In common military parlance, the word missile describes a powered, guided munition, whilst the word "rocket" describes an powered, unguided munition. A common further sub-division is to consider ballistic missile to mean a munition that follows a ballistic trajectory and cruise missile to describe a munition that generates lift. Guided missiles have three key system components:
Missiles may be targeted in a number of ways. The most common method is to use some form of radiation, such as infra-red, lasers or radio waves, to guide the missile onto its target. This radiation may emanate from the target (such as the heat of an engine or the radio waves from an enemy radar), it may be provided by the missile itself (such as a radar) or it may be provided by a friendly third party (such as the radar of the lauch vehicle/platform, or a laser designator operated by friendly infantry). The first two are often known as fire and forget as they need no further support or control from the launch vehicle/platform in order to function. Another method is to use a TV camera - using either visible light or infra-red - in order to see the target. The picture may be used either by a human operator who steers the missile onto its target, or by a computer doing much the same job. Many missiles use a combination of two or more of the above methods, to improve accuracy and the chances of a successful engagement.
Another method is to target the missile by knowing the location of the target, and using a guidance system such as INS, TERCOM or GPS. This guidance system guides the missile by knowing the missile's current position and the position of the target, and then calculating a course between them. This job can also be performed somewhat crudely by a human operator who can see the target and the missile, and guides it using either cable or radio based remote-control.
Whether a guided missile uses a targeting system, a guidance system or both, it needs a flight system. The flight system uses the data from the targeting or guidance system to manoeuver the missile in flight, allowing it to counter inaccuracies in the missile or to follow a moving target. There are two main systems: vectored thrust (for missiles that are powered throughout the guidance phase of their flight) and aerodynamic maneuvering (wings, fins, canards, etc).
There are some similarities between guided missiles and guided bombs. A guided bomb, dropped from an aircraft, is unpowered and uses aerodynamic fins for forward horizontal maneuvering while falling or gliding.
The first missiles to be used operationally were a series of German missiles of WW2. Most famous of these are the V1 and V2, both of which used a simple mechanical autopilot to keep the missile flying along a pre-chosen route. Less well known were a series of anti-shipping and anti-aircraft missiles, typically based on a simple radio control system directed by the operator.
The V2 had demonstrated that a ballistic missile could deliver a warhead to a target city with no possibility of interception, and the introduction of nuclear weapons meant it could do useful damage when it arrived. The accuracy of these systems was fairly poor, but post-war development by most military forces improved the basic inertial platform concept to the point where it could be used as the guidance system on ICBMs flying thousands of miles. Today the ballistic missile represents the only strategic deterrent in most military forces; the USAFs continued support of manned bombers is considered by some to be entirely political in nature.
German experience in WWII demonstrated that destroying a large aircraft was quite difficult, and they had invested considerable effort into air-to-air missile systems to do this. Their Me262's jets often carried R4M rockets, and other types of "bomber destroyer" aircraft had unguided rockets as well. In the post-war period the R4M served as the pattern for a number of similar systems, used by almost all interceptor aircraft during the 1940s and '50s. Lacking guidance systems, such rockets had to be carefully aimed at relatively close range to successfully hit the target. The US Navy and USAF began deploying guided missiles in the early 1950s, most famous being the US Navy's AIM-9 Sidewinder and USAF's AIM-4 Falcon. These systems have continued to advance, and modern air warfare consists almost entirely of missile firing. In the Falklands War technically inferior British Harriers were able to defeat faster Argentinian opponents using AIM-9G missiles provided by the United States as the conflict began. The latest heat-seeking designs can lock onto a target from various angles, not just from behind, where the heat signature from the engines is strongest. Other types rely on radar guidance (either on-board or "painted" by the launching aircraft).
By the end of WWII all forces had widely introduced unguided rockets using HEAT warheads as their major anti-tank weapon (see Panzerfaust, Bazooka). However these had a limited useful range of a 100 m or so, and the Germans were looking to extend this with the use of a missile using wire guidance, the X-7. After the war this became a major design class in the later 1950s, and by the 1960s had developed into practically the only non-tank anti-tank system in general use. During the 1973 Yom Kippur War between Israel and Egypt, the 9M14 Malyutka (aka "Sagger") man-portable anti-tank missile proved potent against Israeli tanks. While other guidance systems have been tried, the basic reliability of wire-guidance means this will remain the primary means of controlling anti-tank missile in the near future.Arrow missile and MIM-104 Patriot for defense against short-range missiles, carry explosives.
However, in the case of a large closing speed, a projectile without explosives is used, just a collision is sufficient to destroy the target. See Missile Defense Agency for the following systems being developed:
The proposed Brilliant Pebbles defense system would use kinetic energy collisions without explosives.
Missile guidance systems generally fall into a number of basic classes, each one associated with a particular role. Modern electronics has allowed systems to be mixed on a single airframe, dramatically increasing the capabilities of the missiles.
See the main article at Missile guidance for details of the types of missile guidance systems.