The type of rocket engine determines the corresponding type of rocket fuel storage and delivery systems. Rocket engines can operate on common fuels such as gasoline, alcohol, kerosene, asphalt or synthetic rubber, plus a suitable oxidizer. Engine designers consider fuel and oxidizer combination's for energy release and so also physical and handling properties needed for desired performance. Selecting propellants for a given rocket requires a complete analysis of rocket - propellant performance, density, storability, toxicity, corrosiveness, availability and cost, size and structural weight of the vehicle and payload weight.
Earlier rockets employed solid rocket fuels. They are simple with a casing (usually iron or steel) and the mixture of fuel and oxidizer. Today there are many chemical combination's that make good solid propellants. Besides gunpowder and metal-powder mixtures (such as zinc and sulfur) which have erratic burning rates and poor physical properties. There are two classes of solid propellants which were originally developed for rockets during and after World War II and are widely used even now, namely double-base (homogeneous) and composite (heterogeneous) propellants. The Scout rocket uses solid propellants exclusively. This four-stage rocket launches small satellites to orbit.
These are a blend of nitrocellulose and nitroglycerin with small quantities of salts, wax, coloring and organic compounds to control burning rates and physical properties. A blast from a small chemical combination can ignite and start rapid recombination of this solid rocket fuels.
As the name implies are mixtures of an oxidizer, usually an inorganic salt such as ammonium perchlorate in a hydrocarbon fuel matrix, such as an asphalt like material. Usually composite propellant are cast to shape so that the rocket fuel is added with small particles of oxidizers that are dispersed throughout. The fuel is called a binder because the oxidizer has no mechanical strength. In both cases fuel and oxidizer are mixed and undergo combustion only on surface, they are placed directly into combustion chamber. Rocket fuel burns until it gets exhausted, which involves changing the effective size and shape during the operation. As it burns only on surface, shape and area of propellant are regulated. Solid rocket fuel burn rapidly by expelling hot gases from nozzle to produce thrust depending on the mass flow rate which in turn depends on the solid rocket fuel consumed per second. The only problem is once ignited the motor can not be shut down until the fuel is exhausted.
The first modern rocket was built with liquid rocket fuel (liquid hydrogen). Though liquid rocket fuels are difficult to store they use liquid rocket fuels in orbital launch vehicles to gain velocity. Liquid fuel rockets are capable of being throttled, shut down and restart. The fuel tankers can be built with less material permitting large mass fraction when ignited.
Liquid rocket propellant units are classified into mono, bi, tri propellants.
Liquid propellants are commonly classified as :
A cryogenic propellant is one that has a very low boiling point and must be kept very low temperatures where as a storable propellant is one that is liquid at normal temperatures and pressures and may be left in a rocket for days, months, or even years.
There are many types of mixtures used as rocket fuels. Some of them are,