The force acting on a rocket, called its thrust, is equal to the mass ejected per second times the velocity of the expelled gases. This force can be understood in terms of Newton's third law of motion, which states that for every action there is an equal and opposite reaction. In the case of a rocket, the action is the backward-streaming flow of gas and the reaction is the forward motion of the rocket. Another way of understanding rocket propulsion is to realize that tremendous pressure is exerted on the walls of the combustion chamber except where the gas exits at the rear; the resulting unbalanced force on the front interior wall of the chamber pushes the rocket forward. A common misconception, before space exploration pointed up its obvious fallacy, holds that a rocket accelerates by pushing on the atmosphere behind it. Actually, a rocket operates more efficiently in outer space, since there is no atmospheric friction to impede its motion.
The key elements in designing a rocket are the propulsion system, which includes the propellant and the exit nozzle, and determining the number of stages required to lift the intended payload. Rocket navigation is usually based on inertial guidance; internal gyroscopes are used to detect changes in the position and direction of the rocket.Rocket Propellants
The most vital component of any rocket is the propellant, which accounts for 90% to 95% of the rocket's total weight. A propellant consists of two elements, a fuel and an oxidant; engines that are based on the action-reaction principle and that use air instead of carrying their own oxidant are properly called jets. Propellants in use today include both liquefied gases, which are more powerful, and solid explosives, which are more reliable; the space shuttle's main engines use liquid propellant, while its boosters are solid-fuel rockets. The chemical energy of the propellants is released in the form of heat in the combustion chamber.
A typical liquid engine uses hydrogen as fuel and oxygen as oxidant; a typical solid propellant is nitroglycerine. In the liquid engine, the fuel and oxidant are stored separately at extremely low temperatures; in the solid engine, the fuel and oxidant are intimately mixed and loaded directly into the combustion chamber. A solid engine requires an ignition system, as does a liquid engine if the propellants do not ignite spontaneously on contact.
The efficiency of a rocket engine is defined as the percentage of the propellant's chemical energy that is converted into kinetic energy of the vehicle. During the first few seconds after liftoff, a rocket is extremely inefficient, for at least two unavoidable reasons: High power consumption is required to overcome the inertia of the nearly motionless mass of the fully fueled rocket; and in the lower atmosphere, power is wasted overcoming air resistance. Once the rocket gains altitude, however, it becomes more efficient. as the trajectory, at first vertical, curves into a suborbital arc or into the desired orbit.
Although all known rockets currently in use derive their energy from chemical reactions, more exotic propulsion systems are being considered. In ion propulsion, a plasma (ionized gas consisting of a mixture of positively charged atoms and negatively charged electrons) would be created by an electric discharge and then expelled by an electric field. The engine could provide a low thrust efficiently for long periods; on a lengthy flight this would produce very high velocities, so that if there is ever a trip to the outer planets an ion drive might be used. Deep Space 1, a space probe launched in 1998 to test new technologies, was propelled intermittently by an ion engine. Even nuclear power has been considered for propulsion; in fact, a nuclear ramjet was developed in the early 1960s before it was realized that because the exhaust gases would be highly radioactive such a drive could never be used in earth's atmosphere.Design of the Exit Nozzle
A critical element in all rockets is the design of the exit nozzle, which must be shaped to obtain maximum energy from the exhaust gases moving through it. The nozzle usually converges to a narrow throat, then diverges to create a form which shapes the hypersonic flow of exhaust gas most efficiently. The walls of the combustion chamber and nozzle must be cooled to protect them against the heat of the escaping gases, whose temperature may be as high as 3,000°C;—above the melting point of any metal or alloy.Staging of Rockets
Although early rockets had only one stage, it was early recognized that no single-stage rocket can reach orbital velocity (5 mi/8 km per sec) or the earth's escape velocity (7 mi/11 km per sec). Hence multistage rockets, such as the two-stage Atlas-Centaur or the three-stage Saturn V, became necessary for space exploration. In these systems, two or more rockets are assembled in tandem and ignited in turn; once the lower stage's fuel is exhausted, it detaches and falls back to earth. Soviet systems clustered several rockets together, operated simultaneously, to obtain a large initial thrust.
The invention of the rocket is generally ascribed to the Chinese, who as early as A.D. 1000 stuffed gunpowder into sections of bamboo tubing to make military weapons of considerable effectiveness. The 13th-century English monk Roger Bacon introduced to Europe an improved form of gunpowder, which enabled rockets to become incendiary projectiles with a relatively long range. Rockets subsequently became a common if unreliable weapon. Major progress in design resulted from the work of William Congreve, an English artillery expert, who built a 20-lb (9-kg) rocket capable of traveling up to 2 mi (3 km). In the late 19th cent., the Austrian physicist Ernst Mach gave serious theoretical consideration to supersonic speeds and predicted the shock wave that causes sonic boom.
The astronautical use of rockets was cogently argued in the beginning of the 20th cent. by the Russian Konstantin E. Tsiolkovsky, who is sometimes called the "father of astronautics." He pointed out that a rocket can operate in a vacuum and suggested that multistage liquid-fuel rockets could escape the earth's gravitation. The greatest name in American rocketry is Robert H. Goddard, whose pamphlet A Method for Reaching Extreme Altitudes anticipated nearly all modern developments. Goddard launched the first liquid-fuel rocket in 1926 and demonstrated that rockets could be used to carry scientific apparatus into the upper atmosphere. His work found its most receptive audience in Germany. During World War II, a German team under Wernher von Braun developed the V-2 rocket, which was the first long-range guided missile. The V-2 had a range greater than 200 mi (322 km) and reached velocities of 3,500 mi (5,600 km) per hr.
After the war, rocket research in the United States and the Soviet Union intensified, leading to the development first of intercontinental ballistic missiles and then of modern spacecraft. Important U.S. rockets have included the Redstone, Jupiter, Atlas, Titan, Agena, Centaur, and Saturn carriers. Saturn V, the largest rocket ever assembled, developed 7.5 million lb (3.4 million kg) of thrust. A three-stage rocket, it stood 300 ft (91 m) high exclusive of payload and with the Apollo delivered a payload of 44 tons to the moon. Rockets presently being used to launch manned and unmanned missions into space include the Brazilian VSV-30; the Chinese Long March 2C, 2E, and 2F; the European Space Agency's Ariane 5 series; the Indian PSLV (Polar Satellite Launch Vehicle); the Israeli Shavit 2; the Russian Soyuz and Proton K and M; the Japanese H-2A; the South Korean-Russian KSLV-1; the U.S. Athena 1 and 2, Taurus, Titan 2 and 4B, Delta 2, 3, and 4, Atlas 2 ,3, and 5, and STS or space shuttle; and the multinational, private Sea Launch Zenit-3SL, which uses a converted oil platform located some 1,400 mi (2,250 km) southeast of Hawaii. The Ares I, a two-stage NASA rocket designed to replace the STS as a launch vehicle on manned missions, underwent its first test flight in Oct., 2009.
See also space science.
See G. P. Sutton, Rocket Propulsion Elements: An Introduction to the Engineering of Rockets (6th ed. 1992); F. H. Winter, Rockets into Space (1993); D. Baker, Spaceflight and Rocketry: A Chronology (1996); M. Stoiko, Pioneers of Rocketry (1997); R. Snedden, Rockets and Space (1998).
Launch vehicle driven by several rocket systems mounted in vertical sequence. The lowest, or first, stage ignites and lifts the vehicle (sometimes assisted by attached booster rockets) at increasing speed until its propellants have been used up. The first stage then drops off, which makes the vehicle lighter, and the second stage ignites and accelerates the vehicle further. The use of additional stages generally follows the same pattern until the payload—the spacecraft—has reached the velocity needed to achieve orbit or leave the vicinity of Earth. The number of stages required depends on the details of the mission, the launch vehicle’s characteristics, and other factors. Some early vehicles needed five stages to reach orbit; most current launch vehicles need only two.
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Type of jet-propulsion device that uses either solid or liquid propellants to provide the fuel and oxidizer needed for combustion. The hot gases provided by combustion are ejected in a jet through a nozzle at the rear of the rocket. The term is also commonly applied to any of various vehicles, including fireworks, skyrockets, guided missiles, and launch vehicles for spacecraft, that are driven by such a propulsive device. Typically, thrust (force causing forward motion) is produced by reaction to a rearward expulsion of hot gases at extremely high speed (see Newton's laws of motion).
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Yellowish-flowered European herbaceous plant (Eruca vesicaria sativa), of the mustard family, cultivated for its foliage, which is used especially in salads. The leaves taste sharp and peppery when young and succulent but become bitter with age. A medicinal oil is extracted from the seeds.
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RPG, or rocket-propelled grenade, is a loose term describing hand-held, shoulder-launched anti-tank weapons capable of firing an unguided rocket equipped with an explosive warhead. RPG is a transliteration of РПГ, the Russian abbreviation of реактивный противотанковый гранатомёт (transliterated as "reaktivniy protivotankoviy granatomyot"), which translates to the English phrase "reactive anti-tank launcher." Thus rocket-propelled grenade is a backronym.
Most modern main battle tanks (MBTs) are not vulnerable to hand-held RPGs, and this was demonstrated in the Israel-Hezbolla war of 2006. RPGs are still used very effectively against lightly-armoured vehicles such as armored personnel carriers (APCs) or unarmored wheeled vehicles, as well as against buildings and bunkers. They can still be a threat to an MBT under certain tactical conditions. One exception is the RPG-29, the most advanced model, which uses a tandem-charge high explosive anti-tank warhead to penetrate explosive reactive armor (ERA). It is capable of destroying some modern MBTs such as the T-90. In August 2006, an RPG-29 round penetrated the frontal ERA of a Challenger 2 tank during an engagement in al-Amarah, Iraq.
The most widely distributed and used RPG in the world is the Soviet Union-developed RPG-7. The Soviets developed the basic design of this RPG during World War II, combining important design features of the US Bazooka and the German Panzerfaust. Today, advanced armies such as that of the United States, have implemented armor on their tanks that are invulnerable to grenades. However, new rounds have been developed to use with the RPG-7 launchers, that can defeat advanced armor types such as ERA.
The RPG launcher is a hollow tube that concentrates the rocket exhaust to create an over-pressure within the tube. This over-pressure propels the warhead at a higher speed than from the specific impulse of the rocket alone. This higher speed is necessary for the rocket to be stable in flight.
The launcher is designed such that the rocket exits the launcher without discharging an exhaust that would be dangerous to the operator. In the case of the RPG-7 the rocket is launched by a gunpowder booster charge, and the rocket motor ignites only after 10 meters. In some other designs the rocket burns completely within the tube.
The high-temperature rocket exhaust is hazardous fifteen to twenty meters to the rear of an RPG launcher. The launcher must be cleaned periodically, as built-up residue will result in an excess of over-pressure, causing the sighting mechanism to be driven into the operator's eye when the rocket is fired. Blindness in one eye often results.
An RPG is an inexpensive way to deliver an explosive payload a distance of 100 yards (91m) with moderate accuracy. Substantially more expensive, wire-guided rockets are used when accuracy is important. These rockets trail a thin wire behind them during firing and can be steered by the operator while in flight. In 1982, British troops were sent to the Falklands War armed with a number of wire-guided MILAN anti-tank missiles even though there were no Argentine tanks in the Falklands Islands. The British used these expensive weapons to destroy Argentine bunkers at longer ranges. The British also used cheap 66 mm M72 LAW unguided rockets and recoilless 84mm against Argentine bunkers. The popularity and usefulness of such weapons prompted the U.S. military to field the SMAW, the U.S. equivalent of the RPG.
The HEAT (high explosive anti-tank) round is a standard shaped charge warhead, similar in concept to those used in tank cannon rounds. In this type of warhead, the shape of the explosive material within the warhead focuses the explosive energy on a copper (or similar metal) lining. This crushes the metal lining and propels some of it forward at a very high velocity. The resulting narrow jet of metal can punch through the armor of most APC's and IFV's. However, the warhead on older RPG systems is too small to penetrate the main armor of most modern battle tanks, although it is still capable of causing secondary damage to vulnerable systems (especially sights, tracks, rear and roof of turrets) and can disable or destroy most lightly armored or unarmored vehicles.
Specialized warheads are available for illumination, smoke, tear gas, and white phosphorus. Russia, China, and many former Warsaw Pact nations have also developed a fuel-air explosive (a/k/a "thermobaric") warhead. Another recent development is a tandem HEAT warhead capable of penetrating reactive armor.
Accuracy limits the standard RPG-7 to a practical range of 50 m, although it can reach 150 or even 300 m in skilled hands. It has an indirect fire (bombardment) range to 920 m, limited by the 4.5-second self-destruct timer.*
So-called PRIGs (Propelled Recoilless Improvised Grenade) were improvised warheads used by the Provisional IRA.
One of the first instances when it was used by terrorists was on 13 January 1975 at the Orly airport in France when Carlos the Jackal together with another member from the PFLP used two Soviet RPG-7 grenades to attack an Israeli El Al airliner. Both missed, and one of them hit a DC-9 of Yugoslav Airlines instead.
Because of the inherent inaccuracy of the RPG, the operator must fire relatively close to the intended target, increasing the chances of being spotted and captured, shot or killed. Most modern armies deploy anti-tank guided missiles (ATGM) as their primary infantry anti-tank weapon, but the RPG can still be effectively employed against tanks under certain tactical conditions, especially urban warfare, where they are favored by low-tech armies. They are most effective when used in restricted terrain as the availability of cover and concealment can make it difficult for the intended target to spot the RPG operator.
The operator must move after firing the RPG as the ignition of the rocket generates a flash visible to the enemy and usually leaves a smoke trail leading back to the firing position. In Afghanistan, Mujahideen RPG shooters who remained in position after firing were often killed by Soviet counter-fire.
When deployed against personnel, the warhead can be aimed at a solid surface to detonate, popular choices being trees or buildings. Another option is an indirect method of firing the warhead over the intended target area at ranges of 800–1000 m where the warhead would detonate automatically. More skilled shooters can use the RPG self-destruct feature to make it explode over the enemy at closer range.
Although they can be used against hovering helicopters, they should not be confused with anti-aircraft shoulder fired surface-to-air missile systems such as the Stinger or SA-7 Grail. Furthermore, firing at steep angles poses a danger to the user, because the backblast from firing reflects off the ground. In Somalia, militia members sometimes welded a steel plate in the exhaust end of an RPG's tube to deflect pressure away from shooter when shooting upwards at US helicopters. RPGs are used in this role only when more effective weapons are not available.
Multiple shooters were also effective against heavy tanks with reactive armor: The first shot would be against the driver's viewing prisms. Following shots would be in pairs, one to set off the reactive armor, the second to penetrate the tank's armor. Favored weak spots were the top and rear of the turret.
Afghans sometimes used RPG-7s at extreme range, exploded by their 4.5- second self-destruct timer, which calculates to an almost 1-km range. This performed expedient indirect antipersonnel bombardment and was sometimes used to discourage reconnaissance by aircraft.
Chechen fighters formed independent "cells" that worked together to destroy a specific Russian armored target. Each cell contained small arms and some form of RPG (RPG-7V or RPG-18, for example). The small arms were used to button the tank up and keep any infantry occupied while the RPG gunner struck at the tank. While doing so other teams would attempt to fire at the target in order to overwhelm the Russians' ability to effectively counter the attack. To further increase the chance of success, the teams took up positions at different elevations where possible. Firing from the third and higher floors allowed good shots at the weakest armor (the top).
When the Russians began moving in tanks fitted with ERA (Explosive Reactive Armor), the Chechens had to adapt their tactics, because the RPGs they had access to were unlikely to result in the destruction of the tank. 2 or more RPG teams would position themselves in such a way that they could all hit the same section of a tank, but from different angles. Usually rebels would first hit the tank with a large IED to blow the tracks off it so it couldn't move then rebels would fire at the reactive armor with an RPG to create a spot where the base armor was exposed. The other team would aim for this spot, since it was now as vulnerable as if there was no ERA on the tank at all. These were crude, but apparently effective, way to get the effect of a tandem warhead without actually having one.
At the time, Soviet helicopters countered the threat from RPGs at landing zones by first clearing them with anti-personnel saturation fire. The Soviets also varied the number of accompanying helicopters (two or three) in an effort to upset Afghan force estimations and preparation. In response, the Mujahideen prepared dug-in firing positions with top cover, and again, Soviet forces altered their tactics by using air-dropped fuel-air bombs on such landing zones. As the U.S.-supplied Stinger surface-to-air missiles became available to them, the Afghans abandoned RPG attacks.
Both of the Black Hawk helicopters lost by the U.S. during the Battle of Mogadishu in Somalia in 1993 were downed by RPG-7s. In Iraq and the second Afghanistan campaign, RPGs were deployed with mixed success against Coalition helicopter forces.
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