Missiles & Rockets: Definition, Types, and Functionality

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Missiles and Rockets

Missiles and rockets are devices designed to travel in air or outer space, propelled by the force of expanding gases.

The word missile is a general term for all such devices, especially those designed as weapons. A rocket is a missile that carries its own supply of oxygen or other oxidizing agent, allowing it to operate beyond the earth's atmosphere. (The term rocket is also applied to the engine propelling such a missile. Rocket engines have also been used to power experimental airplanes.) A rocket used to carry a satellite or other object into space is often called a launch vehicle. A missile that obtains its oxygen from the atmosphere is powered by a jet engine and has no special name.

A true missile is not supported in flight by wings, although it may have winglike fins to guide or stabilize it. Pilotless jet- or rocket-powered craft that have supporting wings are actually airplanes, though they are often called missiles.

A true missile, like a bullet or artillery shell, stays aloft only because of its momentum (movement). It differs from a bullet or shell in that it is self-propelled during at least part of its flight. However, once the engine ceases to operate (which happens in most cases a few minutes after launching), a missile behaves in exactly the same way as an artillery shell. (Some missiles, however, have supplementary engines that can be turned on to make course corrections.)

Missile Engines

General Principles

A missile's engine—either a rocket or jet engine—mixes fuel with an oxidizer (a substance, such as oxygen, in which the fuel can burn). The engine burns the fuel in a chamber, producing hot gases that expand and push against the chamber's walls. Since a jet engine requires oxygen from the air, a jet-powered missile cannot travel above the atmosphere. Jet engines used for missiles are similar to those used on airplanes. A rocket engine can travel above (as well as within) the atmosphere because it carries an oxidizer in addition to the fuel. The propellant (fuel plus oxidizer) may be in solid or liquid form.

The pressure of the gases in any direction tends to move the engine and missile in that direction. The pressure at a point on one side of the engine is balanced by an equal gas pressure on the opposite side. But the pressure on the front is not balanced because of a hole in the rear through which the gases escape. The result is that the engine and missile are pushed forward. This effect illustrates Newton's third law of motion: that for every action (here, the rearward escape of the gases) there is an equal and opposite reaction (the forward thrust).

Rocket Performance

Thrust is the force that the rocket engine generates to move the rocket forward. Thrust is generally stated in pounds—the Saturn V, the largest rocket ever built by the United States, for example, generated 7,500,000 pounds of thrust with the five engines that fired to lift the rocket off the launching pad.

The amount of thrust produced depends on the speed at which gases produced by the burning fuel are ejected from the rocket. This speed, called the exhaust velocity, varies with the burning characteristics of the fuel and with the design of the rocket nozzle. The. most effective design in producing an extremely high exhaust velocity is one in which the nozzle is constricted just below the combustion chamber and then flares out below. The higher the exhaust velocity of a rocket, the greater its thrust.

The ability of a rocket to accelerate itself and its satellite, warhead, or other load depends on its thrust-to-weight ratio; that is, the ratio of the rocket's thrust to the weight of the rocket and its load. The thrust must be greater than the weight if the rocket is to lift off. The higher the thrust-to-weight ratio, the greater the rocket's acceleration. The thrust-to-weight ratio constantly changes during powered flight as the weight of the rocket is reduced through the consumption of fuel. Thus, under normal circumstances, a rocket's acceleration (and hence its speed) continues to increase until all its fuel is consumed. For a rocket being fired upward, the lessening of the earth's gravity also reduces the weight and increases acceleration.

An initial thrust-to-weight ratio of about 5 to 4 was used for manned lunar flights. At the moment of lift-off for the Apollo 11 flight to the moon, for example, the Saturn V rocket developed nearly 5 pounds of thrust for every 4 pounds of weight. The space shuttle is able to develop a thrust-to-weight ratio of about 3 to 2 at lift-off.

Specific impulse is a description of the thrust produced by a particular fuel. It is the number of pounds of thrust produced by each pound of fuel burned during each second of engine firing. Specific impulse is stated in seconds. For example, if 5 pounds of fuel burned in one second of engine operation produces 1,000 pounds of thrust, the fuel has a specific impulse of 200 seconds. Most liquid propellants have specific impulses of about 250 seconds, although liquid hydrogen with liquid fluorine as oxidizer has a specific impulse of about 350 seconds. The specific impulses of solid fuels run somewhat lower, as do those of mono-propellants. (A monopropellant is a single liquid, such as hydrazine [N2H4], that functions as both fuel and oxidizer.)

Solid-fuel Rockets

A solid-fuel rocket consists basically of a cylindrical shell containing a rubbery or plastic substance made up of fuel and oxidizer. When ignited, the fuel burns until it is used up. The same basic structure is used in a wide variety of devices from skyrockets to intercontinental ballistic missiles. Solid-fuel rockets can be fired with a minimum of preparation. They can be safely transported and stored fully fueled and ready to fire.

Whatever its size or shape, a single piece of solid fuel is called a grain, or charge. Most solid-fuel rockets contain a single grain, but a few have more than one. The shape of the grain influences the thrust produced. Some grains are shaped so that the thrust increases as burning time passes; others are designed so that thrust either decreases or remains constant. Further built-in control of the thrust is created by the use of chemical inhibitors, which slow down the rate of burning in specific areas of the grain.

Solid-fuel grains are made opaque to prevent possible explosion. In a translucent grain, electromagnetic radiation produced by the hot exhaust gases penetrates into the grain and produces hot spots—hot enough to ignite the interior of the grain. These hot spots have no vent to release the gases produced and so can cause an explosion.

Among the solid propellants in common use are ammonium nitrate composite, polysulphide-ammonium perchlorate composite, powdered aluminum, and polyurethane ammonium perchlorate composite.

Liquid-fuel Rockets

Liquid-fuel rockets are mechanically much more complex than solid-fuel rockets. This complexity, however, is compensated for by their higher thrust and the fact that they can be turned off and then restarted. A few liquid-fuel rockets, such as Titan II, can be left fueled, but most require that the fuel and oxidizers be kept at extremely low temperatures, and must therefore be fueled just prior to launching. Most of the rockets that have been used as launch vehicles in the space program have been liquid-fueled. The space shuttle uses both liquid-fuel and solid-fuel rockets for launching.

Much of the space inside a liquid-fuel rocket is occupied by fuel tanks and oxidizer tanks. (In rockets designed for monopro-pellants, of course, only one type of tank is required.) The fuel and oxidizer are carried to the combustion chamber through pipelines. Typically, the fuel is carried past the combustion chamber before entering it; this system both cools the chamber and preheats the fuel to improve its burning characteristics.

Most liquid fuels require an ignition system to ignite the fuel and oxidizer as they enter the combustion chamber. Some fuel-oxidizer combinations, however, are hypergolic—that is, they burst into flame spontaneously when they are brought in contact with one another. Hydrogen and fluorine are a hypergolic combination.

Common liquid propellants for United States rockets are kerosene and LOX (liquid oxygen) and liquid hydrogen and LOX.

Stages

The range and speed of a missile can be greatly increased by the use of multistage rockets. These consist of two or more separate rockets, each called a stage, stacked end-to-end. At launching, only the bottom, or first, stage is in operation. When its fuel is used up the first stage drops away from the rest of the rocket and the next stage is ignited. The starting speed of the second stage is equal to the top speed of the first, and the weight of the rocket is greatly reduced.

Additional thrust at lift-off can be obtained by using booster engines in conjunction with the first stage. They are ignited at the same time as the first-stage engine and are jettisoned soon after launching, leaving the main engine in operation by itself. (Somewhat confusingly, the first stage itself, with or without supplementary engines, is sometimes called a booster.) The space shuttle uses a variation of this method—it is not divided into stages, but uses two booster engines with three main engines that operate together.

Military Missiles

Types of Military Missiles

Missiles, like airplanes, can be classified in several ways. The three common methods are (1) by target and launching positions, (2) by guidance, and (3) by range.

By Target and Launching Positions

This classification is based on the place from which a missile is launched and the position of its target.

Air-to-Air

missiles are carried by aircraft and fired at other aircraft.

Surface-to-Air

missiles, or SAM's, are fired from the ground at flying aircraft or missiles. A surface-to-air or air-to-air missile directed at another missile is called an anti-missile missile.

Surface-to-Surface

missiles are launched from the ground or from a ship to a surface target.

Air-to-Surface

rockets are used in place of bombs. Some are fired by aiming the airplane at the target; others can be guided in flight.

Underwater

missiles are of two types. The underwater-to-surface missile is launched from beneath the surface at land or sea surface targets. The U.S. Navy's Poseidon and Trident are examples.

The second type of underwater missile is an antisubmarine missile, launched from a ship or submarine at submarines. One such missile, the U.S. Navy's SUBROC, is launched under water, then flies through the air as a rocket, and finally submerges and acts as a torpedo. The ASROC is a surface-launched antisubmarine missile.

By Guidance

The term guidance refers to the method used in steering a missile to its target. A ballistic missile is aimed during the first few minutes of its flight. After its rocket power is spent it coasts like a bullet in a curved path called a trajectory. The chief advantages of the ballistic missile are its great speed and its resistance to being thrown off course by hostile electronic “jamming.” It can, however, be detected readily by radar and is thus vulnerable to attack by the enemy's defensive weapons.

Guided missiles are designed so their courses can be corrected or altered at any time during the flight to the target. They can be controlled remotely or by a preset internal guidance system. One type of internally guided weapon is called the cruise missile. It is not a true missile but a pilotless airplane with a complex guidance system that allows it to fly close to the earth's surface (to avoid enemy radar) even over hilly terrain.

By Range

A short-range missile is any missile designed to travel less than 600 miles (1,000 km). An intermediate-range ballistic missile, or IRBM, can travel 600 to 1,500 miles (1,000 to 2,400 km). A missile with a range greater than 1,500 miles is an intercontinental ballistic missile (ICBM). Some ICBM's can travel more than 7,000 miles (11,000 km).

Guidance SystemsPre-aimed Missiles

These weapons are generally of the short-range, battlefield type. They can be launched from a tube or rack that is pointed at the target in much the same manner as a rifle or artillery piece. The missile is kept on course by stabilizing fins, and its course cannot be changed in flight. Such a missile does not produce a recoil when fired, so the launching mechanism can be lightweight and simple.

Missiles Guided In Flight

A missile's course can be controlled in several ways. While the missile is within the earth's atmosphere, it can be steered by fins corresponding to an airplane's control surfaces. Another method, which works outside the atmosphere, makes use of small control rockets. These rockets may steer the missile or may cause it to spin like a bullet, keeping its course steady. A third method of guidance is that of regulating the missile's speed by adjusting the rate at which the fuel is consumed.

The various methods of controlling a missile's steering system include the following:

Beam Guidance

A beam of radar signals or a laser beam is aimed at the target. The missile picks up signals from the beam, and electronic devices work the steering mechanisms so that the missile follows the beam to the target. This method is accurate only over short ranges.

Command Guidance

The missile is tracked (followed) with radar from the ground. The target is also tracked. The missile is guided to the target by radio signals broadcast from a ground station. These signals are picked up by a receiver in the missile and operate the controls. Like beam guidance, command guidance is accurate only over short ranges.

Homing Guidance

With this system, the target itself guides the missile. In passive homing, the missile has instruments that can detect radiations (heat, light, or noise) from the target. Information from these instruments is fed into a computer that steers the missile. The disadvantage of this system is that the target can “confuse” the missile by sending out decoy radiations.

Semi-active homing guidance systems do not rely on the target's own radiation, but on radar signals sent out by a ground transmitter. These signals bounce off the target, creating radiation to guide the missile. Missiles with active homing devices send out their own radar signals and are thus independent of ground controls. The disadvantage of these methods is that the enemy may be able to detect the signals and take countermeasures.

Preset Guidance

The missile's control system is set before launching to carry it to a specific target. The target's exact location must be known. If the missile swerves off course, an automatic pilot will make the necessary-correction.

The earliest and simplest method of preset guidance uses a gyroscope system similar to that used on some torpedoes. More advanced systems check the missile's location by using signals from navigation satellites forming the global positioning system.

Another system is called inertial guidance. Once set, it is entirely automatic and does not need outside sources for navigational check points. A typical inertial guidance system contains a small on-board computer, gyroscopes, and devices called accelerometers. An accelerometer consists, essentially, of a weight held between two opposing springs. The weight's inertia causes it to push against the back spring when the missile undergoes acceleration and to push against the forward spring during deceleration. The use of three accelerometers, each perpendicular to the other two, provides a measurement of the missile's acceleration in any direction. From this measurement the computer determines the missile's speed. This information, together with information provided by the gyroscopes concerning the direction of the missile's motion, allows the computer to continually calculate the missile's course.

Television Guidance

A small television camera mounted in the missile's nose is aimed at the target prior to launching. The television picture is observed by the launch operator, who locks the camera onto the target electronically just before launching. After launching, the missile is guided by signals from the camera, which continues to point directly at the target. This system is used mainly over short ranges for air-to-surface missiles.

Terrain-matching

is another complex method of preset guidance. Along the missile's course, ground-scanning radar in the missile's guidance system provides ground-elevation information to a small onboard computer. By comparing data from the radar with map data stored in the computer's memory, the computer can accurately determine the missile's location and can order the automatic pilot to make course corrections if necessary.

Wire Guidance

The missile is linked to its launcher by a thin wire that unwinds from the missile during flight. Tracking is visual, and guidance commands are sent over the wire to direct the missile to its target. Wire-guided missiles are used primarily as battlefield weapons for very short-range attacks against enemy armored vehicles.

ABM System

An anti-ballistic-missile (or ABM) system is designed as a defense against offensive ICBM's. As a result of arms-limitation agreements, the United States and the Soviet Union were each limited to only one ABM site. The United States established its site in North Dakota to protect ICBM's located there. The ABM system installed there consisted of two types of surface-to-air missiles, the Sprint and the Spartan, which were equipped with nuclear warheads; two types of radar, PAR (Perimeter Acquisition Radar) and MSR (Missile Site Radar); computers; and support communications and electronic equipment.

The PAR was designed to detect and track attacking missiles as they came over the horizon, still in space. Spartan missiles would be sent up to intercept the incoming missiles above the atmosphere. The MSR was designed to cope with many targets all at once at close range. The Sprint missiles would intercept in the atmosphere any missiles that passed through the Spartan screen.

The ABM system installed in North Dakota was deactivated in the mid-1970's, primarily because Congress considered its operating costs too high in relation to its potential effectiveness. In 1983, President Ronald Reagan initiated a large-scale research program—the Strategic Defense Initiative (SDI), popularly known as Star Wars—to develop a largely space-based system to destroy enemy ballistic missiles in flight. With the collapse of the Soviet Union in the early 1990's, the SDI program was scaled down. In 1993, it was renamed Ballistic Missile Defense, and work on space-based antimissile weapons was stopped in favor of the development of ground-based weapons. By 2002, the administration of George W. Bush had placed a renewed emphasis on missile defense and the testing of antiballistic interceptor missiles.

When equipped with a warhead (an explosive charge or nuclear bomb), a missile is a devastating weapon. A large missile travels at an enormous speed (more than 15,000 mph [24,000 kmh] is not uncommon), making it difficult to detect and destroy. Some missiles are designed to travel halfway around the world in less than 30 minutes. Small missiles, such as the bazooka rocket developed in World War II, can give a foot soldier the firepower of a tank. Some missiles can change course in flight and seek out and destroy moving targets.

Some military missiles have more than one warhead, each capable of striking a different target. The multiple warhead system is called MIRV (for multiple independently targeted reentry vehicle).

Nonmilitary Rockets

Types of FlightsSuborbital Flights

In a suborbital flight the rocket rises to a given altitude and then returns to earth. Two purposes of such flights are (1) testing and (2) research in the immediate vicinity of the earth. Test flights are made to check the equipment and techniques used on long flights. The rockets used for suborbital research flights are called sounding rockets. Since relatively small and inexpensive rockets may be used, suborbital flights are much less expensive than orbital ones.

Earth Orbital Flights

An earth orbital flight is one in which a satellite is put into orbit around the earth. Usually multistage rockets are used.

Beyond the Earth

Flights beyond earth orbit vary greatly in character. Unmanned probes have been sent to the moon and to most of the planets of the solar system. A vehicle designed to put people or instruments on another body uses a rocket engine upon its arrival to achieve a “soft” (low-speed) landing. The rocket blast is directed toward the landing site. The amount of thrust is such that, instead of rising, the vehicle slowly settles down to the surface. For a manned landing, provision must be made for launching the vehicle from the landing site.

The first step in any of these flights is to escape the earth's gravitation. The mass that a rocket can carry to escape velocity is always much less than it can place into orbit. For example, the Atlas-Agena rocket could send only 1,100 pounds (500 kilograms) to the moon, but could place 6,900 pounds (3,130 kilograms) into orbit around the earth.

Rocket Testing and DevelopmentTesting

The launching of a rocket is an expensive and sometimes dangerous process. It is necessary to determine as nearly as possible how well each rocket will perform before it is launched. Most rockets, therefore, are static tested. In such testing the rocket is bolted down so it cannot move, and the engines are started. Instruments attached to the rocket indicate its performance. Since the gases escaping from the engine are highly destructive, special static-test stands are built to deflect the gases away from structures and equipment.

Development

Rocket development is centered on two main problems—the need for more power and the need for sustained power. The problem of more power must be solved before there is any possibility of long manned flights into interplanetary space. The problem is being met by the development of more powerful rocket engines and by clustering a number of powerful engines together. With the cluster system there is no main engine; the clustered engines together form the propulsion unit. In this respect clustered engines differ from booster engines, which supplement a main engine.

The problems of sustained power, needed for maneuvering in space, require the development of new types of engines that do not depend on large quantities of chemical fuel. Nuclear, electrostatic (ion), and electromagnetic (plasma) propulsion are possibilities. An ion-engine rocket was first tested successfully in 1964.

In addition to military uses, rockets and missiles have an important part in scientific research and the exploration of space. They are also used to launch commercial satellites into orbit. The rockets used for nonmilitary purposes may be single- or multistage rockets. The cargo, or payload, may be a small package of instruments, one or more satellites, or a spacecraft.

History of Missiles and Rockets

Early Rockets

Simple rockets, consisting of paper shells filled with explosive powder, were used in warfare by the Chinese as early as 1232 A.D. Within a short time rockets were being used in Europe to set fire to fortifications and buildings and to frighten the enemy. However, guns proved more accurate and destructive and soon replaced rockets.

Rockets reappeared in warfare in the 18th century. They were refined and developed by William Congreve, an English artillery officer. “The rockets' red glare” seen by Francis Scott Key during the attack on Fort McHenry in 1814 came from Congreve rockets.

In 1838 John Dennet, an English inventor, patented a rocket that would carry rescue line to ships in distress.

In 1846, William Hale, an American devised a rocket stabilized by fins. Hale's rockets, which were solid-fueled, were used in the Mexican War. But once again developments in conventional artillery caused rockets to be outmoded. Until World War II they had little use in warfare except for signaling.

Liquid-fuel rockets were first seriously proposed in the late 19th and early 20th centuries by a Russian writer and teacher, Konstantin E. Tsiolkovsky. He worked out theories for the operation of liquid-fuel rockets and suggested that they could be used for space flight. In 1926, Robert H. Goddard, an American physicist, flew the first liquid-fuel rocket, a device about 4 feet (1.2 m) long and 6 inches (15 cm) in diameter. However, Goddard's work, like that of Tsiolkovsky before him, remained nearly unknown for many years.

In the late 1920's and the 1930's, a German rocket society, which included at various times Hermann Oberth, Fritz von Opel, and Wernher Von Braun, was active in developing liquid-fuel rockets capable of rising one mile (1.6 km) into the air.

World War II

Rockets came back into military use during World War II. They were fired from airplanes at other planes or at ground targets. The bazooka was widely used by United States infantry forces. As artillery, a barrage of rockets proved very effective. Trucks carried banks of rockets that could be fired one at a time or all at once. Rocket barrages were fired from ships to help “soften up” an enemy beach before assault. Large demolition rockets were launched from tanks against concrete barriers and tank traps. Small rockets were developed to help heavily loaded airplanes take off.

German scientists developed long-range bombardment missiles late in World War II. The first, the V-1 or “buzz bomb,” was a jet-propelled pilotless airplane. The second, the V-2, was a true rocket. It was aimed before launching and had a range of nearly 200 miles (320 km). Although inaccurate, it proved the deadly effectiveness of long-range, high-speed missiles.

After World War II

After the war the Western Allies and the Soviets continued and expanded development work, using captured German scientists as well as their own personnel. Early experiments in the United States were conducted at White Sands Missile Range, New Mexico. A program to fire captured V-2 rockets and study them was begun in 1945. In 1949, a multi-stage rocket, using a V-2 as the first stage, was the first to reach space, achieving an altitude of 250 miles (400 km).

The Soviet Union concentrated on powerful boosters needed to carry early thermonuclear devices, which were heavy and bulky. The United States worked on smaller boosters, assuming (correctly) that the size and weight of warheads would be reduced by the time the missiles were ready. The Soviet Union launched the first satellite—Sputnik I—in 1957, several months before the first United States satellite went into orbit. The Soviets were able to launch heavier satellites than the United States in the first years of the space age.

By early 1959, United States politicians were talking about the “missile gap,” the lag in the United States' development of ICBM's compared to the Soviet Union's. Later the same year, however, the first United States ICBM, the Atlas, became operational. In 1960, the first operational Polaris missiles were placed aboard the nuclear submarine George Washington. The Polaris, a solid-fuel ballistic missile designed to be launched underwater, was later largely superseded by the multiple-warhead Poseidon missile. In 1962, the Minuteman, a solid-fuel ICBM designed to be launched from an underground silo, became a part of the United States arsenal.

The age of manned space flight began in April, 1961, when a Soviet cosmonaut, Major Yuri Gagarin, orbited the earth once. In February, 1962, a United States astronaut, John H. Glenn, Jr., made an orbital flight; the launch vehicle was an adapted Atlas missile.

Several launch vehicles were developed for Project Apollo. The Saturn I, used during the testing and first manned flights of the Apollo spacecraft, was a forerunner of the giant Saturn V, the rocket that in July, 1969, launched Apollo 11 on its successful mission to land the first men on the moon.

In the 1970's, the United States began developing the space shuttle—a rocket-powered vehicle capable of lifting off, going into orbit around the earth, and then returning to earth to land like an airplane. The first orbital flight by a space shuttle was made in 1981. The United States built a fleet of four operational space shuttles with the intention of replacing conventional launch vehicles. However, following the explosion of the space shuttle Challenger in 1986, the remaining space shuttles were grounded for more than two years, and the United States began again to use conventional launch vehicles for some satellites. A new space shuttle, the Endeavour, was completed in 1991.

During the 1960's and 1970's, the use of increasingly sophisticated missiles and rockets revolutionized conventional warfare. Foot soldiers were armed with rockets that could destroy the heaviest tank. Missiles replaced artillery as the principal antiaircraft weapon and made obsolete the use of heavy guns on naval vessels. Small boats armed with missiles had the capability of sinking the most powerful warship.

In 1972 the United States and the Soviet Union signed the first Strategic Arms Limitation Treaty (SALT I), which bound each nation to place limits on the size and strength of its arsenal of strategic weapons, especially nuclear ballistic missiles.

During the 1980's the European Space Agency, an organization sponsored by 11 European countries, developed a commercial satellite-launching program for its Ariane rockets. In the 1990's, both ESA and NASA developed more powerful rockets to improve their launching capabilities. NASA continued to use the space shuttle for launching satellites, and in 1998 it began to use the space shuttle for carrying into space many of the materials, crews, and supplies needed to build and operate the International Space Station.

In 2003 the Columbia space shuttle broke apart while returning to earth, and all seven astronauts on board died. The space shuttle was grounded for more than two years. The next space shuttle mission was made by space shuttle Discovery in 2005.