Engineering Chemistry S Datta

346 ENGINEERING CHEMISTRY
form a large volume of gas. Since propellants burn without atmospheric oxygen, the propellant
system must contain the oxidizer along with the fuel. The fuel burn and the large vol. of hot
gases exit through a small opening at a large velocity (as per Newton’s third law of motion)
which can be used for several purposes like charges for guns, rockets, gas generators, propelling
rockets, launching missile. The generated gases can also be used to drive pumps, empty tanks,
actuate valves, inflate air bags etc.
Classification of propellants
Propellants can be classified into solid and liquid propellants broadly.
(A) Solid propellants. Solid propellants may be again categorized into ‘homogeneous’
and ‘composite’.
1. Homogeneous solid propellants are mixed homogeneously in a colloidal state—it is
homogeneous solid propellant
(a) Single-base propellants. It is a type of homogeneous solid propellant consisting of
mainly plasticized nitrocellulose. They contain stabilizers like diphenylamine,
plasticizers like dibutyl phthalate. They can be used only as gun propellants or in
bulk powder fillings for gas generators.
(b) Double-base propellants. General composition is 50–55% nitrocellulose and 40–45%
nitroglycerine, 5% diethyl pthalate as plasticizer giving a smooth mass, 1%
diphenylamine is also added as stabilizer. Flame temperature reaches about 2700°C
and gases liberated at about 1500 times the original volume.
Other compositions are 65% nitrocellulose, 30% nitroglycerine, 5% petroleum jelly, where
the last one functions as plasticizer.
(c) Triple-base propellants. Nitroguanidine, having a low flame temperature is used as
‘cold powders’ for triple-base propellants to reduce the flame temperature. In these
propellants, nitroglycerine is replaced by explosive plasticizers like diethylene glycol
dinitrate or butanetriol trinitrate which have low heat of explosion. They are used as
gun propellants and rocket propellants. For rocket propellant, lead or copper salts
are used as ballistic modifiers to reduce pressure.
2. Composite propellants. Heterogeneously formulated solid propellants that consist of
a polymeric binder in which crystalline oxidizers are incorporated in amounts upto about 90%
by weight. Composite propellants are used as gun and rocket propellants. Propellants for larger
gas generators are generally of the composite type. For small charges, single and double-base
propellants are used. Composite type have low binder content and the crystalline fillers on
burning produces lager amount of gases and also the flame temperature is comparatively low.
Gun powder is the oldest composite propellant. It gives of a flame temperature of 800-1500°C
and the volume of gases is 400 times the volume of the original. Different composite solid
propellants contain 75% potassium perchlorate + 25% asphalt oil, 80% ammonium perchlorate
+ 20% resin binder. The oxidizers are selected which should be stable, non-hygroscopic and
should not produce any corrosive or objectionable product on combustion, viz; ammonium
perchlorate produces HCl on combustion which is corrosive, Aluminium powder in rocket propellant
produces large amount of smoke due to Al 2
O 3
formation.
Ammonium perchlorate is replaced by ammonium nitrate to reduce HCl generation.
Energetic substances (RDX or HMX), explosive plasticizers (nitroglycerine etc.) and energyrich
binders (GAP) are used to improve the performance.