Rocket.

Rocket. I INTRODUCTION Rocket, self-propelled device that carries its own fuel, as well as the oxygen, or other chemical agent, needed to burn its fuel. Most rockets move by burning their fuel and expelling the hot exhaust gases that result. The force of these hot gases shooting out in one direction causes the rocket to move in the opposite direction. A rocket engine is the most powerful engine for its weight. Other forms of propulsion, such as jet-powered and propeller-driven engines, cannot match its power. Rockets can operate in space, because they carry their own oxygen for burning their fuel. Rockets are presently the only vehicles that can launch into and move around in space. A rocket can be as simple and small as a firework, which has a small amount of thrust, or as complex and powerful as the Saturn V rocket, which took humans to the Moon. British Congreve war rockets, which were used in the War of 1812, are referred to in a line of the United States national anthem: "And the rockets red glare..." Rockets have many applications both on Earth and in space. The most common and well-known use of rockets is for missiles--weapons that deliver explosive warheads through the air to specified targets (see Guided Missile). Rockets also have numerous peaceful purposes. Upper atmospheric research rockets, or sounding rockets, carry scientific instruments to high altitudes, helping scientists carry out astronomical research and learn more about the nature of the atmosphere. Jet-Assisted-TakeOff (JATO) rockets help lift heavily loaded planes from runways. Lifesaving rockets carry lifeline ropes to ships stranded offshore. Ships in distress can launch signal rockets to signal for help. Rocket ejection seats safely boost pilots out of jet planes during emergencies. Fireworks have provided entertainment for centuries, and model rockets form the basis of a popular hobby. II ROCKET USES People use all kinds of rockets for the same basic purpose: to carry objects through air and space. Missiles carry explosive devices to targets, while sounding rockets carry scientific instruments into the upper atmosphere. Launch vehicles boost satellites and other spacecraft into space, and smaller thruster rockets steer or stabilize spacecraft in space. A Missiles The term missile actually means any object thrown at an enemy and includes arrows, bullets, and other weapons. In modern military usage, however, missile usually means an explosive device propelled through the air by a rocket or an air-breathing engine. (Air-breathing engines differ from rockets in that rockets carry their own oxygen, while air-breathing engines get their oxygen from the air as they fly through it.) Missiles can be launched from the ground, from airplanes, and even from submarines. Some missiles are designed to hit targets in the air, while others are built to hit targets on the ground. Some missiles, called guided missiles, have steering systems that guide them to their target. B Sounding Rockets Scientists use sounding rockets to carry scientific instruments into the upper atmosphere to take measurements of air quality, radiation from space, and other data. Many countries use sounding rockets to monitor weather and pollution. Engineers enable a rocket to reach its target altitude by shutting down the rocket at a specific height. The rocket then coasts upward until air friction and gravity stop its upward movement and cause it to fall back to Earth. The instruments usually include a radio transmitter that sends measurements back to Earth. Some sounding rockets carry parachutes that allow their controllers to recover the rocket and the instruments, but some fall back to Earth without a parachute. Engineers design a sounding rocket's flight path so that the rocket will fall into the ocean or into an uninhabited area in order to avoid damaging property or hurting people. C Launch Vehicles Launch vehicles send satellites and other spacecraft into space. These vehicles must be far more powerful than other types of rockets, because they carry more cargo farther and faster than other rockets. To place an object into orbit around Earth, the launch vehicle must reach a velocity of about 30,000 km/h (about 18,500 mph). To escape Earth's gravitational pull entirely and head into deep space, these rockets must attain a velocity, called an escape velocity, of about 40,000 km/h (about 25,000 mph). Engineers have found that the most efficient way for launch vehicles to reach these speeds is to use staged rockets, or rockets divided into different stages, one atop another. D Thrusters Many spacecraft use small rockets called thrusters to move around in space. Thrusters can change the speed and direction of a spacecraft. They allow a spacecraft to steer in space, to jump to a higher orbit, or to fall back to Earth. III HOW ROCKETS WORK All rockets--whether small or large, simple or complex--work by the basic principle of action and reaction, which was formulated by English scientist Sir Isaac Newton in 1687. Newton's third law of motion states, "For every action there is an equal and opposite reaction." In the case of the rocket, the expulsion of exhaust gases from the rear is the action, and the forward movement of the rocket is the reaction. A Action and Reaction The motion of a rocket is much like the motion of a balloon losing air. When the balloon is sealed, the air inside pushes on the entire interior surface of the balloon with equal force. If there is an opening in the balloon's surface, the air pressure becomes unbalanced, and the escaping air becomes a backward movement balanced by the forward movement of the balloon. Rockets produce the force that moves them forward by burning their fuel inside a chamber in the rocket and then expelling the hot exhaust that results. Rockets carry their own fuel and the oxygen used for burning their fuel. In liquid-fueled rockets, the fuel and oxygen-bearing substance (called the oxidizer) are in separate compartments. The fuel is mixed with the oxygen and ignited inside a combustion chamber. The rocket, like the balloon, has an opening called a nozzle from which the exhaust gases exit. A rocket nozzle is a cup-shaped device that flares out smoothly like a funnel inside the end of the rocket. The nozzle directs the rocket exhaust and causes it to come out faster, increasing the thrust and efficiency of the rocket. Some early scientists believed that rocket exhaust needed something to push against (such as the ground or the air) in order to move the rocket. Rockets traveling in the vacuum of space, however, demonstrated that this belief was not true. In fact, rockets produce more thrust in the vacuum of space than on Earth. Air pressure and friction with the air reduce a rocket's thrust by about 10 percent on Earth as compared to the rocket's performance in space. B Thrust and Efficiency Thrust is a measurement of the force of a rocket, or the amount of "push" exerted backward to move a rocket forward. Thrusts vary greatly from rocket to rocket. Engineers measure thrust in units of weight or force (newtons [N] in the metric system and pounds [lb] in English measurements). Specific impulse measures the efficiency and power of rocket engines and propellants. Specific impulse (Isp) is the thrust produced per kilogram or pound of propellant per second. Measuring Isp is similar to measuring the efficiency of cars in kilometers per liter or miles per gallon. Modern solid propellants have specific impulses of about 3,400 to 3,900 N per kg per sec (about 350 to 400 lb per lb per sec) and advanced liquid propellants typically have Isps of about 4,200 to 4,400 N/kg/sec (about 425 to 450 lb/lb/sec). Exhaust velocity, or the speed at which exhaust leaves the rocket, is another way to measure rocket performance. The higher the exhaust velocity, the greater the thrust. Propellants with higher exhaust velocities also have higher specific impulses. Exhaust velocities can range from 600 to 900 m/sec (2,000 to 3,000 ft/sec) for gunpowder, 2,000 m/sec (8,000 ft/sec) for a mixture of liquid oxygen and gasoline, to 4,000 m/sec (12,000 ft/sec) or more for a mixture of liquid oxygen and liquid hydrogen. Rocket engine performance also depends on the design of the combustion chamber and nozzle and the pressure of the propellant. C 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. 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. A 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. A1 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. A2 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 liquidfueled 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 courseadjustment or attitude control systems for spacecraft. Most monopropellant rockets used for these applications can be stopped and restarted, and have variable levels of thrust. A3 Hybrid Chemical Rockets Hybrid rocket engines use both liquid and solid fuels. Usually, the liquid oxidizer is injected onto the solid synthetic rubber fuel and ignited in the combustion chamber. Hybrid systems combine advantages of both solid- and liquid-fueled systems. Hybrid propellants are inexpensive, and their burn rate can be controlled by regulating the oxidizer flow. Hybrid rockets are still experimental and have not been widely used, but several rocket manufactures are testing hybrid systems. Hybrid propellants have specific impulses of around 2,900 N/kg/s (300 lb/lb/s), which is comparable to that of cryogenic liquid propellants. B Nuclear Rockets Nuclear rockets are very powerful rockets that are theoretically capable of acting as launch vehicles and long-distance space travel systems. No nuclear rocket has yet made it into space, but experimental rockets have undergone tests on Earth. The complexity of building safe nuclear rockets and worries about using rockets that are carrying radioactive materials have limited the practical use of nuclear rockets. Nuclear rockets generate thrust by using nuclear reactions to heat liquid hydrogen to a superheated gas, or a gas heated well beyond its boiling point, that shoots out of the rocket nozzle. In the nuclear reactions that occur, called fission reactions, heavy atoms such as uranium and plutonium split apart to produce lighter elements and energy. Nuclear rockets could produce much higher specific impulses than chemical systems, because nuclear rockets heat propellants to higher temperatures. Specific impulses of nuclear rockets are 7,800 N/kg/sec (800 lb/lb/sec) or more. In one form of nuclear rocket engine, a small nuclear reactor (similar to one used to produce electricity on the ground) superheats liquid hydrogen circulated through the reactor. Another type of nuclear rocket, called a gaseous fission nuclear rocket, offers specific impulses of 14,000 N/kg/sec (1,400 lb/lb/sec) or more. Gaseous fission rockets create an intensely hot fireball by splitting atoms of uranium-233 gas or a similar fuel. As before, liquid hydrogen is pumped in and converted into a superheated gas that exits the nozzle. See Nuclear Energy. A fission reaction releases most of its energy in the form of heat, which helps power the rocket. Fission reactions also release other types of radiation in the form of gamma rays and fast-moving neutrons. Both gamma rays and these fast neutrons can be harmful to the rocket body and to any living things nearby. The intense heat of both kinds of reactors can also be quite destructive to the rocket's structure. Engineers of nuclear rockets surround the reactor with heavy metals, such as lead, in order to contain radiation. Engineers also design extensive cooling systems--usually with circulated water or cold liquid hydrogen--to control the heat. The National Aeronautics and Space Administration (NASA) in the United States is investigating nuclear propulsion. This extremely powerful source of propulsion energy holds much promise for both piloted and unpiloted space exploration within and beyond the solar system. C Electric Rockets Electric rocket engines use batteries, solar power, or some other energy source to accelerate and expel charged particles. These rocket engines have extremely high specific impulses, so they are very efficient, but they produce low thrusts. The thrusts that they produce are sufficient only to accelerate small objects, changing the object's speed by a small amount in the vacuum of space. However, given enough time, these low thrusts can gradually accelerate objects to high speeds. This makes electric propulsion suitable only for travel in space. Because electric rockets are so efficient and produce small thrusts, however, they use very little fuel. Some electric rockets can provide thrust for years, making them ideal for deep-space missions. Satellites or other spacecraft that use electric rockets for propulsion must be first boosted into space by more powerful chemical rockets or launched from a spacecraft. Rocket manufacturers in the United States began experimenting with electric propulsion in the early 1960s. The first electric rocket engines shot a stream of cesium atoms through an electric field that was generated by an onboard battery. The electric field stripped off electrons from the atoms, creating ions--or atoms with a net electric charge. The charge of the ions made them more susceptible to being directed by electric fields. The stream of ions was directed into another electric field that accelerated them to very high speeds, then expelled them from the rocket nozzle. The steady stream of ions exiting the rocket produced the rocket's thrust. Plasma engines, another type of electric rocket engines, use a strong electric current to turn a normal gas into a plasma. Plasma is a state of matter in which many atoms have been ionized, or stripped of at least one of their electrons. This conversion turns the gas into a sea of ions, free electrons, and neutral atoms, with fairly equal numbers of positively charged ions and negatively charged electrons. The most common type of plasma rocket engines uses a cathode, or a positive electric terminal, that extends into a cylindrical chamber. One edge of the chamber is an anode, or negative electrical terminal. Injectors feed a neutral gas into the chamber. A strong electric current is put on the cathode. The current ionizes some of the gas (turning it into plasma) and uses the traveling ions to carry electricity between the cathode and the anode. This ionization sets up an electric field between the cathode and the anode, and a magnetic field around the cathode. These fields act to accelerate the charged particles out of the rocket nozzle. Collisions between the charged and neutral particles make the particles move faster and give the rocket even more thrust. In 1992 Russian and American aerospace engineers began developing electric rockets called Hall thrusters, or Stationary Plasma Thrusters (SPTs). Hall thrusters act much like the plasma thrusters described above, except Hall thrusters have an external magnetic field. The chamber of a Hall thruster is surrounded by a magnet. A cathode extends into the chamber, and an anode forms the outer edge of the chamber. A neutral gas is fed through the back of the chamber. The electric field created by the cathode and the anode turns the gas into plasma, and the electric and magnetic fields accelerate the plasma out of the rocket. Hall rockets are especially useful for keeping satellites in the correct orbit, or station keeping. The electricity for most Hall thrusters comes from solar cells. Such rockets last for years and are much lighter and less expensive than chemical thrusters. Electric rockets work well for station keeping, but the amount of thrust they produce must be greatly increased if these rockets are to be used for primary propulsion systems or for long distance voyages. The photon rocket is another potential means of rocket propulsion. Theoretically, photon rockets move by emitting a beam of light with an exhaust velocity of the speed of light. Photons (packets of light) have no mass, but their speed is so great that they could theoretically produce a tiny amount of thrust. The thrust of a photon rocket would be so small that such rockets would be of use only outside of the gravitational influence of the solar system. V ROCKET FLIGHT Rockets are used for many different applications, but they share some aspects of their flight profiles (the actions and the order of the actions that they perform during flight). All rockets require some structure or method with which they can be launched. They also require a design that provides stability and control in flight. A Launching Controllers can launch a rocket in a variety of ways, depending on the rocket's use. For some applications, such as the military duty of missiles, rockets need to be protected from enemy detection and enemy attack while controllers ready them for launch. In the case of rockets carrying piloted spacecraft, the most important concern is the safety of the people aboard the spacecraft. A1 Missiles The most important consideration in launching missiles is minimizing the opportunity that the enemy will have to attack the missile while it is on the ground. Missiles launched from open ground are usually solid-fueled or storable liquid-fueled rockets, because they require much less preparation time than cryogenic liquid-fueled rockets. Cryogenic liquid-fueled rockets take too long to fuel to be safe in the open. Some missiles are launched from within silos (covered, bombproof underground tubes). Cryogenic liquid-fueled missiles are often stored in and launched from silos. Some rockets that perform as missiles can be launched from airplanes. Air-launched missiles are fired from special racks called pylons underneath the plane. When ready to launch, the missiles fall from the pylons until they are a safe distance from the airplane, then they ignite. This method prevents the missile's hot exhaust from harming the aircraft. Some surface-to-air missiles, such as the Hawk, are carried and launched on mobile launchers. Trident missiles are launched from huge, upright tubes inside a submerged submarine. A blast of gas forces the rocket through the top of the tube and out of the water. When safely clear from the submarine, the missile automatically ignites and heads toward its target. A2 Sounding Rockets Solid-fueled sounding rockets are far simpler to launch than missiles. Sounding rockets are usually light and portable, often requiring only a rail to stabilize the rocket for its first seconds of flight. An early type of sounding rocket called Aerobees used launch towers that were small enough to carry aboard ships. The ships carried the rockets to good positions from which to observe solar eclipses or other phenomena that scientists wished to study. A3 Launch Vehicles Space launch vehicles, especially those used to launch piloted missions, have the most complex launch arrangements, because absolute safety is critical. Engineers prepared early U.S. launch vehicles, such as the Mercury-Redstone rocket, days before launch in huge, movable, bridgelike frame structures called gantry towers. These towers had elevators that enabled workers to make necessary adjustments along the length of the rocket and to fuel it. After fueling and boarding of the astronauts, the crane slowly rolled away. Workers disconnected electrical hookups, or "umbilical cords," just prior to launch, or the rocket's motion broke these connections upon launch. Launches to the Moon of the Saturn V vehicles, which were 111 m (363 ft) tall, used larger, more streamlined mobile launch platforms. Today, the space shuttle uses the fixed Launch Umbilical Tower (LUT), which has elevators and swing arms for servicing the shuttle. The largest land vehicles ever built, the 2,700-metric-ton giant crawler transporters, carried the Saturn V to the launch pad. Giant crawlers still carry the space shuttle to its launch pad. B Stability and Control Rockets need some means of stabilization to help them fly in an even flight path through changes in air pressure, wind, uneven burning of the propellant, or slight irregularities in the balance of the rocket itself. Engineers apply Sir Isaac Newton's first law of motion to control rocket stability. This law states that an object in motion tends to stay in motion. The specific case of this law used in rocket stability is that a spinning body tends to keep spinning in the same orientation. One application of this law is to make the rocket spin. A spinning rocket is resistant to directional changes, making its flight more stable. Spin-stabilized rockets use special fins, or vanes, in the path of their exhaust. The vanes are oriented so that the rocket spins as the hot exhaust passes over the vanes. Many rockets use gyroscopes, instruments that also employ Newton's first law, to track their orientation. Gyroscopes consist of a spinning disk mounted in a base that allows the disk to move freely, but the mounted base moves with the rocket. A power source, such as a battery, keeps the disk spinning. Because spinning objects tend to maintain their orientation, the angle of the disk to the base changes when the rocket and subsequently the gyroscope base change orientation. Automatic systems track the relative positions of the parts of the gyroscope to track changes in the rocket's orientation. The system can make movable exhaust vanes direct the flow of the exhaust to change the rocket's direction based on gyroscope readings. Some rockets can change the orientation of their engines to direct the flow of the rocket exhaust. This technique, called gimballing, can be used with movable exhaust vanes and small thrusters along the rocket edge to provide even better stability and control. Large, long-range solid-fueled rockets use two other methods of steering and stabilizing themselves: thrust vector control (TVC), in which the nozzles can swivel, and jetavators-movable tabs in the exhaust path that can slightly alter the rocket's direction. Today, computers aboard rockets sense the slightest movement off course and instantly correct the direction through links with small motors that adjust the movable vanes on the fins or activate other control methods. For more information about rocket and missile guidance and control, see Guided Missile. VI HISTORY Rockets appear to have originated in China in about the mid-11th century. The first rockets were probably crude gunpowder-filled parchment paper tubes tied to arrow shafts. The arrow shafts gave the rockets stability in flight. By the end of the 13th century, knowledge of the rocket had reached Arabia and soon spread to Europe. People used these first rockets mainly as fireworks and signals. The first firearms appeared at around the same time that rockets reached Europe; the early rockets could not compete as weapons with the power and accuracy of guns. In India, however, rockets became a favorite weapon and remained so for several centuries. A Early Military Rockets In the early 1800s, British inventor William Congreve noted reports of Indian rockets employed against British forces. Congreve greatly improved rockets as weapons by attaching warheads, or bombs that would explode after the rocket was launched, and by increasing the ranges of rockets. These Congreve rockets were accurate and powerful enough to use against the firearms of the early 1800s. The rockets still used poles, or sticks, to stabilize the rockets in flight. Britain used Congreve rockets against the United States in the War of 1812 (1812-1815), and other countries copied the rocket design. Despite their stabilizing poles, Congreve rockets were often inaccurate. In 1844 British inventor William Hale invented the stickless or spin-stabilized rocket, in which the exhaust gases caused the rocket to spin in flight. The spinning helped stabilize the rocket, eliminating the need for the clumsy guidestick and making the rocket more accurate. By the 1890s, the gunpowder war rocket finally fell out of use as guns improved and again became more accurate weapons than rockets. B Developing More Powerful Rockets In the 1880s, Russian teacher Konstantin Tsiolkovsky theorized that rockets might be useful for spaceflight. Although Sir Isaac Newton wrote his third law of motion in the 1680s, few scientists recognized that this law applied to rocket motion. Most scientists still believed that rockets moved because their exhaust gases pushed against air, so rockets could not be used in the vacuum of space. B1 Early Ideas In 1903 Tsiolkovsky began publishing his theories, but his early writings were not circulated outside his native Russia. In World War I (1914-1918), rockets were used only as signals and simple antiballoon weapons. Meanwhile, American physicist Robert H. Goddard evolved his own theories, independently of Tsiolkovsky, about the use of the rocket for spaceflight. Goddard also began experimenting with new solid-fueled rockets. In 1919 the Smithsonian Institution published Goddard's findings in a small booklet called A Method of Reaching Extreme Altitudes. In this booklet, Goddard wrote about his use of smokeless powder as an improvement over gunpowder and how instrumented rockets could help explore the upper atmosphere. He also briefly mentioned the theoretical possibility of an unpiloted solid-fueled rocket reaching the Moon. Goddard's theory was widely published in newspapers and helped make the world conscious of the possibility of rocket-powered spaceflight. Goddard, a shy man, continued his experiments with more secrecy. In 1921 he began experimenting with liquid propellants. On March 16, 1926, Goddard launched the world's first liquid-fueled rocket, though few people knew about it at the time. Goddard's overall impact was therefore less than generally believed. During this same time period in Germany, Rumanian-born mathematics teacher Hermann Oberth independently developed his own theories on spaceflight. In 1923 Oberth published Die Rakete zu den Planetenräumen (The Rocket into Interplanetary Space), which was about liquid-propellant rockets for piloted spaceflight. Die Rakete had an even larger impact than Goddard's booklet and led to an international spaceflight movement, which was especially strong in Germany. In the 1920s and 1930s spaceflight and rocketry clubs sprang up in Europe (especially Germany) and the United States and undertook their own experiments. The most important of these groups was the Verein für Raumschiffahrt (VfR, or Society for Spaceship Travel). The VfR started their experiments in 1930. During the same year, Goddard moved his experimental work away from populated areas to a location near Roswell, New Mexico. He was looking for privacy, safety, and good launching weather. B2 World War II In 1932 the German army hired Wernher von Braun, a bright young member of the VfR, for its own secret rocket program. The program started modestly, but funding increased with the approach of World War II (1939-1945). In 1937 the German Rocket Research Center opened at Peenemünde with von Braun as its technical director. Contrary to a popular misconception, the Germans were unaware of the details of Goddard's work and developed their rockets independently. During World War II, the Germans developed a variety of solid- and liquid-fueled missiles that were more sophisticated than those of the Allies. The most important of these missiles was the A-4, later called the V-2, the world's first large-scale liquid-fueled rocket with a thrust of 250,000 N (56,000 lb) and a range of about 300 km (about 200 mi). At the war's end, both the United States and the Union of Soviet Socialist Republics (USSR) scrambled to capture V-2 parts, plans, and scientists. United States troops brought V-2 material and personnel, including von Braun, back to the United States. C Rockets of the Cold War Shortly after the end of World War II, the USSR and the United States disagreed over the control of Europe and entered a period of tense relations called the Cold War. The Cold War included a race to develop rockets as weapons and as launch vehicles for the space race, a contest for "firsts" in space. The Cold War also gave rise to increasingly advanced missiles, which led to an uneasy balance of power between the two nations for several decades. C1 Stability and Control In order to build safe launch vehicles and accurate missiles, engineers needed to improve rocket stability and control. Robert Goddard used aerodynamic air vanes for his early liquid-fueled rockets. These air vanes helped stabilize and steer rockets by deflecting in desired directions the air through which the rockets moved. Goddard also succeeded with another control--a battery-operated gyroscope within the rocket. The gyroscope was linked to exhaust vanes and straightened the rocket when it began to tilt. The V-2 rocket used a similar method of control. The exhaust gases passed over a set of four heat-resistant, movable, gyro-controlled graphite exhaust vanes. When the rocket swerved, the vanes were moved to deflect the exhaust, forcing the rocket back to a straight path. In 1948 the experimental U.S. MX-774 missile pioneered the technique of gimballing, in which the liquid-fueled rocket engine could be tilted for precise steering and stability in its flight. The following year, the Viking sounding rocket started using small maneuvering thrusters around the vehicle.This method was widely adopted and is often used in conjunction with gimballing. C2 Rockets for Spaceflight In the early 1950s, more than 60 captured V-2 rockets were tested at the U.S. Army's White Sands Proving Grounds in New Mexico. The V-2s gave the Americans valuable experience in handling large rockets, while von Braun's team helped the Americans develop their own missile program. The first American von Braun rocket was the Redstone, developed in 1951. The engine in the Redstone was a great improvement over that in the V-2. The V-2 had a cumbersome arrangement of 16 cupshaped injectors, leading some rocket engineers to dub the V-2 "a plumber's nightmare." The Redstone used an engine that was originally meant for the Navaho missile and had a flat plate into which the injectors were set. The USSR also test-fired captured V-2 rockets and accelerated the Soviet rocket development program. The USSR developed its first intercontinental ballistic missile, the V-2 inspired R-7, around 1954. The world was shocked when, on October 4, 1957, the USSR used a modified version of the R-7 to put Sputnik 1, the world's first artificial satellite, into orbit around Earth. The United States responded by attempting to launch a satellite with the Vanguard rocket, developed in 1957. The Vanguard fell back to its launch pad and exploded after a few seconds of flight. The United States turned to von Braun's Redstone, modifying it and renaming it Jupiter-C, to place America's first satellite, Explorer 1, into orbit on January 31, 1958. In 1961, after more space "firsts" by the USSR, United States president John F. Kennedy declared a national goal to send a man to the Moon and bring him back safely. Von Braun's team started work on the huge Saturn V rocket in 1962. On July 20, 1969, Saturn V allowed two Apollo 11 astronauts to land on the Moon. The Saturn V was a magnificent engineering achievement that successfully placed a dozen men on the Moon. C3 Missiles of the Cold War At the same time that the USSR and the United States were racing to build rockets to get them farther into space, the two countries were constantly striving to build bigger ballistic missiles. Ballistic missiles with the power to travel between the two countries are typically three-stage rockets carrying nuclear warheads. Ballistic missiles are designed to destroy targets in enemy countries, but the sheer number and power of the missiles that both countries had in their possession acted as a deterrent to either country ever launching one. The energy put into the ballistic missile programs did benefit the space program, because many rockets designed for missiles were ultimately used as launch vehicles. The first U.S. intercontinental ballistic missiles (ICBMs), such as the Atlas and the Titan, used liquid propellants. The preparation time, including fueling, of these missiles was long, causing military planners to consider the missiles vulnerable to attack. The next generation Titan II saw improvements in its safe, ordinary temperature, hypergolic (meaning that the oxidizer and fuel ignite on contact) liquid propellant, which cut down the preparation time to a minute. Titan IIs were also kept and launched from underground bombproof structures called silos. Sliding doors in the silo roof opened just prior to launch. The next generation ICBM, the solid-fueled Minuteman, required even less maintenance than its liquid-fueled predecessors, but also launched from silos. During the Cold War, plans were made to carry and launch missiles from specially equipped trains to make detection of the missiles' location more difficult for the enemy. These schemes were never enacted. D Reusable Rockets--The Space Shuttle Rockets such as the large missiles and launch vehicles in the U.S. Atlas or Titan families, first introduced in the 1950s, were expendable. Each rocket could be used only one time, and each was very expensive. The world's first reusable rocket engines were those that propel the space shuttle, which was first flown in 1981. The solid rocket boosters that launch the shuttle into orbit can be retrieved and refurbished but are not really reusable. The reusable engines are actually part of the orbiter (the planelike craft often thought of as the shuttle). The space shuttle's main engine has a built-in electronic controller computer that automatically monitors, regulates, and records all phases of the engine. This computer insures utmost reliability and makes the engine the most sophisticated liquid-fueled rocket engine ever developed. Each of the shuttle's three engines, clustered at the rear of the orbiter, generates about 1.65 million N (about 375,000 lb) of thrust. Today, the U.S. space program relies on the fleet of space shuttle orbiters and a number of expendable launch vehicles. Private companies play an increasingly larger role in the design and use of rockets. The European Space Agency (ESA) has developed several launch vehicles, including the advanced Ariane family of rockets. Despite fiscal problems after the breakup of the USSR, Russia still designs and uses rockets, such as the Proton. Contributed By: Frank H. Winter Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.