Rocket propellant is used as a reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.
Overview Rockets create thrust by expelling mass rearward, at high velocity. The thrust produced can be calculated by multiplying the mass flow rate of the propellants by their exhaust velocity relative to the rocket (specific impulse). A rocket can be thought of as being accelerated by the pressure of the combusting gases against the combustion chamber and nozzle, not by "pushing" against the air behind or below it. Rocket engines perform best in outer space because of the lack of air pressure on the outside of the engine. In space it is also possible to fit a longer nozzle without suffering from flow separation. Most chemical propellants release energy through redox chemistry, more specifically combustion. As such, both an oxidizing agent and a reducing agent (fuel) must be present in the mixture. Decomposition, such as that of highly unstable peroxide bonds in monopropellant rockets, can also be the source of energy. In the case of bipropellant liquid rockets, a mixture of reducing fuel and oxidizing oxidizer is introduced into a combustion chamber, typically using a turbopump to overcome the pressure. As combustion takes place, the liquid propellant mass is converted into a huge volume of gas at high temperature and pressure. This exhaust stream is ejected from the engine nozzle at high velocity, creating an opposing force that propels the rocket forward in accordance with Newton's laws of motion. Chemical rockets can be grouped by phase. Solid rockets use propellant in the solid phase, liquid fuel rockets use propellant in the liquid phase, gas fuel rockets use propellant in the gas phase, and hybrid rockets use a combination of solid and liquid or gaseous propellants. In the case of solid rocket motors, the fuel and oxidizer are combined when the motor is cast. Propellant combustion occurs inside the motor casing, which must contain the pressures developed. Solid rockets typically have higher thrust, less specific impulse, shorter burn times, and a higher mass than liquid rockets, and additionally cannot be stopped once lit.
Rocket stages In space, the maximum change in velocity that a rocket stage can impart on its payload is primarily a function of its mass ratio and its exhaust velocity. This relationship is described by the rocket equation. Exhaust velocity is dependent on the propellant and engine used and closely related to specific impulse, the total energy delivered to the rocket vehicle per unit of propellant mass consumed. The mass ratio can also be affected by the choice of a given propellant. Rocket stages that fly through the atmosphere usually use lower-performing, high-molecular-mass, high-density propellants due to the smaller and lighter tankage required. Upper stages, which mostly or only operate in the vacuum of space, tend to use the high-energy, high-performance, low-density liquid hydrogen fuel. For future planetary missions the use of local resources and solar energy for in situ propellant production is considered.
Solid chemical propellants Solid propellants come in two main types. "Composites" are mostly a mixture of granules of solid oxidizer, such as ammonium nitrate, ammonium dinitramide, ammonium perchlorate, or potassium nitrate in a polymer binding agent, with flakes or powders of energetic fuel compounds such as RDX, HMX, aluminium or beryllium. Plasticizers, stabilizers, and burn rate modifiers (e.g. iron oxide or copper oxide) can be added. Single-, double-, or triple-bases are homogeneous mixtures of one to three primary ingredients, which must include fuel and oxidizer, and often include binders and plasticizers. All components are macroscopically indistinguishable and often blended as liquids and cured in a single batch. Ingredients can have multiple roles: RDX is both fuel and oxidizer, while nitrocellulose is fuel, oxidizer, and structural polymer. Many propellants contain elements of double-base and composite propellants, and often contain energetic additives homogeneously mixed into the binder. In the case of gunpowder (a pressed composite without a polymeric binder), the fuel is charcoal, the oxidizer is potassium nitrate, and sulfur serves as a reaction catalyst while also being consumed to form reaction products such as potassium sulfide. The newest nitramine solid propellants based on CL-20 (HNIW) can match the performance of NTO/UDMH storable liquid propellants, but cannot be throttled or restarted. Extraterrestrial on-site production is being explored by combining aluminium and ice (ALICE).
Advantages Solid-propellant rockets are much easier to store and handle than liquid-propellant rockets. High propellant density makes for compact size as well. These features plus simplicity and low cost make solid-propellant rockets ideal for military applications. Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid-fueled rockets in their boost stages (solid rocket boosters) for this reason.
Disadvantages
… excerpt ends here. Continue reading the full article.
