Fire-control system


Fire-control system

:"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 gunner firing a weapon, 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 could work out how far they missed by and in which direction, and this information could 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 torpedo data computers were added to dramatically improve the speed of these calculations.

The Dreyer Table fire control system was already fitted to British capital ships by mid 1916. It was claimed to been plagiarised from earlier work by Arthur Pollen, but given the differences between the two designs this is now disputed. Pollen's own system, the Argo Mark IV was installed on ships of the Imperial Russian Navy. An improved development the "Admiralty Fire Control Table" was in use in 1927.

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.

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. Naval gun fire control potentially involves three levels of complexity. Local control originated with primitive gun installations aimed by the individual gun crews. Director control aims all guns on the ship at a single target. Coordinated gunfire from a formation of ships at a single target was a focus of battleship fleet operations. Corrections are made for surface wind velocity, firing ship roll and pitch, powder magazine temperature, drift of rifled projectiles, individual gun bore diameter adjusted for shot-to-shot enlargement, and 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 [http://www.gwpda.org/naval/usnfirec.htm 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 [http://ahoy.tk-jk.net/GentlemansCordite/AglimpseatNavalGunnery..html] .

The use of Director controlled firing together with the fire control computer moved the control of the gun laying from the individual turrets to a central position; although individual gun mounts and multi-gun turrets may retain a local control option for use when battle damage limits Director information transfer. Guns could then be fired in planned salvos, with each gun giving a slightly different trajectory. Dispersion of shot caused by differences in individual guns, individual projectiles, powder ignition sequences, and transient distortion of ship structure was undesirably large at typical naval engagement ranges. Directors high on the superstructure had a better view of the enemy than a turret mounted sight, and the crew operating it where distant from the sound and shock of the guns.

Unmeasured and uncontrollable ballistic factors like high altitude temperature, humidity, barometric pressure, wind direction and velocity required final adjustment through observation of fall of shot. Visual range measurement (of both target and shell splashes) was difficult prior to availability of RADAR. The British favoured coincident rangefinders while the Germans stereoscopic type. The former were less able to range on an indistinct target but easier on the operator over a long period of use, the latter the reverse.

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.

Aircraft use

An early use of fire-control systems was in bomber aircraft, with the use of computing bombsights that accepted altitude and airspeed information to predict and display the impact point of a bomb released at that time.

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.

By the start of the Vietnam War, a new computerized bombing predictor called the Low-Altitude Bombing System began to be integrated into the systems of aircraft equipped to carry nuclear armaments. This new bomb computer was revolutionary in that the actual release command for the bomb was given by the computer, not the pilot; the pilot designated the target using the radar or other targeting system, then "consented" to release the weapon, and the computer then did so at a calculated "release point" some seconds later. This is very different from previous systems which, though they had also become computerized, still calculated an "impact point" showing where the bomb would fall if the bomb were released at that moment. The key advantage is that the weapon can be released accurately even when the plane is making a maneuver such as a climb or dive. Most bombsights until this time required that the plane maintain a constant attitude (usually level, though dove-bombing sights wre also common).

The LABS system was originally designed to facilitate a tactic called toss bombing, to allow the aircraft to remain out of range of a weapon's blast radius. The principle of calculating the release point, however, was eventually integrated into the Fire Control Computers of later bombers and strike aircraft, allowing level, dive and toss bombing. In addition, as the fire control computer became integrated with ordinance systems, the computer can take the flight characteristics of the weapon to be launched into account.

ee also

* Predicted impact point
* Fire-control radar
* Counter-battery radar

References

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