How are propellants classified ?

The propellants are classified depending on their physical into solid and liquid propellants
Solid propellants are classified as homogeneous and heterogeneous propellants homogeneous solid propellant is solid propellant or mixture of solid propellants thoroughly mixed in a colloidal state. Stabilizers and plasticizers are used to increase chemical stability and properties of propellant it is further classified as-

• Single– base propellant contains only one propellant.
  e.g. nitrocellulose or gum cotton
• Double – base propellant contains two propellants it is used in small arms, mortars, cannons and rockets.
  E.g. ballistite (nitrocellulose+ nitroglycerine),cordite (65% nitroglycerine+30%nitrocelluloss +5% petroleum jelly as plasticizer)
• Heterogeneous solid propellants or composite is solid propellant where the oxidant is dispersed in the fuel mass. Oxidants should be non-hygroscopic, stable and non-corrosive in contact with fuel.
  e.g. 75% potassium perchlorate + 52% asphalt oil

80 % ammonium perchlorate + 20% resin binder
46% ammonium picrate +48% sodium nitrate + plastic resin binder

Liquid propellants are more versatile and engine calibration is easier. However, engines using liquid propellants are delicate. Liquid propellants are classified as-

• Monopropellant has fuel and oxidant in the same molecule or a solution containing both.
  e.g. hydrogen peroxide, nitromethane, hydrazine, 21.4% methanol + 78.6% hydrogen peroxide
• Bipropellant has liquid fuel and oxidant stored and injected separately in the combustion chamber. They are commonly used.
  e.g. liquid fuels: liquid hydrogen, ethanol, hydrazine, ethanol+25% water (water reduced lame temperature as well as molecular weights of combustion gas)
• E.g. oxidants: liquid oxygen (non-toxic, safe but have to be stored under pressure in insulated cylinders), ozone(toxic and explodes at higher concentration), liquid fluorine (volatile, toxic, corrosive but good oxidant), hydrogen peroxide ( good oxidant but difficult to store and handle).