Rocket

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Staging

Rockets are very powerful, but it is often more efficient to use several rockets, rather than a single rocket, to move an object to the desired place. Launch vehicles often use more than one rocket engine, or stage, during a mission. In rockets that use stages, the stages are stacked on top of each other. The stage on the bottom of the stack is the first one to fire. In some rockets that use stages, the first stage has additional rockets attached to the outside, acting as boosters to further increase the thrust. Rockets can theoretically use any number of stages, but the complications caused by coordinating the firing times of the stages make it impractical to have too many. The huge Saturn V rocket that sent Apollo astronauts to the Moon had four stages, including the Apollo spacecraft’s own rocket.

The first and most powerful stage lifts the launch vehicle into the upper atmosphere. The first stage then separates from the rest of the rocket and falls toward Earth. Some first stages, such as the space shuttle’s booster rockets, can be recovered. Others, such as the first stage of the huge Saturn V Moon rocket, burn up in the atmosphere once their fuel is expelled and they drop off the launch vehicle.

The second stage carries less weight than the first stage, because the first stage has dropped off of the rocket. When the second stage takes over, the vehicle reaches a much higher speed; the second stage, however, also uses up its fuel and drops off. The third stage fires and places the spacecraft into orbit (for a mission designed to orbit Earth). On deep space missions, the third stage allows the spacecraft to reach escape velocity and head away from Earth. For some missions, three stages are not adequate.

Engineers can reduce the number of stages a launch vehicle needs by getting a rocket closer to its destination through some other means. For example, an airplane carries the Pegasus rocket, which sends spacecraft into space, to a high altitude first. The rocket then fires and carries its cargo into orbit.

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IV

Types of Rocket Propulsion

There are three basic types of rocket propulsion: chemical, nuclear, and electrical. Chemical rockets use chemicals, in solid or liquid form, for fuel and oxidizer, or the chemical that contains the oxygen needed to burn the fuel (together, the fuel and oxidizer are called the propellant). Nuclear rockets use the heat of nuclear reactions to heat chemical propellants for combustion. Electrical rockets use electric and magnetic fields (regions of space affected by electrical and magnetic energy) to accelerate and expel ions and elementary particles. Ions are atoms with positive or negative electrical charges, and elementary particles such as protons, neutrons, and electrons are the tiny building blocks of matter that make up atoms.

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Chemical Rockets

Chemical rockets are suitable for many purposes. Large solid-fueled and liquid-fueled chemical rockets act as launch vehicles or as missiles that are capable of traveling from continent to continent. People use smaller chemical rockets as sounding rockets, as missiles with shorter ranges, or as the upper stages of launch vehicles. Small liquid-fueled chemical rockets make good thrusters, because the burning of their fuel can be stopped and restarted whenever the spacecraft needs a course correction. Solid-fueled rockets and liquid-fueled rockets that use fuel at ordinary temperatures are the best chemical rockets for missiles.

Combustion, or burning, takes place inside a cup-shaped container called the combustion chamber at the rear of the rocket. The exhaust nozzle, which is engineered to provide the greatest thrust for the particular propellant used, leads from the combustion chamber to the bottom of the rocket. The narrow part of the nozzle, between the hemispherical combustion chamber and the nozzle itself, is called the throat. Nozzles are made with material that is resistant to heat, because they must be able to withstand very high temperatures.

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Solid-Fueled Rockets

Solid-fueled rockets are the simplest rockets. They have two main parts: the body, or case, where the propellant is stored, and the combustion chamber with its attached nozzle. The case holds the propellant and opens to the combustion chamber at one end. Most cases are cylindrical, but the cases of some rockets that are used to move objects through space are spherical. The solid mass of propellant is called the charge, or grain. Solid-fueled rockets often use electrically heated wires called igniters to heat the propellant to its ignition point (the temperature at which the propellant catches fire). Igniters are threaded through the nozzle to the bottom of the propellant or through a hole in the propellant farther up in the grain.

Solid rocket fuels of the past included gunpowder and mixtures containing nitroglycerin and nitrocellulose that were called double-base propellants. Current fuels are called composite fuels and are composed of synthetic rubbers or plastics with additives. These additives include binders that hold the fuel together, powdered metals that increase specific impulses, and chemicals that control the speed at which the propellant grain burns. Usually, the faster a rocket burns, the more thrust it produces. The rocket also uses up its fuel faster if the fuel burns faster. Engineers must take the burning rate into account when they design solid-fueled rockets, because stopping the propellant from burning once it has ignited is very difficult. Rockets such as booster rockets, which must produce large amounts of thrust in a short period of time, use chemicals to increase the burning rate. Other rockets that need to produce less thrust over a longer period of time use chemicals to decrease the burning rate. The longer-burning rockets are called sustainers. A few types of rockets have small tanks and pumps that can spray water or another extinguisher on the propellant to stop its burning.

Engineers can make composite fuels in several separate segments, then stack and join them together in the rocket case to produce extremely large, powerful, and long-duration motors. The huge solid rocket boosters of the space shuttle are put together in sections and are capable of about 13 million N (about 3 million lb) of thrust. The shuttle’s solid rocket boosters are presently the world’s largest solid-fueled rockets. Star-shaped cavities in the propellant blocks increase thrust by increasing the surface area of fuel available for burning. This increase in surface area allows the propellant to burn faster.

Engineers seek to make rockets as light as possible in order to maximize their efficiency. About 90 percent of the weight of a modern solid-fueled rocket is propellant, but decreasing the weight of the case still increases the rocket’s efficiency. Using heat-resistant fiberglass and heat-resistant plastic helps lighten the materials used in the case, and special techniques for building the cases help reduce the amount of material needed while maintaining the cases’ strength.

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Liquid-Fueled Rockets

Liquid-fueled rockets carry their own fuel and oxidizer in liquid form. The liquids are stored in tanks in the rocket case and are pumped into the combustion chamber as needed. Liquid fuels generally provide greater specific impulses than solid fuels, mainly because the liquid fuels are denser. Engineers can control combustion in liquid-fueled rockets by simply changing the rate at which the pumps move the liquid. Engineers can stop combustion by stopping the pumps completely. Stopping and restarting combustion can be very useful in space missions, because course corrections or steering may require only short bursts from the rockets.

Liquid-propellant systems are more complex to handle than solid-fueled systems. Liquid-fueled rockets require separate oxidizer and fuel tanks, and many systems need high speed, lightweight pumps and injectors to spray fuel into the combustion chamber. The simplest liquid-fueled rockets use a non-reactive pressurized gas, such as nitrogen gas, to force the propellants into the combustion chamber. The non-reactive gas is held under pressure in a tank above the fuel tanks. Valves between the tanks open when fuel is needed in the combustion chamber. The pressure of the gas entering the fuel tank forces the liquid propellant into the chamber. More complicated liquid-fueled rocket systems use pumps to move the fuel and oxidizer between their holding tanks and the combustion chamber.

Liquid-fueled rockets use several types of fuels and oxidizers. Some rockets use familiar liquid fuels such as alcohols, gasoline, and kerosene. The oxidizer used with these fuels is most often liquid oxygen—oxygen gas that is cooled and compressed to a liquid form. Kerosene is the most popular fuel for modern rockets.

Other compressed and cooled gases, such as hydrogen, perform as fuels in some liquid-fueled rockets. When a substance stays in liquid form even though its temperature is colder than its freezing point, or the point at which it should become a solid, the substance is called supercooled. Supercooled gases used as liquid fuels are called cryogenic (from the Greek word cryo for “cold”) fuels (see Cryogenics). Some liquid-fueled rockets use oxidizers and fuels that begin burning as soon as they come in contact with each other. Such propellants are called hypergolic, and they greatly simplify a rocket’s ignition system. Some cryogenic fuel-oxidizer combinations are also hypergolic. Monopropellant rockets mix and store the fuel and oxidizer together. When ignited, monopropellants supply their own oxygen for burning.

Rockets that use non-hypergolic propellants need an igniter or some other way of lighting the fuel and oxidizer. Some early liquid-fueled rockets used spark plugs as their igniters. A spark plug consists of two wires separated by a small gap and connected to a power source. When an electrical voltage is applied to the wires, a current jumps, or arcs, between the wires, producing a spark. The spark plug igniters in early liquid-fueled rockets were placed in the path of the injection streams or built into injectors that moved the liquids to the combustion chamber.

Other types of igniters include small explosive powder charges and pieces of metal that heat up when an electric current flows through them until they ignite the propellant. Some rockets that do not use hypergolic propellants as their main source of power may use small amounts of hypergolic propellants to ignite their main propellant. The combustion of the hypergolic propellant often takes place in a small chamber that opens into the main combustion chamber. Another method of igniting propellants is to use catalysts (chemicals that encourage certain chemical reactions to occur) to start a reaction that produces enough heat to ignite the propellant.

Liquid propellants burn in rocket engines at an average temperature of about 3,000° C (about 5,400° F). By comparison, the melting point of steel is about 1,370° C (about 2,500° F). Engineers must provide a way to cool the combustion chamber in order to keep the rocket engines from melting if the rocket will burn for more than a few seconds. Early liquid-fueled rockets of the 1920s and 1930s often experienced premature burnouts and explosions. Early groups of researchers tried using water jackets, heat-absorbing aluminum blocks, and other heat-resistant materials to protect the rocket body from the intense heat.

A cooling technique called regenerative cooling involves circulating the fuel around the outside of the rocket engine before burning the fuel. The heat of the combustion in the engine transfers to the circulating fuel, cooling the engine surfaces and warming the fuel. Many fuels burn more efficiently if they are heated before burning. In a process called film–cooling, special fuel injectors spray the fuel and oxidizers on the interior walls of the combustion chamber. The heat of the walls causes the liquid to evaporate, cooling the walls in the same way as sweat cools a human body. The propellant vapor then burns in the center of the combustion chamber.

Most modern large liquid-fueled engines, such as the Space Shuttle’s Main Engine (SSME), use a design of combustion chamber called the spaghetti design. These chambers are called spaghetti chambers because hollow cooling tubes resembling strips of pasta form the walls of the combustion chambers. These chambers are well cooled and much lighter, yet stronger, than previous chambers.

Cryogenic propellants pose many of the same challenges to engineers that storable propellants do. The combustion temperatures of cryogenic propellants are generally higher than those of storable propellants, so the techniques for cooling the rocket engines need to be even more efficient. In addition, rockets that use cryogenic propellants must have ways of keeping the fuel cold enough to keep it from evaporating. Liquid hydrogen and other liquefied gases are usually made by compressing the gases under extreme pressure and at low temperatures. The gases are cooled in steps using special equipment. Liquefied gases must also be stored and transported in leak-free insulating containers to maintain their cryogenic temperature and prevent the liquid from evaporating, or turning back into gas and escaping into the atmosphere.

Hypergolic and monopropellant liquid-fueled rockets have only slight differences from the other types of liquid-fueled rockets. Systems in which an inert gas presses the fuel into the combustion chamber (pressure-fed systems) often use hypergolic propellants. Hypergolic propellants burn at about the same temperature as storable propellants, so rockets that use hypergolic propellants still need to provide a way to keep the rocket engines cool.

Monopropellant rockets generate much lower thrusts than those generated by all types of bipropellant rockets, or rockets that use a separate fuel and oxidizer. Monopropellant rockets are very useful, however, because they are simple, lightweight, and have only one propellant tank. Monopropellants burn at significantly lower temperatures (well beneath the melting point of steel) than other propellants, so cooling structures are not as important. Small monopropellant rockets serve as course-adjustment or attitude control systems for spacecraft. Most monopropellant rockets used for these applications can be stopped and restarted, and have variable levels of thrust.