Propellant Characteristics

The propellant selection is critical to rocket motor design. The desirable propellant characteristics are listed below and are discussed again in other parts of this book. The requirements for any particular motor will influence the priorities of these characteristics:

1. High performance or high specific impulse; really this means high gas temperature and/or low molecular mass.

2. Predictable, reproducible, and initially adjustable burning rate to fit the need of the grain design and the thrust-time requirement.

3. For minimum variation in thrust or chamber pressure, the pressure or burning rate exponent and the temperature coefficient should be small.

4. Adequate physical properties (including bond strength) over the intended operating temperature range.

5. High density (allows a small-volume motor).

6. Predictable, reproducible ignition qualities (such as reasonable ignition overpressure)

7. Good aging characteristics and long life. Aging and life predictions depend on the propellant's chemical and physical properties, the cumulative damage criteria with load cycling and thermal cycling (see page 461), and actual tests on propellant samples and test data from failed motors.

8. Low absorption of moisture, which often causes chemical deterioration.

9. Simple, reproducible, safe, low-cost, controllable, and low-hazard manufacturing.

10. Guaranteed availability of all raw materials and purchased components over the production and operating life of the propellant, and good control over undesirable impurities.

11. Low technical risk, such as a favorable history of prior applications.

12. Relative insensitivity to certain energy stimuli described in the next section.

13. Non-toxic exhaust gases.

14. Not prone to combustion instability (see next chapter).

Some of these desirable characteristics will apply also to all materials and purchased components used in solid motors, such as the igniter, insulator, case, or safe and arm device. Several of these characteristics are sometimes in conflict with each other. For example, increasing the physical strength (more binder and or more crosslinker) will reduce the performance and density. So a modification of the propellant for one of these characteristics can often cause changes in several of the others.

Several illustrations will now be given on how the characteristics of a propellant change when the concentration of one of its major ingredients is changed. For composition propellants using a polymer binder [hydroxyl-ter-minated polybutadiene (HTPB)] and various crystalline oxidizers, Fig. 12-3 shows the calculated variation in combustion or flame temperature, average product gas molecular weight, and specific impulse as a function of oxidizer concentration; this is calculated data taken from Ref. 12-2, based on a thermochemical analysis as explained in Chapter 5. The maximum values of Is and occur at approximately the same concentration of oxidizer. In practice the optimum percentage for AP (about 90 to 93%) and AN (about 93%) cannot be achieved, because concentrations greater than about 90% total solids (including the aluminum and solid catalysts) cannot be processed in a mixer. A castable slurry that will flow into a mold requires 10 to 15% liquid content.

TABL-E 12-2. Characteristics of Selected Propellants

Propellant Type

Advantages

Disadvantages

Double-base (extruded)

Double-base (castable)

Composite-modified double-base or CMDB with some AP and Al

Composite AP, Al, and PBAN or PU or CTPB binder

Composite AP, Al, and HTPB binder; most common composite propellant today

Modified composite AP, Al, PB binder plus some HMX or RDX

Modest cost; nontoxic clean exhaust, smokeless; good burn rate control; wide range of burn rates; simple well-known process; good mechanical properties; low temperature coefficient; very low pressure exponent; plateau burning is possible Wide range of burn rates; nontoxic smokeless exhaust; relatively safe to handle; simple, well-known process; modest cost; good mechanical properties; good burn rate control; low temperature coefficient; plateau burning can be achieved Higher performance; good mechanical properties; high density (sp. gr. 1.83-1.86); less likely to have combustion stability problems; intermediate cost; good background experience

Reliable; high density; long experience background; modest cost; good aging; long cure time; good performance; usually stable combustion; low to medium cost; wide temperature range; high density; low to moderate temperature sensitivity; good burn rate control; usually good physical properties; class 1.3 Slightly better solids loading % and performance than PBAN or CTPB; widest ambient temperature limits; good burn-rate control; usually stable combustion; medium cost; good storage stability; widest range of burn rates; good physical properties; good experience; class 1.3 Higher performance; good burn-rate control; usually stable combustion; high density; moderate temperature sensitivity; can have good mechanical properties

Free-standing grain requires structural support; low performance, low density; high to intermediate hazard in manufacture; can have storage problems with NG bleeding out; diameter limited by available extrusion presses; class 1.1 NG may bleed out or migrate; high to intermediate manufacture hazard; low performance; low density; higher cost than extruded DB; class 1.1

Storage stability can be marginal; complex facilities; some smoke in exhaust; high flame temperature; moisture sensitive; moderately toxic exhaust; hazards in manufacture; modest ambient temperature range; the value of n is high (0.8 to 0.9); moderately high temperature coefficient Modest ambient temperature range; high viscosity limits at maximum solid loading; high flame temperature; toxic, smoky exhaust; some are moisture sensitive; some burn-rate modifiers (e.g. aziridines) are carcinogens Complex facilities; moisture sensitive; fairly high flame temperature; toxic, smoky exhaust

Expensive, complex facilities; hazardous processing; harder-to-control burn rate; high flame temperature; toxic, smoky exhaust; can be impact sensitive; can be class 1.1; high cost; pressure exponent 0.5-0.7

Composite with energetic binder and plasticizer such as NG, AP, HMX Modified doublebase with HMX Modified AN propellant with HMX or RDX added

Ammonium nitrate plus polymer binder (gas generator)

RDX/HMX with polymer

Highest performance; high density

(1.8 to 1.86); narrow range of burn rates

Higher performance; high density

(1.78 to 1.88); stable combustion; narrow range of burn rates Fair performance; relatively clean; smokeless; nontoxic exhaust

Clean exhaust; little smoke; essentially nontoxic exhaust; low temperature gas; usually stable combustion; modest cost; low pressure exponent

Low smoke; nontoxic exhaust; lower combustion temperature

Expensive; limited experience; impact sensitive; high pressure exponent

Same as CMDB above; limited experience; most are class 1.1; high cost Relatively little experience; can be hazardous to manufacture; need to stabilize AN to limit grain growth; low burn rates; impact sensitive; medium density; class 1.1 or 1.3 Low performance; low density; need to stabilize AN to limit grain growth and avoid phase transformations; moisture sensitive; low burn rates

Low performance; low density; class 1.1

TABLE 12-3. Representative Propellant Formulations

Double-Base Composite Composite Double-Base

(JPN Propellant) (PBAN Propellant) (CMDB Propellant)

TABLE 12-3. Representative Propellant Formulations

Double-Base Composite Composite Double-Base

(JPN Propellant) (PBAN Propellant) (CMDB Propellant)

Ingredient

Wt %

Ingredient

Wt %

Ingredient

Wt %

Nitrocellulose

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