Ballistic Missiles

Any missile that lofts an explosive payload which descends to its target as a ballistic projectile—that is, solely under the influence of gravity and air resistance—is a ballistic missile. Missiles that do not deliver a free-falling payload, such as engine powered-cruise missiles (which fly to their targets as robotic airplanes), are not "ballistic."

A ballistic missile has two basic components: a package contains guidance systems and explosives (the payload), and the rocket that lofts the payload into the upper atmosphere or into space (the booster). Ballistic missiles traverse distance rapidly; a long-range ballistic missile can to the other side of the world in 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 highly destructive and difficult to defend against.

Boosters

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 7000 miles (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.