Fire-control system

From Wikipedia, the free encyclopedia

Note: the term "fire control" may also refer to means of stopping a fire, such as sprinkler systems.

A fire-control system is a computer, often mechanical, which is designed to assist a weapon system in hitting its target. It performs the same task as a human firing a weapon, for example, an archer or rifleman, but attempts to do so faster and more accurately.

The original fire-control systems were developed for ships. When gunnery ranges increased dramatically in the late 19th century it was no longer a simple matter of calculating the proper aim point given the flight times of the shells. Increasingly sophisticated mechanical calculators were employed for proper gunlaying, typically with various spotters and distance measures being sent to a central plotting station deep within the ship. There the fire direction teams fed in the location, speed and direction of the two ships, as well as various adjustments for Coriolis effect, weather effects on the air, and other adjustments. The resulting directions, known as a firing solution, would then be fed back out to the turrets for laying. If the rounds missed, an observer can work out how far they missed by and in which direction, and this information can be fed back into the computer along with any changes in the rest of the information and another shot attempted.

Submarines were also equipped with fire control computers for the same reasons, but their problem was even more pronounced; in a typical "shot", the torpedo would take several minutes to reach its target. Calculating the proper "lead" given the relative motion of the two ships was very difficult, and computers were added to dramatically improve the speed of these calculations.

By the start of World War II, aircraft altitude performance had increased so much that anti-aircraft guns had similar predictive problems, and were increasingly equipped with fire-control computers. The main difference between these systems and the ones on ships was size and speed. The High Angle Control System, or HACS, of Britain's Royal Navy is an example of a system that was not predictive and was therefore limited in its usefulness. The Kerrison Predictor is an example of a system that was built to solve laying in "real time", simply by pointing the director at the target and then aiming the gun at a pointer it directed. It was also deliberately designed to be small and light, in order to allow it to be easily moved along with the guns it served.

Simple systems, known as lead computing sights also made their appearance inside aircraft late in the war. These devices used a gyroscope to measure turn rates, and moved the gunsight's aim-point to take this into account. The only manual "input" to the sight was the target distance, which was typically handled by dialing in the size of the target's wing span at some known range. Small radar units were added in the post-war period to automate even this input, but it was some time before they were fast enough to make the pilots completely happy with them.

The United States Navy deployed the Mark I Fire Control Computer on many of its vessels constructed during World War II.

Modern fire-control computers, like all high-performance computers, are digital. The added performance allows basically any input to be added, from air density and wind, to wear on the barrels and distortion due to heating. These sorts of effects are noticeable for any sort of gun, and fire-control computers have started appearing on smaller and smaller platforms. Tanks were one early use, automating gun laying using a laser rangefinder and a barrel-distortion meter. Fire-control computers are not just useful for large cannons. They can be used to aim machine guns, small cannons, guided missiles, rifles, grenades, rockets—any kind of weapon which can have its launch or firing parameters varied. They are typically installed on ships, submarines, aircraft, tanks and even on some rifles, for example the Fabrique Nationale F2000. Fire-control computers have gone through all stages of technology that computers have, with some designs being based upon analogue technology and vacuum tubes which were later replaced with transistors.

Fire-control systems are often interfaced with sensors (such as sonar, radar, infra-red search and track, laser range-finders, anemometers, wind vanes, thermometers, etc.) in order to cut down or eliminate the amount of information which has to be manually inputted in order to calculate an effective solution. Sonar, radar, IRST and range-finders can give the system the direction to and/or distance of the target. Alternatively, an optical sight can be provided and an operator can point it at the target, which is easier than having someone input it using other methods and gives the target less warning that it is being tracked. Typically, weapons fired over long ranges need the environmental information—the longer a munition travels, the more the wind, temperature etc. will affect its trajectory, so the more important having accurate information becomes to getting a good solution. Sometimes, for very long-range rockets, environmental data has to be obtained at high altitudes or in between the launching point and the target. Often, satellites or balloons are used to gather this information.

Once the firing solution is calculated, many modern fire-control systems are also able to aim and fire the weapon(s). Once again, this is in the interest of speed and accuracy, and also in the case of a vehicle like an aircraft or tank, in order to allow the pilot/gunner/etc. to perform other actions simultaneously, such as tracking the target or flying the aircraft. Even if the system is unable to aim the weapon itself, for example the fixed cannon on an aircraft, it is able to give the operator cues on how to aim. Typically, the cannon points straight ahead and the pilot must maneuver the aircraft so that it points in the right direction before firing. In most aircraft the aiming cue takes the form of a "pipper" which is projected on the heads-up display (HUD). The pipper shows the pilot where the target must be relative to the aircraft in order to hit it. Once the pilot maneuvers the aircraft so that the target and pipper are superimposed, he or she fires the weapon, or on some aircraft the weapon will fire automatically at this point, in order to overcome the reaction delay of the pilot. In the case of a missile launch, the fire-control computer may give the pilot feedback about whether the target is in range of the missile and how likely the missile is to hit if launched at any particular moment. The pilot will then wait until the probability reading is satisfactorily high before launching the weapon.

[edit] Naval fire control

The situation for naval fire control was more complex because of the need to control the firing of several guns at once. In naval engagements both the firing guns and target are moving, and the variables are compounded by the greater distances and times involved. Corrections are made for temperature, rate of change of range with additional modifications to the firing solution based upon the observation of preceding shots.

Rudimentary naval fire control systems were first developed around the time of World War I. For a description of one, see US Naval Fire Control, 1918.

For the UK, their first system was built before the Great War. At the heart was an analogue computer designed by Frederic Charles Dreyer (to retire later as Admiral Dreyer) that calculated rate of change of range. The Dreyer Table was to be improved and served into the interwar period at which point it was replaced by the Admiralty Fire Control Table, the AFCT described in action here.

[edit] Ground use

Once an engagement has begun, it is also possible for a fire control radar to track incoming fire, trace back the trajectories to their source, and produce the coordinates of an enemy unwise enough to fire ballistic rounds. This return-fire capability has been included in some systems since the 1970s. Returning fire to the location of the rounds' origin is known as counter-battery fire.

See also: Fire-control radar, Counter-battery radar