A rocket, in its conventional form, is an internal combustion engine that needs no outside air to operate. It carries both fuel and oxidizer, which are burned together in a combustion chamber and produce hot gases that are discharged through a nozzle. Inside the combustion chamber the burning gases exert pressure in all directions. If the chamber were sealed, all these pressures would be balanced and the rocket would not move. The gases are allowed to escape at high speed through the nozzle, however, causing an imbalance in the chamber. Because the pressure exerted on the rocket in the forward direction is much greater than in the backward direction, the rocket shoots forward. It obeys Newton's third law of motion: for every action there is an equal and opposite reaction. In the rocket the escaping exhaust gases are the action, and the forward pressure, or thrust, is the reaction. Because a rocket carries its own fuel and oxidizer, and because Newton's law is valid everywhere, the rocket can operate both within the Earth's atmosphere and in the vacuum of space. Rockets can thus be used to launch artificial satellites (see \Tsatellite, artificial\t), probes, or manned spacecraft for \Tspace exploration\t, or to power a variety of short- or long-range missiles for military purposes. EARLY HISTORY The first rockets were probably made in China. When Mongol hordes besieged the town of Kai-feng-fu in AD 1232, the townsfolk repulsed them with "arrows of flying fire." Tied to the shafts were tubes containing an early form of gunpowder, which produced a fiery exhaust. That these rockets were not purely incendiary devices is implied in an ancient manuscript that describes them as "making a noise like thunder" and "travelling a great distance." In the Middle East, and Europe, the art of rocketry appeared soon afterward. In 1242 the English Franciscan monk Roger Bacon produced a secret formula for gunpowder, specifying 41.2 parts saltpeter, 29.4 parts charcoal, and 29.4 parts sulfur. He also succeeded in distilling saltpeter--an oxygen-producing ingredient--to achieve the faster rates of burning that would make rockets more practicable. In The Book of Fighting on Horseback and With War Engines, composed about 1280, the Syrian scholar al-Hassan-al-Rammah gave instructions for making gunpowder and rockets, which he called "Chinese arrows." Rockets are also mentioned in a German chronicle of 1258, and the Italian historian Muraroti describes how, during the siege of Chiozzia (near Venice) in 1379, a defending tower was set ablaze by a black powder rocket, thereby eliminating the last pocket of resistance. The British first encountered rocket warfare in India, to which the secret of rocket manufacture had probably been brought during the 17th century by Arab traders. Manuscripts suggest that the 18th-century Indian ruler \THyder Ali\t employed thousands of men for throwing rockets during warfare. Constructed of a thick stalk of bamboo 2.5 to 3 m (8 to 10 ft) long attached to a tube of iron weighing from 2.5 to 5.5 kg (6 to 12 lb) containing the fuse and powder, these rockets were said to be able to reach a distance of 2.5 km (1.5 mi). When the first examples of the Indian rockets reached England around 1770, Captain Thomas Desaguliers examined their structure at the Royal Laboratory at Woolwich, but he failed to reproduce their range or accuracy. The matter rested there until 1804, when William Congreve took up the challenge and asked Woolwich to have several large rockets made to his specifications. Because a rocket has no recoil, Congreve thought that they might find application both at sea and on land. Within a year he had produced a 24-pounder with a range of about 1,800 m (6,000 ft). Congreve rockets were first used in battle during the Napoleonic Wars on the night of Oct. 8, 1806, when 18 rocket boats were quietly slipped from mother ships and rowed into Boulogne harbor (France) to enable officers and men of the Royal Marine Artillery to attack the French invasion fleet. The rockets--32-pounders measuring 1.06 m (3.5 ft) long by 10 cm (4 in) in diameter--had balancing sticks 4.5 m (15 ft) long and a range of about 2,700 m (9,000 ft). Some of the warheads contained ball-shot to create a shrapnel effect; others had a liquid incendiary compound that squeezed out a flame from holes in the pointed nose cap and impaled wooden ships and buildings. The year 1810 saw the publication in London of W. Moore's treatise On the Motion of Rockets, which contained a mathematical study of rocket motors and trajectories. By 1844, William Hale, an Englishman, had invented spin-stabilized rockets, which eliminated the cumbersome guide sticks. These rockets were set into rotation by deflecting the exhaust through offset nozzles drilled in the baseplate, and, later, by restricting the expanding exhaust gases on one side of the nozzles by the use of semicircular vanes. DEVELOPMENT OF MODERN ROCKETRY Rocketry had to wait half a century before further big advances were made, and these were of a theoretical nature. Tsiolkovsky A Russian schoolteacher, Konstantin E. \TTsiolkovsky\t, established in 1883 that a rocket would work in the vacuum of space; in 1903 he published his first treatise on space travel, advocating the use of liquid propellants. His notebooks of the first quarter of the 20th century contain sketches of spaceships fueled with liquid oxygen and liquid hydrogen, or liquid oxygen and kerosene. The sketches showed valves for controlling the flow of liquids to the combustion chamber and vanes in the exhaust for steering. The occupants were depicted in a supine position to withstand the full force of acceleration, and the pressurized cabin had double-wall protection against meteoroids. Tsiolkovsky also advocated step rockets--rocket stages arranged in tandem that dropped off as soon as the next rocket fired, thus reducing deadweight. In this way, he suggested, the high speeds necessary to place satellites in orbit could be attained. He advocated the use of gyroscopes and stabilizing rockets and even went so far as to describe spinning habitats in which people could live under artificial gravity, obtaining food and oxygen from closed-cycle biological systems. Goddard A long road still had to be traveled before rockets could be applied so ambitiously. Robert H. \TGoddard\t, the founder of U.S. rocketry, had invented a bazooka-type solid-fuel rocket during World War I and in 1919 published A Method of Reaching Extreme Altitudes. Two years later he began the experiments with liquid fuels that Tsiolkovsky had never attempted. On Mar. 16, 1926, at Auburn, Mass., Goddard became the first to launch a liquid-propellant rocket. Fueled by gasoline and liquid oxygen, it rose to a height of 12.5 m (41 ft), reached a top speed of 100 km/h (60 mph), and landed 56 m (184 ft) from the launch stand. Although few people recognized it as such at the time, it was a turning point in history. Later, at his lonely test station at Roswell, N.M., Goddard succeeded in launching (May 1935) a rocket with gyro-controlled exhaust vanes to an altitude of 2,300 m (7,500 ft) at a top speed of about 1,100 km/h (700 mph). Germany Meanwhile, in Germany, Hermann \TOberth\t had published, in 1923, Die Rakete zu den Planetenraumen (The Rocket into Planetary Space). Like Goddard, he favored liquid fuels because they were more energetic and controllable. Inspired by Oberth's enthusiasm, German rocket enthusiasts in July 1927 founded the Verein fur Raumschiffahrt (VfR, or Society for Space Travel). Its members included Willy Ley, Johannes Winkler, Hermann Oberth, Max Valier, Walter Hohmann, Guido von Pirquet, Klaus Riedel, Kurt Heinish, and Rudolf Nebel, all of whom passionately desired to improve the performance of the often-erratic liquid-fueled rockets. On Feb. 21, 1931, Winkler became the second person to launch a liquid-fuel rocket. Powered by liquid methane and liquid oxygen, the rocket made a disappointingly brief hop of just 3 m (9 ft); three weeks later, however, it ascended to more than 600 m (2,000 ft). The entire VfR built a series of small test models known as Mirak and Repulsor. A young enthusiast who joined the group at this time was the 18-year-old Wernher \Tvon Braun\t. The group tested its rockets at the Raketenflugplatz (Rocket Flying Field), in a suburb of Berlin, and soon came into conflict with the city authorities. Late-model Repulsors, although not fueled to capacity, could reach altitudes of 1.6 km (1 mi); they did not, however, always fly true. The economic depression of the early 1930s took its toll; money ran low, and it became obvious that further useful work would be impossible without additional funds. Nebel, Riedel, and von Braun therefore approached the German army and demonstrated one of their Repulsors at the army proving grounds at Kummersdorf, about 100 km (60 mi) south of Berlin. The artillery experts were interested but unconvinced. They wanted measured thrusts and other data. Von Braun gathered what data he could find and returned to Kummersdorf with results that were to change the course of history. The army realized that the rocket fell outside the scope of the Versailles Treaty--which barred the Germans from building aircraft--and in 1933, the same year in which Adolf Hitler came to power, a special section of the Army Weapons Department was established at Kummersdorf with Captain (later General) Walter Dornberger at its head. The young von Braun was placed in charge of rocket development while he was still studying for his doctoral degree, and work began on a series of experimental liquid-fuel rockets. Within a few years highly improved rockets were being fired in secret by von Braun's small team at the North Sea island of Borkum, near Emden. In December 1934 two A-2 rockets, called Max and Moritz, ascended approximately 2.5 km (1.5 mi). The VfR, meanwhile, finally disbanded because of financial problems, and the Raketenflugplatz returned to its original use as an army ammunition depot. In April 1937 a major rocket research station was completed near the village of \TPeenemunde\t on the Baltic coast. Former VfR stalwarts Klaus Riedel, Hans Hueter, Kurt Heinish, and Helmut Zoike were now able to resume their work in rocketry alongside von Braun. At Peenemunde the large A-4 rocket was developed as an artillery weapon. This rocket was later used to bombard London, Antwerp, and other targets in 1944-45. The German High Command called it the V-2 (for Weapon of Vengeance No. 2; see V-2). The first such weapon, the V-1 (see V-1), had been the Fieseler Fi 103 flying bomb, a small, pilotless aircraft powered by a simple pulse-jet engine, and it was also directed against London and southeastern England. After the war the V-2 was used as a \Tsounding rocket\t for upper-atmosphere research; other sounding rockets such as Viking and \TAerobee\t were built using the same technology. \TUSSR\t The Soviets, too, had not been idle. On Aug. 17, 1933, a group of Soviet researchers led by Mikhail K. Tikhonravov launched the GIRD 09 rocket, which flew to a height of about 400 m (1,310 ft). The 09 was fueled with liquid oxygen and a mixture of gasoline and colophony, a dark-colored rosin obtained from turpentine. Its builders included a young man, Sergei P. \TKorolev\t, who many years later would develop the rocket that launched Sputnik 1, the world's first artificial satellite. A more conventional Soviet rocket, the GIRD X, designed by Friedrich A. Tsander, was powered by gasoline and liquid oxygen. On Nov. 25, 1933, it soared nearly 4,900 m (16,000 ft). The Soviets had also test-flown a rocket-propelled flying bomb (Project 212) in 1939, but it never went into production. Designed by Korolev, it had a small nitric acid-kerosene engine designed by Valentin Glushko. Glushko also developed a series of pump-fed nitric acid-kerosene engines for the auxiliary propulsion of Soviet military aircraft. MILITARY MISSILES From these beginnings grew the immense challenge of the postwar era; as tension developed between the \TUSSR\t and the Western Allies, the demand for weapons of even greater power increased. In little more than a decade missiles that at best could carry chemical explosives a few hundred kilometers were superseded by multistage ballistic missiles capable of lobbing thermonuclear warheads into the heart of another continent. These gave rise to the \Tantiballistic missile\t and also focused attention on the merits of short-range tactical missiles. Ballistic missiles (the name is derived from the ballistic path they follow after the initial propulsion phase) usually make use of the multistage principle, a major breakthrough in rocket technology. The multistage rocket consists of two or more rockets mounted parallel or in tandem. The first rocket carries the upper stages to a certain altitude and separates when its propellants are exhausted. The next stage fires and lifts the remaining stage or stages still higher. The final speed attained by the last stage is equal to the sum of the net changes in velocity accomplished by each stage. In this way unnecessary structure is discarded as soon as it is no longer accelerating the vehicle, and the payload can attain the high speeds necessary to reach a distant target. Any long- or short-range missile equipped with a \Tguidance and control system\t is called a guided missile. To achieve accuracy ballistic missiles usually depend on inertial guidance systems using precision \Lgyroscope\ls and other devices that sense deviations from a preset flight path and restore the vehicle to that path, usually by swiveling the engines to produce offset thrust. Because this type of guidance system does not give out a signal, it cannot be jammed by radio waves. Once engine thrust is terminated by the guidance system, the vehicle follows a ballistic path like an artillery shell, usually without further power or guidance. The guidance systems of tactical missiles may be either external or internal. External guidance, which is effective only over short ranges, makes use of a homing system or of a light beam or radar beam, aimed at the target, that the missile follows. Internal guidance, however, does not permit control of the trajectory after the missile has been launched. Such a guidance system may consist, for example, of an electronic memory storing a preprogrammed course. Ballistic missiles that are launched from land include the intercontinental ballistic missile (\TICBM\t), with a range exceeding 8,000 km (5,000 mi); the intermediate-range ballistic missile (IRBM), with a range between 2,500 and 8,000 km (1,500 and 5,000 mi); the medium-range ballistic missile (MRBM), with a range between 800 and 2,500 km (500 and 1,500 mi); and the short-range ballistic missile (SRBM), with a range up to 800 km (500 mi). \TICBM\t development was spurred in 1954 by the development of the hydrogen bomb and by reductions in the dimensions of atomic bombs, which made possible the design of warheads that could be carried by missiles. The first ICBMs successfully launched in the United States were the \TAtlas\t (1958), the \TTitan\t (1959), and the \TMinuteman\t I (1961). These were followed by Minuteman II (1965) and Minuteman III (1970), which together with the Titan make up the land-based, strategic nuclear force of the United States. Development of the \TMX missile\t, a 10-warhead \TICBM\t, began in 1979. Original plans called for a mobile basing system whereby the missiles would be shuttled around underground, concealing their location. Deemed too costly, the basing mode was replaced by the "dense pack" configuration in which all the missiles would be clustered together in underground silos. After congressional rejection of the dense-pack plan, the bipartisan presidential Commission on Strategic Forces recommended in 1983 that 100 MX missiles be deployed in newly hardened existing Minuteman silos. The commission also recommended the further study of a new, smaller, single warhead missile, the so-called \TMidgetman\t.. The United States began to develop the submarine-launched ballistic missile (SLBM) in 1954, the same year that the first nuclear submarine, the Nautilus, was launched. Under the Fleet Ballistic Missile (FBM) Program, 41 submarines had been built by 1968, each armed with 16 \TPolaris\t missiles. The latest version is the Polaris A3, which has a range of 4,600 km (2,800 mi). The \TPoseidon\t SLBM, the development of which was begun in 1964, is twice as heavy as the Polaris A3, has twice its payload capacity, and is armed with ten 50-kiloton bombs. The approaching end of the Poseidon development program and the strategic demand for an SLBM with a much longer range led to the initiation of the \TTrident\t program in 1975. A Soviet submarine, armed with 16 missiles comparable in range to the Polaris A1, became operational only in 1966. An effort comparable to the FBM Program began in the \TUSSR\t in 1970. The military significance of the SLBMs lies in their near-invulnerability when the submarines carrying them are submerged (see \Tnuclear strategy\t). A system for the use of multiple warheads on a single missile first became operational in 1964. At the end of 1967 the multiple independently targeted reentry vehicle (\TMIRV\t) concept was originated, whereby multiple warheads are independently targeted near the end of the ballistic missile flight (see \TMIRV missile\t). A further refinement was the installation of a homing system in the \TMIRV\t warhead for navigation and target recognition. The \TMIRV\t has the advantage of increasing the possibility of penetrating an antiballistic missile system and therefore is more lethal. A disadvantage is the reduced possibility of \Tarms control\t, because spy satellites can verify the number of strategic weapons but not the number and type of bombs with which they are armed. The U.S. Minuteman III and Poseidon missiles are equipped with \TMIRV\t warheads; the capability of the \TUSSR\t's missiles is unknown. Enhancing the advantages and exacerbating the problems of MIRVs is the development of the maneuvering reentry vehicle (see \TMARV\t), devised to deliver multiple nuclear warheads with pinpoint accuracy. Using radar scanning devices and computerized guidance systems, MARVs are designed for accuracy to within 37 m (120 ft) after a 1,609-km (1,000-mi) flight. Tactical missiles are intended for battlefield use rather than against distant targets. They include surface-to-surface missiles, surface-to-air missiles (SAMs), air-to-surface missiles, and air-to-air missiles. Surface-to-surface missiles were first used on a large scale in World War II and were still being developed during the 1960s. The best-known types are the American Honest John, which has a range of more than 35 km (22 mi) and a conventional or nuclear warhead, and its Soviet counterpart, known by the NATO code name Frog. A few guided missiles with ranges comparable to those of the V-1 (220 km/137 mi) and the V-2 (3,200 km/2,000 mi) have been developed. Examples are the American Lance (48 km/30 mi range), which superseded the Honest John; the smaller Little John (16 km/10 mi range); the Sergeant (150 km/93 mi range); and the Pershing (200 to 800 km/125 to 500 mi range). These missiles are intended for battlefield support, especially as antitank weapons. \Lsurface-to-air missile\ls, used primarily against enemy aircraft and rockets, had been developed during World War II by both the Allies and Germany. After the war the Americans developed the Nike Ajax and Nike Hercules; the Talos, Tartar, and Terrier; the Hawk; and the Redeye, which can be fired from the shoulder. The \TUSSR\t has also developed a large number of SAMs. The range of SAMs is usually no more than 50 km (30 mi). Some can intercept targets at a distance of 100 km (62 mi); others, especially shoulder-fired missiles, have a considerably shorter range. Air-to-surface rockets are fired by aircraft against ground targets and were used during World War II by many nations. The Germans were the first to use successfully rockets fired from aircraft against other planes, notably with the Messerschmitt fighter. Many such air-to-air missiles were developed after World War II, some equipped with radar or infrared guidance systems. These missiles can generally be used against both air and surface targets. The American \TSidewinder\t, Sparrow Super Falcon, and Phoenix, and the Soviet Ash and Atoll, are important examples. All are guided missiles, with ranges of up to 20 km (12 mi). The \Tcruise missile\t is more accurately classified as an unmanned airplane rather than a rocket, because it is propelled by a jet engine and travels over a flat, nonballistic path. Early cruise missiles developed by the United States during the 1950s were superseded by faster and less-vulnerable ballistic missiles. The development of more efficient engines and better guidance systems, however, has recently made possible the development of medium- to long-range cruise missiles capable of carrying conventional or nuclear warheads. SPACE ROCKETS Many of the rockets that opened the space age were straightforward adaptations of ballistic missiles to which upper stages were added to give the higher speeds necessary to achieve Earth orbit or to project payloads on lunar and interplanetary missions. The first Sputniks, as well as the Soviet manned spacecraft, were launched by rockets adapted from a Soviet \TICBM\t with the NATO code name Sapwood. Even the powerful Soviet \TProton\t rocket, first introduced in 1965 and ultimately used to launch the Salyut space station, embodied a good deal of military technology. In the United States the \TJuno\t I rocket that launched Explorer 1 was developed from the army's \TJupiter\t C rocket, which in turn was a successor of the \TRedstone\t. The \TThor\t IRBM was used in combination with Able, \TAgena\t, and \TDelta\t stages. A modified Atlas \TICBM\t was used to launch the four manned Mercury orbital flights and was later combined with Agena and \TCentaur\t stages for heavier payloads. The Titan \TICBM\t was developed for use in the manned Gemini program and a variety of other missions. In addition, the National Aeronautics and Space Administration (\TNASA\t) developed or contracted for new launch vehicles, including the Delta, \TScout\t, and \TSaturn\t rockets. Delta, the first rocket to be built specifically for \TNASA\t, was developed by the Douglas Aircraft Company and grew into a large family of vehicles that has provided launch reliability since 1960. Scout, the first rocket developed by \TNASA\t itself, also became operational in 1960 and was a low-cost means of launching lighter payloads. The Saturn I rocket, conceived by Wernher von Braun in 1958, was first launched with all stages operational in January 1964. After a number of interim improvements in the Saturn I, the Saturn V rocket was developed. Test-launched in 1967 and 1968, it performed flawlessly in the Apollo program. With the growth of space exploration and its commercial applications, a number of countries have developed their own launch capability. Among other rockets developed for the space age are France's \TDiamant\t, the British Black Arrow, and the European Space Agency's \TAriane\t. After 20 years these expendable rockets are at last giving way to reusable vehicles such as the U.S. \TSpace Shuttle\t, which takes off vertically like a rocket, carries crew and cargo into Earth orbit, and returns to land on a runway like an airplane. The shuttle lifts off with its own main engines firing in conjunction with two large solid-propellant rocket boosters, which jettison at a height of about 43 km (27 mi) and are recovered by parachute. A large propellant tank, jettisoned over the ocean just before the craft arrives in orbit, burns up as it reenters the atmosphere. Reusable vehicles could eventually cut the cost of space launching by 50 to 70 percent. Kenneth Gatland PRINCIPLES OF ROCKET PROPULSION A rocket propulsion system produces a force, known as the thrust, that acts (according to Newton's third law) in the direction exactly opposite to the flow of ejected propellants. According to Newton's second law, this thrust force is equal to the rate of change of momentum of the ejected matter, which depends on both the rate at which the propellants are burned in the engine and the effective exhaust velocity at which the resulting gases are expelled. The effective exhaust velocity for chemical propellants is usually between 1,500 and 4,500 m/sec (4,900 and 14,700 ft/sec). Its value increases with the square root of the combustion temperature (hotter flames are better) and inversely as the square root of the average molecular weight of the exhaust gas (low-molecular-weight elements such as hydrogen are best). The Saturn space-launch vehicle has a takeoff thrust of about 3.45 million kg (7.6 million lb) and consumes about 12,700 kg (28,600 lb) of propellant per second ax an effective exhaust velocity of 2,600 m/sec (8,600 ft/sec) for about 2 1/2 minutes. In contrast, a small attitude-control rocket engine may produce only 0.045 kg (0.1 lb) of thrust with a very small propellant flow for short, pulsed durations of 0.02 to 0.2 seconds. The impulse, or total impulse, of a rocket is the product of thrust and the effective firing duration. A typical shoulder-launched short-range rocket may have an average thrust of 300 kg (660 lb) for an effective duration of 0.2 seconds, giving a total impulse of 60 kg-sec (132 lb-sec). In contrast, the Saturn rocket has a total impulse of 510 million kg-sec (1,140 million lb-sec). Specific impulse is the amount of thrust derived from each pound of propellant in one second of engine operation. It is equal to the exhaust velocity (in ft/sec) divided by the acceleration of gravity (32.2 ft/sq sec). Specific impulse is the common measure of propellant and propulsion-system performance, and is somewhat analogous to the reciprocal of the specific fuel consumption used with conventional automobile or aircraft engines. The larger the value of this specific impulse, the better a rocket's performance. Improving specific impulse by using propellants of higher energy means that more thrust will be obtained for each pound of propellant consumed. Specific impulse is often expressed in terms of the number of seconds for which 1 pound mass of propellant will produce a thrust of 1 pound force. The most important parameter affecting the ultimate maximum flight velocity is a rocket's mass ratio, the relationship between a rocket vehicle and the amount of propellant it can carry. The mass ratio is obtained by dividing the total mass at lift-off by the total mass remaining after the propellants have burned. In general, a high mass ratio means that a maximum amount of propellant is pushing a minimum amount of inert vehicle mass, resulting in a high vehicle velocity. High values of specific impulse (high-energy propellants and low-molecular-weight exhaust gases) and mass ratio are necessary for difficult missions. ROCKET PROPULSION SYSTEMS A rocket propulsion system is an engine or powerplant that produces thrust by ejecting matter stored within a moving vehicle. This stored matter, or propellant, becomes the working fluid of the rocket. Rocket propulsion systems can be classified according to their energy source (chemical combustion, nuclear, solar), the types of vehicles they are used on (missiles, spacecraft, sounding rockets), the amount of thrust produced, or the type of propellant (see \Tpropellant, rocket\t). Liquid-Propellant Rockets A liquid-propellant rocket engine system consists of one or more thrust chambers, one or more vehicle tanks that contain the propellants, a feed mechanism to force the liquids into the thrust chamber, a power source to furnish the energy required by the feed mechanism, suitable valves and piping to transfer the liquids, a structure to transmit the thrust forces, and control devices to start and regulate propellant flow rates. The thrust chamber, consisting of an injector, a combustion chamber, and a nozzle, is the device where the liquid propellants are metered, injected, atomized, mixed, and burned to form hot, gaseous reaction products, which in turn are accelerated and ejected at a high velocity to impart thrust. The injector is usually an intricate assembly of pipes and accurately oriented injection holes that introduce the propellants into the combustion chamber, atomizing and mixing them in such a way as to create a relatively uniform mixture of fuel and oxidizer in droplets that will readily evaporate and burn in the combustion chamber. The chamber may be cooled by circulating one of the propellants (usually the fuel) through cooling jackets or passages. Heat may also be absorbed by ablative materials, ceramics, or special metals. Alternatively, certain special high-temperature materials, such as molybdenum metal, can be used to radiate away excess heat. The exhaust nozzle allows the hot gas to expand and accelerate to supersonic velocities. A convergent-divergent nozzle with smooth internal contours is commonly used. In some applications the nozzle axis is moved (by hinging or gimballing the thrust chamber, or sometimes the complete engine) so as to steer the vehicle by changing the direction of the thrust vector. Two principal types of feed systems are used for liquid-propellant rocket engines: those which use pumps for moving the propellants from their tanks to the thrust chamber (this type is usually found in high-thrust booster applications), and those which use high-pressure gas for expelling or displacing the propellants from their tanks (usually used in spacecraft attitude-control and maneuvering applications). Because liquid propellants float in the zero-gravity environment of space, special devices are necessary to ensure that the outlet pipe will always be filled with liquid. For low-thrust attitude-control applications, rocket engines are usually mounted in pairs at the perimeter of a spacecraft; two thrust chambers pointing in opposite directions are fired simultaneously to give a true turning moment to the vehicle. A minimum of 12 thrust chambers is needed to allow rotational control in each of two directions about three perpendicular axes. For precise angular position control in space, only a small impulse need be applied at one time; position-control rockets typically operate for pulsed durations of from 20 to 100 milliseconds. Solid-Propellant Rockets Solid-propellant rocket engines, commonly called rocket motors, come in many different types and sizes. The solid propellant to be burned is contained within the combustion chamber, or case. The propellant charge is called the grain and contains the chemical elements for complete burning. Once ignited, it usually burns smoothly on all of its exposed surfaces. By changing the design of the internal exposed grain surfaces, it is possible to vary the amount of propellant exposed and thus the amount of propellant that will burn. The burning rate of the solid propellant--usually between 0.3 and 3.3 cm/sec (0.1 and 1.3 in/sec) in a direction perpendicular to the burning surface--depends on the propellant ingredients. The rate increases with chamber pressure (which in turn is determined by the nozzle design and the grain configuration) and the ambient temperature of the propellant grain prior to ignition. The objective of a good design is to pack as much solid propellant as possible into a given chamber volume; the ideal unit is an end-burning grain, where the grain burns in cigarettelike fashion from one end to the other; this grain type has been used in past jet-assisted takeoff rockets for aircraft. For higher thrust and shorter duration a more complex initial internal surface is chosen, such as a two-dimensional internal star grain (used in air-launched missiles). Electric Rockets Three basic types of on-board electric propulsion systems for rockets and missiles are possible--electrothermal, electrostatic, and electromagnetic--as well as combinations of these types. Their use could include changing the orbits or attitudes or overcoming the perturbations of artificial satellites, correcting spaceflight trajectories, or achieving interplanetary transfers or solar-system escape. In an electrothermal system the propellant is heated or vaporized by electric resistance heaters or electric arcs, and the heated gas is expanded through a nozzle as in a chemical rocket. The electrostatic system achieves acceleration through the interaction of electrostatic fields on charged propellant particles such as small liquid droplets or colloidal particles. Electrostatic rockets making use of ions of atoms or molecules are known as ion rockets. An electromagnetic system achieves acceleration through the action of an electromagnetic field with a propellant \Tplasma\t, a high-temperature, electrically natural gas that contains electrons, ions, and neutral molecular species. All these types of on-board electric propulsion require a relatively large and heavy power source. They also require heavy power conversion and conditioning equipment to transform the power yield into the proper voltage and frequency. The weight of the equipment can become excessive, even when solar energy is employed, particularly if the efficiency in converting electricity to thrust is low. On the other hand, such systems perform well, as proved by the operation of the relatively few electric-propulsion units that have already been used in spacecraft. They have high values of specific impulse, but they are limited to a very low thrust of 0.45 g to 0.9 kg (0.001 to 2.0 lb). The more direct use of electric propulsion systems, without involving on-board units in rockets, and missiles, is also being explored and developed (see \Trailgun\t). Such systems are designed to launch objects as rockets do and may prove of great use in future space operations. Nuclear Rockets The nuclear rocket does not generate its power by chemical combustion, as do almost all rocket engines today. Instead it heats a propellant such as hydrogen in a fission reactor and expels the propellant at a high velocity. In this way, exhaust velocities twice those of the best chemical rockets can be reached. Extensive efforts have been directed toward nuclear-fission-reactor rocket propulsion, notably the now-abandoned \TNerva\t (Nuclear Engine for Rocket Vehicle Application) project in the United States. However, none of the different concepts and approaches has as yet been selected for a practical propulsion system. Part of the problem with the concept of nuclear rockets is that of the potential for disaster and long-term contamination of an area in case of a launch failure. Nuclear reactors are already used in rockets, however, as sources of electric power for on-board systems--primarily for deep-space probes by the United States, but for Earth satelites as well by the Soviet Union. Future Trends It is unlikely that further major performance improvements can be achieved with conventional chemical propellants, although relatively minor advances in power and efficiency continue to be made. For certain deep-space missions, electric rockets and nuclear rockets both have good potential. Several other advanced and imaginative concepts for rocket power systems have been proposed. Such concepts include the unfolding of gigantic solar sails to make use of the Sun's energy as a propellant on long space journeys, and, for still longer journeys, the devising of some sort of matter-annihilation device that would release a beam of photons to achieve velocities approaching that of the speed of light. A somewhat more likely rocket engine of the future is one based on the use of nuclear fusion power, once scientists have learned how to harness such power in practical form. George P. Sutton Bibliography: Baker, David, The Rocket (1979); Ordway, Frederick I., III, and Sharpe, Mitchell, The Rocket Team (1979); Ryan, J.W., Guns, Mortars and Rockets (1982); Sutton, George, Rocket Propulsion Elements, 5th ed. (1986); von Braun, Wernher, et al., Space Travel: A History, 4th ed. (1985); Winter, Frank, Rockets into Space (1990).