Function: A solid propellant is a monopropellant fuel-a single mixture of several chemicals (the oxidizing agent and the reducing agent or fuel).  This fuel, as implied, is in its solid state and has a preformed or molded shape.  The propellant grain, this interior shape of the core is an important factor in determining a rocket's performance.  The variables determining grain-relative performance are core surface area and specific impulse.

Surface area is the amount of propellant exposed to interior combustion flames, existing in a direct relationship with thrust.  An increase in surface area will increase thrust but will reduce burn-time since the propellant is being consumed at an accelerated rate.  The optimal thrust is typically a constant one, which can be achieved by maintaining a constant surface area throughout the burn.  Examples of constant surface area grain designs include: end burning, internal-core and outer-core burning, and internal star core burning (figure 3). Various shapes are used for the optimization of grain-thrust relationships since some rockets may require an initially high thrust component for takeoff while a lower thrust will suffice its post-launch regressive thrust requirements.  Such a compromise has implications as seen, but it optimizes overall rocket performance. Complicated grain core patterns, in controlling the exposed surface area of the rocket's fuel, often have parts coated with a non-flammable plastic (such as cellulose acetate). This coat prevents internal combustion flames from igniting that portion of fuel, ignited only later when the burn reaches the fuel directly.

Specific Impulse, the thrust per unit propellant burned each second, measures rocket performance and more specifically, internal thrust production a product of pressure and heat.  Thrust in chemical rockets (inclusive of both solid and liquid fueled rockets, is a product of the hot and expanding gasses created in the combustion of an explosive fuel (a reduction-oxidation reaction).  The degree of the fuel's explosive power coupled with the rate of combustion is the specific impulse.  In designing the rocket's propellant grain specific impulse must be taken into account since it can be the difference betwixt a conflagration of failure (explosion), and a successfully optimized thrust producing rocket.  If a propellant with a high specific impulse is used as the fuel for a rocket whose grain design offers a high surface area ratio, high amounts of thrust will ensue ignition.  And if the engine grain casing cannot withstand the extreme pressure and temperature it will rupture and explode.  Thus, the function involving the variables of both specific impulse and surface area must be considered in grain design.

The departure from the use of gunpowder to more powerful fuels (higher specific impulses) marks the development of modern solid fueled rockets.  Once the chemistry behind rocket fuels (fuels provide their own "air" to burn) was discovered, scientists sought the evermore-powerful fuel, constantly approaching new limits. A composite propellant is a mechanically mixed combination of the oxidizer and the fuel.  Some common solid oxidizers are: ammonium perchlorate (NH4-ClO4) and ammonium nitrate (NH4-KNO3), chemicals providing far more oxygen than potassium nitrate (KNO3), the oxidizing agent in gunpowder.  These oxidizers are often mixed, in making composite propellants, with synthetic rubbers such as: polystyrenes, polysulfides, and polyurethanes.  Another type of propellant is homogeneous where the oxidizer and the fuel are combined as one molecule.  Propellants of this type often use a double-base (combination of two propellants) of nitrocellulose and nitroglycerin (C3H5(ONO2)3).
 

Advantages/Disadvantages: Solid fueled rockets are relatively simple rockets.  This is their chief advantage, but it also has its drawbacks.  Once a solid rocket is ignited it will consume the entirety of its fuel, without any option for shutoff or thrust adjustment.  Another key disadvantage is the danger involved in the premixed fuels of monopropellant rockets.  In the double-base homogeneous nitrocellulose-nitroglycerin propellant, for example the nitroglycerin is too unstable (sufficient shock will detonate) to use individually add thus a more stable propellant like nitrocellulose (a form of gunpowder) is added.  Composite engines, having the fuel and oxidizer as separately mixed elements, are less sensitive to shock and therefore safer to use.  A relatively low specific impulse limits the use of solid rockets when large amounts of thrust precondition.  The Saturn V moon rocket used nearly 8 million pounds of thrust that would not have been feasible with the use of solid propellant, requiring a high specific impulse liquid propellant.  The ease of storage of solid propellant rockets is another main advantage employing a high level use in the military.  Some of these rockets are small missiles such as Honest John and Nike Hercules; others are large ballistic missiles such as Polaris, Sergeant, and Vanguard.  Liquid propellants may offer better performance, but the difficulties in propellant storage and handling of liquids near absolute zero (0 degrees Kelvin) has limited their use unable to meet the stringent demands the military requires of its firepower.