Composite Propellant, also known as heterogeneous propellant, consists of separate fuel and oxidiser chemicals. However, most explosives employ single compounds of carbon, hydrogen, nitrogen and oxygen. On detonation, the bonds between these elements within the compound are broken, For example, the explosive nitroglycerine is formed by treating glycerine with nitric and sulphuric acid in order to attach nitrate esters to carbon atoms within the glycerine. Nitroglycerine contains enough oxygen for complete oxidation to occur, with no need for a separate oxidiser. However, as with all nitrate compounds, nitroglycerine is highly unstable unless mixed with another material. This results in double-base propellants. The most common double base propellents combine the explosives nitroglycerine and nitrocellulose to form a colloid. Double-base propellants are also known as homogeneous propellants.
Probably the most famous double-base composition is cordite. This was used as the fuel in the duel thrust (boost and sustain) motors of the guided Malkara anti-tank missile and the Sealyham motors of the British Tigercat and Seacat surface-to-air missiles. Both Seacat and Tigercat missiles saw action in the Falklands War. Seacat was the most numerous SAM employed by the Royal Navy, while Tigercat was deployed by the Argentinians to defend Port Stanley airport.
As a general rule of thumb, double-base propellants are used in battlefield missiles such as anti-tank weapons and composite propellants are used in larger weapons such as ballistic missiles. However, there are many exceptions to this rule.
In addition to composite and double-base propellants, it is possible to combine the two by using a matrix of nitrocellulose and nitroglycerine as a binder supportingthe ammonium perchlorate oxidiser and powdered aluminium fuel. This is sometimes reffered to as composite modified double base (CMDB) propellant. The most common application for this propellent has been submarine launched ballistic missiles. This propellant is very energetic, but produces very dense white smoke.
One of the most significant users of combined composite and double base propellant is the trident SLBM. The solid component of the trident one (C4) propellant consisted of HMX (His Majesty's Explosive or tetramethylene tetranitramine - which replaces some of the ammonium perchlorate in high energy propellants), aluminium and ammonium perchlorate. The binder of these solids was Polyglycol Adipate (PGA), nitrocellulose (NC), nitroglycerine (NO), and hexadiisocryanate (HDI). This propellant was called XLDB-70 (cross-link double-base 70 percent solid fuels). The trident two (D5) replaced the PGA with polythylene glycol (PEG). This made the mixture more flexible and allowed an increase in the solids to 75%. This propellant has the name NEPE-75.