Classification

Processed modern propellants can be classified in several ways, as described below. This classification is not rigorous or complete. Sometimes the same propellant will fit into two or more of the classifications.

1. Propellants are often tailored to and classified by specific applications, such as space launch booster propellants or tactical missile propellants; each has somewhat specific chemical ingredients, different burning rates, different physical properties, and different performance. Table 11-1 shows four kinds of rocket motor applications (each has somewhat different propellants) and several gas generator applications. Propellants for rocket motors have hot (over 2400 K) gases and are used to produce thrust, but gas generator propellants have lower-temperature combustion gases (800 to 1200 K) and they are used to produce power, not thrust.

Historically, the early rocket motor propellants used to be grouped into two classes: double-base (DB*) propellants were used as the first production propellants, and then the development of polymers as binders made the composite propellants feasible.

2. Double-base (DB) propellants form a homogeneous propellant grain, usually a nitrocellulose (NC*), a solid ingredient which absorbs liquid nitroglycerine (NG) plus minor percentages of additives. Both the major ingredients are explosives and function as a combined fuel and oxidizer. Both extruded double-base (EDB) and cast double-base (CDB) propellant have found extensive applications, mostly in small tactical missiles of older design. By adding crystalline nitramines (HMX or RDX)* the performance and density can be improved; this is sometimes called cast-modified double-base propellant. A further improvement is to add an elastomeric binder (rubber-like, such as crosslinked polybutadiene), which improves the physical properties and allows more nitramine and thus improves the performance slightly. The resulting propellant is called elastomeric-modified cast double-base (EMCDB). These four classes of double base have nearly smokeless exhausts. Adding some solid ammonium perchlorate (AP) and aluminum (Al) increases the density and the specific impulse slightly, but the exhaust gas is smoky. The propellant is called composite-modified double-base propellant or CMDB.

3. Composite propellants form a heterogeneous propellant grain with the oxidizer crystals and a powdered fuel (usually aluminum) held together in a matrix of synthetic rubber (or plastic) binder, such as polybutadiene (HTPB)*. Composite propellants are cast from a mix of solid (AP crystals, Al powder)* and liquid (HTPB, PPG)* ingredients. The propellant is hardened by crosslinking or curing the liquid binder polymer with a small amount of curing agent, and curing it in an oven, where it becomes hard and solid. In the past three decades the composite propellants have been the most commonly used class. They can be further subdivided:

(1) Conventional composite propellants usually contain between 60 and 72% ammonium perchlorate (AP) as crystalline oxidizer, up to 22%

'Acronyms, symbols, abbreviations, and chemical names of propellant ingredients are explained in Tables 12-6 and 12-7 in Section 12.4.

aluminum powder (Al) as a metal fuel, and 8 to 16% of elastomeric binder (organic polymer) including its plasticizer.

(2) Modified composite propellant where an energetic nitramine (HMX or RDX) is added for obtaining a little more performance and also a somewhat higher density.

(3) Modified composite propellant where an energetic plasticizer such as nitroglycerine (used in double-base propellant) is added to give a little more performance. Sometimes HMX is also added.

(4) A high-energy composite solid propellant (with some aluminum), where the organic elastomeric binder and plasticizer are largely replaced by energetic materials (such as certain explosives) and where some of the AP is replaced by HMX. Some of these are called elastomer-modified cast double-base propellants (EMCDB). Most are experimental propellants. The theoretical specific impulse can be between 270 and 275 sec at standard conditions.

(5) A lower-energy composite propellant, where ammonium nitrate (AN) is the crystalline oxidizer (no AP). It is used for gas generator propellant. If a large amount of HMX is added, it can become a minimum smoke propellant with fair performance.

Figures 12-1 and 12-2 show the general regions for the specific impulse, burning rate, and density for the more common classes of propellants. Composite propellants give higher densities, specific impulse, and a wider range of burning rates. The ordinate in these figures is an actual or estimated specific impulse at standard conditions (1000 psi and expansion to sea-level atmosphere). It does not include any pressure drops in the chamber, any nozzle erosion, or an assumption about combustion losses and scaling. The composite propellants are shown to have a wide range of burning rates and densities; most of them have specific gravities between 1.75 and 1.81 and burning rates between 7 and 20 mm/sec. Table 12-1 lists performance characteristics for several propellants. The double-base (DB) propellants and the ammonium nitrate (AN) propellants have lower performance and density. Most composite propellants have almost the same performance and density but a wide range of burning rates. The highest performance is for a CMDB propellant whose ingredients are identified as DB/AP-HMX/A1, but it is only four percent higher.

Several of the classifications can be confusing. The term composite-modified double-base propellant (CMDB) has been used for (1) a DB propellant, where some AP, Al, and binder are added; (2) alternatively, the same propellant could be classified as a composite propellant to which some double-base ingredients have been added.

4. Propellants can be classified by the density of the smoke in the exhaust plume as smoky, reduced smoke, or minimum smoke (essentially smoke-

Multi Fin Solid Propellant

10 20 30 40 50 60

Burning rate (mm/sec)

FIGURE 12-1. Estimated actual specific impulse and burning rate for several solid propellant categories. (Adapted and reproduced from Ref. 12-1 with permission of the American Institute of Aeronautics and Astronautics [AIAA].)

10 20 30 40 50 60

Burning rate (mm/sec)

FIGURE 12-1. Estimated actual specific impulse and burning rate for several solid propellant categories. (Adapted and reproduced from Ref. 12-1 with permission of the American Institute of Aeronautics and Astronautics [AIAA].)

less). Aluminum powder, a desirable fuel ingredient, is oxidized to aluminum oxide, which forms visible small solid smoke particles in the exhaust gas. Most composite propellants are smoky. By reducing the aluminum content in composite propellant, the amount of smoke is also reduced. Carbon (soot) particles and metal oxides, such as zirconium oxide or iron oxide, can also be visible if in high enough concentration. This is further discussed in Chapter 18.

5. The safety rating for detonation can distinguish propellants as a potentially detonable material (class 1.1) or as a nondetonable material (class 1.3), as described in Section 11.3. Examples of class 1.1 propellant are a number of double-base propellants and composite propellants containing a significant portion of solid explosive (e.g., HMX or RDX), together with certain other ingredients.

6. Propellants can be classified by some of the principal manufacturing processes that are used. Cast propellant is made by mechanical mixing of solid and liquid ingredients, followed by casting and curing; it is the most common process for composite propellants. Curing of many cast

Density (g/cm3)

FIGURE 12-2. Estimated actual specific impulse and specific gravity for several solid propellant categories. (Adapted and reproduced from Ref. 12-1 with permission of the AIAA.)

Density (g/cm3)

FIGURE 12-2. Estimated actual specific impulse and specific gravity for several solid propellant categories. (Adapted and reproduced from Ref. 12-1 with permission of the AIAA.)

propellants is by chemical reaction between binder and curing agent at elevated temperature (45 to 150°C); however, there are some that can be cured at ambient temperatures (20 to 25°C) or hardened by a nonchemi-cal process such as crystallization. Propellant can also be made by a solvation process (dissolving a plasticizer in a solid pelletized matrix, whose volume is expanded). Extruded propellant is made by mechanical mixing (rolling into sheets) followed by extrusion (pushing through a die at high pressure). Solvation and extrusion processes apply primarily to double-base propellants.

7. Propellants have also been classified by their principal ingredient, such as the principal oxidizer (ammonium perchlorate propellants, ammonium nitrate propellants, or azide-type propellants) or their principal binder or fuel ingredient, such as polybutadiene propellants or aluminized propellants. This classification of propellants by ingredients is described in Section 12.4 and Table 12-8.

8. Propellants with toxic and nontoxic exhaust gases. This is discussed in more detail in Section 12.3.

TABLE 12-1. Characteristics of Some Operational Solid Propellants

Propellant Type0

Range (sec)4

Flame Temperature''

Density or Spec. Gravity®

Metal Content (wt %)

Burning Rate" (in./sec)

Pressure Exponent® n

Hazard Classification^

Stress (psi)/Strain (%)

Processing Method

(°F)

CK)

(lb/in3) (sp

gr.)

—60°F

+ 150°F

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