Solid propellant rockets were invented before liquid fueled rockets. The solid propellant type began with contributions by scientists Zasiadko, Constantinov, and Congreve. Now in an advanced state, solid propellant rockets remain in wide spread use today, including the Space Shuttle dual booster engines and the Delta series booster stages.
How a Solid Propellant FunctionsA solid propellant is a monopropellant fuel, a single mixture of several chemicals i.e. the oxidizing agent and the reducing agent or fuel. This fuel 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.
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. 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 ImpulseSpecific impulse is the thrust per unit propellant burned each second, it measures rocket performance and more specifically, internal thrust production a product of pressure and heat. Thrust in chemical rockets is a product of the hot and expanding gasses created in the combustion of an explosive fuel. 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 failure (explosion), and a successfully optimized thrust producing rocket.
Modern Solid Fueled RocketsThe 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.
Advantages/DisadvantagesSolid 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. 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 danger involved in the premixed fuels of monopropellant rockets i.e. sometimes nitroglycerin is an ingredient.
One advantage, is the ease of storage of solid propellant rockets. 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.