Rubber Binder Chemistry
Some composite propellant formulations and characteristics:
Molded composite: potassium nitrate (20-50%), elastomer/synthetic rubber binder (10%), ammonium picrate-NH4C6H2NO7-(70-40%). Specific impulse s.l.: 160 to 200 sec. Abundant smoke. Hard to brittle.
Castable composite: ammonium nitrate (80%), elastomer/synthetic rubber binder
(18%), catalyst(2%). Specific impulse s.l.: 185 to 198 sec. Little smoke. Soft
and resilient to hard and tough.
Castable composite: ammonium perchlorate (50-85%), elastomer/synthetic rubber
binder (50-15%). Specific impulse s.l.: 175 to 240 sec. Much smoke at low oxidiser;
little at high oxidiser; mist at relative humidity greater than 80%. Soft and
resilient to hard and tough.
Castable composite: potassium perchlorate (50-80%), elastomer/synthetic rubber
binder (50-20%). Specific impulse s.l.: 165 to 210 sec. Abundant smoke. Soft
and resilient to hard and tough.
OXIDISERS
Ammonium perchlorate is the most widely used today. It is characterised by high
heat, is a good gas producer (not a smoke producer), percent of oxygen by weigh:
34 percent, specific gravity: 1.9.
Potassium perchlorate is used for fast burning rates. It is characterised by
high heat, is a low gas producer, percent of oxygen by weight: 46 percent, specific
gravity: 2.5.
Ammonium nitrate is used for slower burning rates. It is characterised by low
heat, is a high gas producer, and is good for gas generator propellants. It
requires a greater amount of binder (fuel) to make castable, but too much binder
produces excessive smoke. Ammonium nitrate's percent of oxygen by weight: 20
percent, specific gravity: 1.9. It may be the oxidiser for the future. It contains
no toxic elements and no solid elements, produces no solids by decomposition,
and therefore, together with a high energy non-polluting fuel, could provide
a more 'environmentally friendly' solid propellant.
Lithium perchlorate, a proposed oxidiser, is very hygroscopic and may be used
in some high-temperature propellants. Percent of oxygen by weight: 60 percent,
specific gravity:2.4.
PROPELLANT ADDITIVES
To help provide the high-energy propellants that were required for the more
efficient space vehicles and missiles, many propellants used special fuel additives
such as powdered metals. Powdered aluminium was used extensively in propellant
formulation for the extra energy it contributed and for the help it gave in
promoting stable burning. Although powdered beryllium had a higher theoretical
energy value than aluminium, it was seldom used because of its extreme toxicity,
relative scarcity, and higher cost. In addition, beryllium had a poor combustion
efficiency with most of the hydrocarbon binder-fuels available. However, this
could be improved by using it with unique and advanced binders like fluorocarbons.
