Rocket Propellants

Propellants are the fuels used in rockets for propulsion. For example, alcohol, liquid hydrogen, liquid ammonia, kerosene, hydrazine and paraffin can be used as propellants.

To burn them, rocket fuels require an oxidising agent. Examples are: liquid oxygen, liquid fluorine, dinitrogen tetroxide (N₂O₄), nitric acid (HNO₃), a nitrate, chlorate or perchlorate.

There are two types of propellants:

Solid Propellants

Solid propellants are a mixture of solid hydrocarbon and an oxidising agent. This oxidising agent is stable at room temperature. For example, a mixture of paraffin and potassium nitrate (KNO₃). Paraffin is the solid hydrocarbon and KNO₃ acts as the oxidising agent. These are divided into:

• Composite and

• Double base propellants

Composite Propellants

They consist of a polymeric binder (polyurethane or polybutadiene) and ammonium perchlorate (oxidiser). Additives like finely divided aluminium or magnesium enhance their performance.

Double Base Propellants

Double base propellants mainly use nitroglycerine and nitrocellulose. These two together constitute a gel to give a semisolid mass.

Disadvantage

Fire caused due to the burning of solid propellants is very difficult to control.

Liquid Propellants

Liquid propellants are widely used as rocket fuels. They can be monopropellants or biliquid propellants based on the number of liquids in the propellant mixture.

Advantages

• They give better thrust than solid propellants

• By propellant flow regulation, thrust can be controlled

A mixture of liquid oxygen and liquid hydrogen is one of the most important liquid propellants. Oxygen burns liquid hydrogen to produce heat energy.

Liquid propellant examples are alcohols, liquid ammonia, kerosene, hydrazine (unsymmetrical dimethyl hydrazine - UDMH and monomethyl hydrazine - MMH)

Oxiding agents can be liquid oxygen, liquid fluorine, hydrogen peroxide and nitric acid.

Hybrid Propellant

When the propellant has a solid fuel and a liquid oxidiser composition, it is called a hybrid propellant.

Example: A mixture of acrylic rubber and liquid dinitrogen tetraoxide

Characteristics of Propellants

A good rocket propellant must

• produce large volumes of gases for every gram of fuel which undergoes combustion

• burn at a fast rate

• burn completely without leaving behind residue or ash (dead weight)

• have high calorific value for high efficiency of fuel

Principle

When the fuel is ignited, combustion occurs. The liberated gases pass through the nozzle of the rocket motor, providing the necessary thrust for the rocket to lift and take off.