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 (A1) 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,
A1 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%aluminum powder (A1) 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.