Any missile that lofts an explosive which descends to its target as a ballistic projectile—that is, solely under the influence of gravity and aerodynamics—is a ballistic missile. Missiles that do not deliver a free-falling payload, such as cruise missiles (which fly to their targets as powered robotic airplanes), are not ballistic missiles.
A ballistic missile has two basic components: a package containing guidance systems and explosives (the payload), and the rocket that lofts the payload into the upper atmosphere or into space (the booster). Ballistic missiles travel rapidly; a long-range ballistic missile can fly to the far side of the world in about 30 minutes. Because they give so little advance warning and deliver small, fast-moving payloads that may contain nuclear weapons capable of destroying entire cities, ballistic weapons are both highly destructive and difficult to defend against.
With the exception of submarine-launched ballistic missiles (SLBMs), ballistic missiles are categorized according to range. Five commonly-accepted categories of ballistic missile, with their associated ranges, are as follows: (1) battlefield short range ballistic missiles (BSRMBs:>93 mi [150 km]); (2) short range ballistic missiles (SRBMs: 93–497 mi [150–800 km]), (3) medium range ballistic missiles (MRBMs: 497–1, 490 mi [800–2,400 km]), (4) intermediate range ballistic missiles (IRBMs: 1,490–3416 mi [2, 400–5,500 km]), and (5) intercontinental range ballistic missiles (ICBMs: > 3,416 mi [> 5,500 km]).
Alternatively, the U.S. Department of Defense defines ballistic missiles with ranges less than 683 mi (1,100 km) as SRBMs, those with ranges between 683 and 1,708 mi (1,100–2,750 km) as MRBMs, those with ranges between 1,708 and 3,416 mi (1,100–5,500 km) as IRBMs.
Ballistic missiles can be launched from submarines, silos (i.e., vertical underground tubes), ships, or trailers. All ballistic missiles launched from submarines, regardless of range, are categorized as SLBMs; modern SLBMs have ranges comparable to those of ICBMs. The purpose of mounting ballistic missiles on submarines is to make them secure from attack. Modern missile submarines, such as those in the U.S. Trident class, are difficult to locate and can launch their missiles without surfacing.
The flight of a ballistic missile can be divided into three phases: boost phase, cruise phase, and descent (terminal) phase. Boost phase begins with the ignition of the missile’s booster rocket. The booster lofts the missile at a steep angle, imparting a high speed to the payload before burning out. The payload and booster then separate, beginning the cruise phase. The spent booster falls back to Earth while the payload, starting to lose speed, continues to gain altitude. If the missile is sufficiently long-range, its payload rises above the Earth’s atmosphere during cruise phase, where it jettisons its aerodynamic protective shroud and arcs under the influence of gravity. The payload may be a single cone-shaped warhead or a flat “bus” with several warheads attached to it like upside-down ice-cream cones arranged circularly on a plate.
Individual warheads are not propelled downward toward their targets on the ground, but follow ballistic paths determined by gravity and aerodynamics, gaining speed as they lose altitude. Modern reentry vehicles usually feature small external fins or other steering devices that enable them to control their course, within limits, as they fall through the atmosphere; though such maneuverable reentry vehicles (MARVs) are not, strictly speaking, ballistic objects, missiles delivering them are still termed “ballistic” missiles for convenience. Maneuverability increases accuracy; a modern MARV delivered by ICBM or SLBM can land within a few hundred feet of its target after a journey of thousands of miles. Warheads may explode in the air high above their targets, on the surface, or under the surface after striking into the ground.
The booster rockets of early ballistic missiles were powered by liquid fuels. A liquid-fuel rocket carries fuel (hydrazine, liquid hydrogen, or other) and liquid oxygen in tanks. Pressurized streams of fuel and oxygen are mixed and ignited at the top of a bell-shaped chamber: hot, expanding gases rush out of the open end of the bell, imparting momentum to the rocket in the opposite direction. Liquid fuels are unwieldy, as they must be maintained at low temperatures and may leak fuel or oxygen from tanks, pipes, valves, or pumps. Early U.S. ICBMs such as the Atlas and Titan I required several hours of above-ground preparation, including fueling, before they could be launched.
Since the late 1950s, ballistic-missile design has concentrated on solid-fuel boosters, which require less maintenance and launch preparation time and are more reliable because they contain fewer moving parts. Solid-fuel rockets contain long, hollow-core casts of a fuel mixture that, once ignited, burn from the inside out in an orderly way, forcing gases out the rear of the rocket. Starting in the early 1960s, liquid-fuel ballistic missiles were gradually phased out of the U.S. and Russian arsenals in favor of solid-fuel missiles. The first U.S. solid-fuel ICBM was the Minuteman I missile (so-called because of its near-instant response time), which was deployed to underground silos in the Midwest starting in 1962.
Today, the ballistic-missile fleet of the United States consists almost entirely of solid-fuel rocket boosters. The Minuteman III, for example, like the Minuteman I, and II it replaces, has a three-stage solid-fuel booster and a range of over 7,000 mi (11,265 km). (Stages are independent rockets that are stacked to form a single, combined rocket. The stages are burned from the bottom up; each is dropped as it is used up, and the stage above it is ignited. The advantage of staging is that the booster lightens more rapidly as it gains speed and altitude. There are single-stage, two-stage, and three-stage ballistic missiles; the greater the number of stages, the longer the range of the missile.)
Payloads, warheads, and MIRV
As mentioned above, the payload of a ballistic missile may be either a single warhead or a bus bearing several warheads which can each be sent to a different target in the same general area (e.g., the eastern United States). Such a payload is termed a multiple independently targetable reentry vehicle (MIRV) system, and missiles bearing multiple independently targetable warheads are said to be MIRVed. The first MIRVed missiles were deployed the U.S. in 1970; only long-range ballistic missiles (ICBMs and SLBMs) are MIRVed. After a MIRV bus detaches from the burnt-out upper stage of its booster, it arcs through space in its cruise phase. It may possess a low-power propulsion system that enables it to impart slightly different velocities to each of its warheads, which it releases at different times. (Slight differences between individual warhead trajectories in space can translate to relatively large differences between trajectories later on, when the individual warheads are approaching their targets.) The U.S. Minuteman III ICBM is a modern MIRVed missile carrying up to three warheads; other MIRVed missiles, such as the MX, have been capable of carrying up to ten warheads.
Regional or approximate targeting for each MIRVed warhead is achieved by bus maneuvering and release timing during cruise phase. During descent phase, the warhead may steer itself to its precise target by means of inertial guidance, radar, or a combination of the two. Inertial guidance is based on the principle that every change in an object’s velocity can be sensed by that object as an acceleration. By knowing its exact prelaunch location and state of motion (e.g., by consulting the Global Positioning System) and by precisely measuring all accelerations during and after launch, an inertial guidance system can calculate its location at all times without needing to make further observations of the outside world. Ballistic-missile payloads rely primarily on inertial guidance to strike their targets; MARVs may refine their final course by consulting the Global Positioning System (as is done, for example, by the Chinese CSS-6 SRBM) or by using radar to guide themselves during final approach (as was done, for example, by the Pershing II IRBM deployed the U.S. in Europe during the 1980s).
The nuclear warheads mounted on modern long-range ballistic missiles are usually thermonuclear warheads having yields in the range of several hundred kilotons to several megatons. (One kiloton equals the explosive power of one thousand tons of the chemical explosive TNT; one megaton is equivalent to a million tons of TNT.) Those nations that do not possess nuclear weapons mount conventional-explosive warheads on their ballistic missiles.
The world’s first ballistic missile was the V-2, developed by Nazi Germany during World War II. The V-2, which was first test-launched on October 3, 1942, could deliver a 1,650-lb (750-kg) warhead to a target 225 miles away. Germany launched approximately 3,000 V-2s during the war, but with little military effect; the V-2, lacking the sophisticated guidance computers of later ballistic missiles, were inaccurate. Only 50% of V-2s aimed at a given point would, on average, land within 11 mi (17 km) of that point. The V-2 was therefore not aimed at military installations but, like its predecessor the V-1 (the first cruise missile, also developed by Nazi Germany), at the city of London. Some 518 V-2s struck London during the final years of World War II, killing over 20,000 people and making the V-2 the deadliest ballistic missile in history—so far. (The “V” in V-1 and V-2 stands for Vergeltungswaffe, German for “retaliation weapon,” reflecting the fact that the V-2’s primary purpose was not victory but vengeance.)
The United States and Soviet Union were far behind Germany in the design of large rockets during World War II, but both captured V-2 technicians and information at the end of the war and used them to accelerate their own missile programs. The U.S. began by experimenting with captured V-2s, and during the late 1940s built several new rockets of its own based on the V-2. During the 1950s both the Soviet Union and the United States turned their attention to the development of ballistic-missile boosters that could reach the other country’s heartland from anywhere in the world. The Soviet Union flight-tested the world’s first ICBM, the R-7, in August, 1957. Two months later the R-7 was soon used to launch the world’s first artificial satellite, Sputnik I, and four years later launched the world’s first orbital manned space flight. The U.S. was not far behind, and by 1959 had deployed its own ICBMs, the liquid-fueled Atlas and Titan missiles. The Americans also used their ICBMs for early space-flight efforts; the first manned U.S. space flights (Mercury and Gemini programs) used the Redstone, Atlas, and Titan II missile boosters.
Today, dozens of countries have the technological ability to develop ballistic missiles or have already done so. A relatively small number, including most recently India, Israel, Pakistan, and North Korea, have also developed nuclear weapons that they could deliver using their ballistic missiles.
See also Nuclear weapons.
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