of the powered flight. Figure 11-19 shows grains which give two or more discrete thrust periods in a single burn operation. The configurations are actually combinations of the configurations listed in Table 11-4.

FIGURE 11-18. Schematic diagram of end-burning grain coning effect. In larger sizes (above approximately 0.5 m diameter) the burning surface does not remain flat and perpendicular to the motor axis, but gradually assumes a conical shape. The lines in the grain indicate successively larger-area burning surface contours.

Equilibrium conical burning

Initial burning surface

FIGURE 11-18. Schematic diagram of end-burning grain coning effect. In larger sizes (above approximately 0.5 m diameter) the burning surface does not remain flat and perpendicular to the motor axis, but gradually assumes a conical shape. The lines in the grain indicate successively larger-area burning surface contours.

Case

Equilibrium conical burning

Initial burning surface

Case

Dual end burning grains with two propellants of different burning rates. Not used today, because the manufacture is more expensive

Dual end burning grains with two propellants of different burning rates. Not used today, because the manufacture is more expensive

Single grain. Boost with large burning area, sustain with smaller burning area (both radial)

Single grain. Boost-sustain-boost, with different burning areas (all radial burning)

Single grain. Boost-sustain-boost, with different burning areas (all radial burning)

FIGURE 11-19. Several simplified schematic diagrams of grain configurations for an initial period of high thrust followed by a lower-thrust period.

In a single-propellant dual-thrust level solid rocket motor, factors relating to the sustain portion usually dominate in the selection of the propellant type and grain configuration if most of the propellant volume is used during the longer sustain portion.

A restartable rocket motor has advantages in a number of tactical rocket propulsion systems used for aircraft and missile defense applications. Here two (or sometimes three) grains are contained inside the same case, each with its own igniter. The grains are physically separated typically by a structural bulkhead or by an insulation layer. One method for accomplishing this is shown in Fig. 11-20. The timing between thrust periods (sometimes called thrust pulses) can be controlled and commanded by the missile guidance system, so as to change the trajectory in a nearly optimum fashion and minimize the flight time to target. The separation mechanism has to prevent the burning-hot pressurized gas of the first grain from reaching the other grain and causing its inadvertent ignition. When the second grain is ignited the separation devices are automatically removed, fractured, or burned, but in such a manner that the

Grain for second pulse .

Rib support structure

Metal case with internal insulation

Grain for first pulse

Grain for second pulse .

Rib support structure

Metal case with internal insulation

Grain for first pulse

Burst diaphragm with insulation on right side

Igniter

FIGURE 11-20. Simplified diagram of one concept of a two-pulse experimental rocket motor with two grains separated by a bulkhead. During the first pulse operation the metal diaphragm is supported by a spider-web-like structure made of high temperature material. Upon ignition of the second stage, the scored diaphragm is loaded in the other direction; it breaks and its leaves peel back. The bulkhead opening has a much larger area than the nozzle throat.

Burst diaphragm with insulation on right side

Igniter

FIGURE 11-20. Simplified diagram of one concept of a two-pulse experimental rocket motor with two grains separated by a bulkhead. During the first pulse operation the metal diaphragm is supported by a spider-web-like structure made of high temperature material. Upon ignition of the second stage, the scored diaphragm is loaded in the other direction; it breaks and its leaves peel back. The bulkhead opening has a much larger area than the nozzle throat.

fragments of hardware pieces will not plug the nozzle or damage the insulation (see Refs. 11-18 and 11-19).

Slivers

Any remaining unburnt propellant is known as slivers. Figure 11-5 and the figure in Problem 11-6 show small slivers or pieces of unburnt propellant remaining at the periphery of the grain, because the pressure went below the deflagration limit (see Ref. 11-20). About 25 years ago grain designs had 2 to 7% propellant slivers; this useless material caused a reduction in propellant mass fraction and vehicle mass ratio. The technology of grain design has advanced so that there are almost no slivers (usually less than 1 %). If slivers were to occur in a new unusual grain design, the designer would try to replace the sliver volume with lower-density insulator, which gives less of a mass ratio penalty than the higher-density propellant residue. This is shown in Fig. 11-17.

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