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A guided missile is a military rocket that can be directed in flight to change its flight path. In typical usage the term "missile" refers to guided rockets, and "rockets" to unguided ones. The differences between the two may be fairly minor other than the guidance system.
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.
German systems tended to be based on liquid fuel rocket engines, typically derived from Hellmuth Walter's hydrogen peroxide based systems. These were impractical for general use, notably due to the danger of handling the corrosive fuel, and almost all post-war research turned to the simpler solid fuel designs. In addition the quality and ability of various electronic systems improved dramatically in the immediate post-war era, allowing a number of more advanced automatic guidance systems to be introduced. By the 1950s almost every "major power" had a series of missile development programs underway, generally divided into a number of basic classifications.
- ballistic missiles
- After the boost-stage ballistic missiles follow a trajectory mainly determined by ballistics, the guidance is for relatively small deviations from that.
- 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 most to be entirely political in nature.
- cruise missiles
- The V1 had been successfully intercepted during the war, but this did not make the cruise missile concept entirely useless. After the war the US deployed a small number of nuclear armed cruise missiles in Germany, but these were considered to be of limited usefulness. Continued research into much longer ranged and faster versions led to the US's Navaho missile, and its Soviet counterparts, the Burya and Buran. However these were rendered largely useless by the ICBM, and none were used operationally. Instead shorter-range developments have become widely used as highly accurate attack systems, such as the US Tomahawk missile.
- Another major German missile development project was the anti-shipping class, intended to stop any attempt at a cross-channel invasion. However the British were able to render their systems useless by jamming their radios, and missiles with wire guidance were not ready by D-Day. After the war the anti-shipping class slowly developed, and became a major class in the 1960s with the introduction of the low-flying turbojet powered cruise missiles known as "sea-skimmers". These became famous during the Falklands War when Argentine Exocet missiles sank several Royal Navy ships.
- By early 1944 US and British bombers were flying essentially unhindered over the German heartland, and the Luftwaffe became desperate to get some sort of useful ground-based anti-aircraft system into operation. Several systems were under development, but none had reached operational status before the war's end. The US Navy also started missile research to deal with the Kamikaze threat. By 1950 systems based on this early research started to reach operational service, including the US Army's Nike Ajax, the Navy's "3T's" (Talos, Terrier, Tartar), and soon followed by the Soviet SA-2 and SA-2 and French and British systems.
- 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. While their research never reached fruition, the US Navy and USAF used their superior electronics to deliver a number of such designs in the early 1950s, most famous being the 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.
- By the end of WWII all forces had widely introduced unguided rockets using HEAT warheads as their major anti-tank weapon. However these had a limited useful range of a 100m 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.
- Like most missiles, the 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:
- Kinetic Energy Interceptor (KEI)
- Aegis Ballistic Missile Defense System (Aegis BMD) - a SM-3 missile with Lightweight Exo-Atmospheric Projectile (LEAP) Kinetic Warhead (KW)
- anti-satellite weapon (ASAT) - also the proposed Brilliant Pebbles defense system would use 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.
- inertial guidance
- Inertial guidance systems (INS) use a series of gyroscopes to detect acceleration, and electronics integrate the acceleration to solve for the current location. Early mechanical systems were fairly inaccurate, but continued development allowed for dramatic improvements, and the introduction of the ring laser gyro improved that even more. Today's INS systems offer accuracies of less than 100m at intercontenental ranges, allowing shorter range systems to attack point targets like buildings. INS systems are often combined with other systems to allow the missile to fly to the target system, and then the terminal guidance system takes over for "fine tuning".
- Almost all German systems used manual command to line-of-sight, or MCLOS, in which the missile was directed via either radio control by an operator watching both the missile and the target. MCLOS relies on their being a high speed differential between the missile and the target, otherwise manually calculating "lead" becomes difficult. In the case of glide bombs missiles against ships or the supersonic Wasserfall against slow-moving B-17 bombers this system worked fine, but as speeds increased MCLOS was quickly rendered useless for most roles.
- SACLOS, semi-automatic command to line-of-sight, is a modification of MCLOS systems, in which the operator tracks only the target, and electronics calculate the needed "lead". Commands are then sent to the missile as before, either over radio control or wire guidance. SACLOS dramatically reduces the operator workload, typically to simply keeping a piece of equipment pointed at the target. SACLOS is the most common form of guidance against ground targets such as tanks and bunkers.
- active radar homing
- Active radar systems mount a small radar unit in the missile and use it to track targets, typically the "largest return". Radar resolution is based on the size of the antenna, so in a smaller missile these systems are useful for attacking only large targets, ships or large bombers for instance. Active radar systems remain in widespread use in anti-shipping missiles, and in "fire and forget" air-to-air missile systems such as AMRAAM and R-77
- semi-active radar homing
- SARH systems combine a radar receiver on the missile with a radar broadcaster located "elsewhere". Since the missile is typically being launched after the target was detected using a powerful radar system, it makes sense to use that same radar system to track the target, thereby avoiding problems with resolution or power. SARH is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground and air launched.
- contrast seeker
- Contrast seekers use a television camera, typically black and white, to image a field of view in front of the missile, which is presented to the operator. When launched, the electronics in the missile look for the spot on the image where the contrast changes the fastest, both vertically and horizontally, and then attempts to keep that spot at a constant location in its view. Contrast seekers have been used for air-to-ground missiles, including the famous AGM-65 Maverick, because most ground targets can be distinguished only by visual means. However they rely on there being strong contrast changes to track, and even traditional camouflage can render them unable to "lock on".
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