any of a variety of weapons systems that deliver explosive warheads to their targets by means of rocket propulsion. Rocket is a general term used broadly to describe a variety of jet-propelled missiles in which forward motion results from reaction to the rearward ejection of matter (usually hot gases) at high velocity. The propulsive jet of gases usually consists of the combustion products of solid or liquid propellants. In a more restrictive sense, rocket propulsion is a unique member of the family of jet-propulsion engines that includes turbojet, pulse-jet, and ramjet systems. The rocket engine is different from these in that the elements of its propulsive jet (that is, the fuel and oxidizer) are self-contained within the vehicle. Therefore, the thrust produced is independent of the medium through which the vehicle travels, making the rocket engine capable of flight beyond the atmosphere or propulsion underwater. The turbojet, pulse-jet, and ramjet engines, on the other hand, carry only their fuel and depend on the oxygen content of the air for burning. For this reason, these varieties of jet engine are called air-breathing and are limited to operation within the Earth's atmosphere. For the purposes of this article, a rocket engine is a self-contained (i.e., non-air-breathing) propulsion system of the type described above, while the term rocket refers to any free-flight (unguided) missile of the types used since the beginning of rocketry. A guided missile is broadly any military missile that is capable of being guided or directed to a target after having been launched. Tactical guided missiles are shorter-ranged weapons designed for use in the immediate combat area. Long-range, or strategic, guided missiles are of two types, cruise and ballistic. Cruise missiles are powered by air-breathing engines that provide almost continuous propulsion along a low, level flight path. A ballistic missile is propelled by a rocket engine for only the first part of its flight; for the rest of the flight the unpowered missile follows an arcing trajectory, small adjustments being made by its guidance mechanism. Strategic missiles usually carry nuclear warheads, while tactical missiles usually carry high explosives. Additional reading On missile systems, see Kenneth W. Gatland, Development of the Guided Missile, 2nd ed. (1954), a somewhat technical examination of early efforts. Other historical works include Homer E. Newell, Guide to Rockets, Missiles, and Satellites, 2nd rev. ed. (1961); Eric Burgess, Long-Range Ballistic Missiles (1961); E. Michael Del Papa, From SNARK to SRAM: A Pictorial History of Strategic Air Command Missiles (1976); Norman Polmar, Strategic Weapons: An Introduction, rev. ed. (1982); and Kenneth P. Werrell, The Evolution of the Cruise Missile (1985). Characteristics of strategic weapons are covered in Michael J.H. Taylor, Missiles of the World, 3rd ed. (1980); and Philip Birtles and Paul Beaver, Missile Systems (1985). Some insight into future developments is suggested in Max Walmer, An Illustrated Guide to Strategic Weapons (1988). The latest developments in the field are covered in Jane's Weapon Systems (annual) and The International Countermeasures Handbook (annual). Stephen Oliver Fought John F. Guilmartin, Jr. Strategic missiles Strategic missiles represent a logical step in the attempt to attack enemy forces at a distance. As such, they can be seen as extensions of either artillery (in the case of ballistic missiles) or manned aircraft (in the case of cruise missiles). Ballistic missiles are rocket-propelled weapons that travel by momentum in a high, arcing trajectory after they have been launched into flight by a brief burst of power. Cruise missiles, on the other hand, are powered continuously by air-breathing jet engines and are sustained along a low, level flight path by aerodynamic lift. Although experiments were undertaken before World War II on crude prototypes of the cruise and ballistic missiles, the modern weapons are generally considered to have their true origins in the V-1 and V-2 missiles launched by Germany in 194445. Both of those Vergeltungswaffen, or Vengeance Weapons, defined the problems of propulsion and guidance that have continued ever since to shape cruise and ballistic missile development. Given the extremely long ranges required of strategic weapons, even the most modern guidance systems cannot deliver a missile's warhead to the target with consistent, pinpoint accuracy. For this reason, strategic missiles have almost exclusively carried nuclear warheads, which need not strike a target directly in order to destroy it. By contrast, missiles of shorter range (often called tactical- or battlefield-range) have been fitted with both nuclear and conventional warheads. For example, the SS-1 Scud, a ballistic missile with ranges of up to 185 miles (300 kilometres), was fielded with nuclear warheads by Soviet troops in eastern Europe from the 1950s through the 1980s; but in the war of the cities during the IranIraq conflict of the 1980s, many SS-1s armed with conventional warheads were launched by both sides, killing thousands of civilians. Other dual-capable short-range ballistic missiles are the U.S. Lance, with a range of about 80 miles, and the Soviet SS-21 Scarab, with a range of 75 miles. (In this section, missile systems of the former Soviet Union are referred to by their NATO designations.) The exclusively nuclear capacity of strategic-range weapons confined serious development of cruise and ballistic missile technology to the world's nuclear powersparticularly the United States and the former Soviet Union. These two countries took different paths in exploiting missile technology. Soviet cruise missiles, for instance, were designed mostly for tactical antiship use rather than for threatening strategic land targets (as was the U.S. emphasis). Throughout the ballistic missile arms race, the United States tended to streamline its weapons, seeking greater accuracy and lower explosive power, or yield. Meanwhile, the Soviet Union, perhaps to make up for its difficulties in solving guidance problems, concentrated on larger missiles and higher yields. Most U.S. systems carried warheads of less than one megaton, with the largest being the nine-megaton Titan II, in service from 1963 through 1987. The Soviet warheads often exceeded five megatons, with the largest being a 20- to 25-megaton warhead deployed on the SS-7 Saddler from 1961 to 1980 and a 25-megaton warhead on the SS-9 Scarp, deployed from 1967 to 1982. (For the development of nuclear weapons, see nuclear weapon.) Most other countries pursuing missile technology have not developed strategic weapons to the extent of the United States and the former Soviet Union. Nonetheless, several other nations have produced them; their emphasis, however, has been on ballistic rather than cruise missiles because of the extremely sophisticated guidance systems required of cruise missiles. Also, as with any technology, there has occurred a transfer of ballistic missile technology to less-developed countries. Combined with the widespread capacity to produce chemical warheads, such weapons represent a potent addition to the arsenals of emerging powers of the Third World. Ballistic missiles Design principles Strategic ballistic missiles can be divided into two general categories according to their basing mode: those that are launched from land and those launched at sea (from submarines beneath the surface). They also can be divided according to their range into intermediate-range ballistic missiles (IRBMs) and intercontinental ballistic missiles (ICBMs). IRBMs have ranges of about 600 to 3,500 miles, while ICBMs have ranges exceeding 3,500 miles. Modern land-based strategic missiles are almost all of ICBM range, whereas all but the most modern submarine-launched ballistic missiles (SLBMs) have been of intermediate range. Prelaunch survivability (that is, the ability to survive an enemy attack) has been a long-standing problem with land-based ICBMs. (SLBMs achieve survivability by being based on relatively undetectable submarines.) At first, they were considered safe from attack because neither U.S. nor Soviet missiles were sufficiently accurate to strike the other's launch sites; hence, early systems were launched from above ground. However, as missile accuracies improved, above-ground missiles became vulnerable, and in the 1960s both countries began to base their ICBMs below ground in concrete tubes called silos, some of which were hardened against nuclear blast. Later, even greater improvements in accuracy brought ICBM basing strategy back to above-ground systems. This time, prelaunch survivability was to be achieved by mobile ICBMs that would confound an attacker with multiple moving targets. Most U.S. silos are designed for one-time hot-launch use, the rocket engines igniting within the silo and essentially destroying it as the missile departs. The Soviets pioneered the cold-launch method, in which the missile is expelled by gas and the rocket engine ignited after the missile clears the silo. This method, essentially the same system used with SLBMs, allows silos to be reused after minor repair. In order to increase their range and throw weight, ballistic missiles are usually multistaged. By shedding weight as the flight progresses (that is, by burning the fuel and then discarding the pumps, flight controls, and associated equipment of the previous stage), each successive stage has less mass to accelerate. This permits a missile to fly farther and carry a larger payload. The flight path of a ballistic missile has three successive phases. In the first, called the boost phase, the rocket engine (or engines, if the missile contains two or three stages) provides the precise amount of propulsion required to place the missile on a specific ballistic trajectory. Then the engine quits, and the final stage of the missile (called the payload) coasts in the midcourse phase, usually beyond the Earth's atmosphere. The payload contains the warhead (or warheads), the guidance system, and such penetration aids as decoys, electronic jammers, and chaff to help elude enemy defenses. The weight of this payload constitutes the missile's throw weightthat is, the total weight that the missile is capable of placing on a ballistic trajectory toward a target. By midcourse the warheads have detached from the remainder of the payload, and all elements are on a ballistic path. The terminal phase of flight occurs when gravity pulls the warheads (now referred to as the reentry vehicles, or RVs) back into the atmosphere and down to the target area. Most ballistic missiles use inertial guidance to arrive at the vicinity of their targets. This technology, based on Newtonian physics, involves measuring disturbances to the missile in three axes. The device used to measure these disturbances is usually composed of three gyroscopically stabilized accelerometers mounted at right angles to one another. By calculating the acceleration imparted by external forces (including the rocket engine's thrust), and by comparing these forces to the launch position, the guidance system can determine the missile's position, velocity, and heading. Then the guidance computer, predicting the gravitational forces that will act on the reentry vehicle, can calculate the velocity and heading required to reach a predetermined point on the ground. Given these calculations, the guidance system can issue a command to the missile thrust system during boost phase to place the payload at a specific point in space, on a specific heading, and at a specific velocityat which point thrust is shut off and a purely ballistic flight path begins. Ballistic missile guidance is complicated by two factors. First, during the latter stages of the powered boost phase, the atmosphere is so thin that aerodynamic flight controls such as fins cannot work and the only corrections that can be made to the flight path must come from the rocket engines themselves. But, because the engines only provide a force vector roughly parallel to the missile's fuselage, they cannot be used to provide major course corrections; making major corrections would create large gravitational forces perpendicular to the fuselage that could destroy the missile. Nevertheless, small corrections can be made by slightly gimballing the main engines so that they swivel, by placing deflective surfaces called vanes within the rocket exhaust, or, in some instances, by fitting small rocket engines known as thrust-vector motors or thrusters. This technique of introducing small corrections into a missile's flight path by slightly altering the force vector of its engines is known as thrust-vector control. A second complication occurs during reentry to the atmosphere, when the unpowered RV is subject to relatively unpredictable forces such as wind. Guidance systems have had to be designed to accommodate these difficulties. Errors in accuracy for ballistic missiles (and for cruise missiles as well) are generally expressed as launch-point errors, guidance/en-route errors, or aim-point errors. Both launch- and aim-point errors can be corrected by surveying the launch and target areas more accurately. Guidance/en-route errors, on the other hand, must be corrected by improving the missile's designparticularly its guidance. Guidance/en-route errors are usually measured by a missile's circular error of probability (CEP) and bias. CEP uses the mean point of impact of missile test firings, usually taken at maximum range, to calculate the radius of a circle that would take in 50 percent of the impact points. Bias measures the deviation of the mean impact point from the actual aim point. An accurate missile has both a low CEP and low bias. Tactical guided missiles Guided missiles were a product of post-World War II developments in electronics, computers, sensors, avionics, and, to only a slightly lesser degree, rocket and turbojet propulsion and aerodynamics. Although tactical, or battlefield, guided missiles were designed to perform many different roles, they were bound together as a class of weapon by similarities in sensor, guidance, and control systems. Control over a missile's direction was most commonly achieved by the deflection of aerodynamic surfaces such as tail fins; reaction jets or rockets and thrust-vectoring were also employed. But it was in their guidance systems that these missiles gained their distinction, since the ability to make down-course corrections in order to seek or home onto a target separated guided missiles from purely ballistic weapons such as free-flight rockets and artillery shells. Guidance methods The earliest guided missiles used simple command guidance, but within 20 years of World War II virtually all guidance systems contained autopilots or autostabilization systems, frequently in combination with memory circuits and sophisticated navigation sensors and computers. Five basic guidance methods came to be used, either alone or in combination: command, inertial, active, semiactive, and passive.

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