Fire-control system

• 1. Introduction

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  A fire-control system does the job of a human gunner, but it tries to do it faster and more accurately. It is an integrated system of components that helps a gunner fire with full or semi automation. Before 1800, most naval engagements were fought at ranges of just 20 to 50 yards. A gunner could practically see the whites of the enemy's eyes. Within a century, that intimacy was gone. Guns could throw shells so far that the hardest part was simply seeing where they were aimed. How does a machine learn to hit a moving target from a moving platform, across miles of open water, in the dark? The answer runs from the deck of a battleship to the cockpit of a bomber, from a fort guarding an American harbor to the targeting computer inside a modern rifle's grenade launcher.

• 2. The Range Problem At Sea

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  Rifled guns firing explosive shells changed the geometry of naval combat in the late 19th century. These shells were lighter in relative weight than all-metal balls, and the guns hurled them far enough that the central difficulty became aiming while the ship rolled on the waves. The gyroscope solved that. It corrected the ship's motion and delivered sub-degree accuracies, freeing guns to grow without limit. By the 1890s they had surpassed 10 inches in calibre.

  The steam turbine made the problem harder again. A capital ship driven by a reciprocating engine managed perhaps 16 knots, but the first large turbine ships could exceed 20 knots. Now a target could travel several ship lengths between the firing of a shell and its landing. Eyeballing the aim no longer worked.

  Naval gun fire control sorted itself into three levels of complexity. Local control came from individual gun crews aiming their own weapons. Director control pointed every gun on the ship at one target. Coordinated gunfire pulled a whole formation of ships onto a single target, a goal of battleship fleet operations. The math behind a firing solution had to account for surface wind velocity, the firing ship's roll and pitch, powder magazine temperature, the drift of rifled projectiles, and even the gradual enlargement of each gun's bore from shot to shot.

  The earliest method was artillery spotting. A gunner fired, watched where the projectile fell, and corrected from there. As gun ranges stretched, that fall of shot grew harder and harder to see, which pushed shipbuilders to mount ever higher masts so observers could spot the target at all.

• 3. Rival Inventors Of The Rangekeeper

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  Arthur Pollen and Frederic Charles Dreyer each independently built the first true fire-control systems. Pollen took up the problem after watching the poor accuracy of naval artillery at a gunnery practice near Malta in 1900. Lord Kelvin, regarded as Britain's leading scientist, had first proposed using an analogue computer to solve the equations created by the relative motion of two ships and the time delay of a shell in flight.

  Pollen wanted a combined mechanical computer paired with an automatic plot of ranges and rates. He built a plotting unit to capture the target's position and relative motion, then added a gyroscope to allow for the yaw of his own ship. The primitive gyroscope of the day needed heavy development to give continuous, reliable guidance. Trials in 1905 and 1906 failed, yet they showed promise. Pollen drew encouragement from Admiral Jackie Fisher, Admiral Arthur Knyvet Wilson, and John Jellicoe, then the Director of Naval Ordnance and Torpedoes.

  Around 1905, mechanical aids such as the Dreyer Table, the Dumaresq, and the Argo Clock began to appear, though they took years to spread widely. These were early rangekeepers. Dreyer's rival group designed a similar system, and the Royal Navy ordered both for new and existing ships. The Navy ultimately favored the Dreyer system in its definitive Mark IV form. With director control added, a full and practicable fire control system was ready for World War I, and most Royal Navy capital ships carried one by mid 1916. That arrangement was later replaced by the improved Admiralty Fire Control Table for ships built after 1927.

• 4. Inside The Battleship

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  Twenty-seven crew packed the Transmitting Station that held the Dreyer table for the main guns of HMS Hood. Even with heavy mechanization, fire control still demanded a large human element. In a typical World War II British ship, the system linked the gun turrets to the director tower and to an analogue computer buried in the heart of the vessel.

  In the director tower, operators trained their telescopes on the target. One telescope measured elevation, the other bearing, while rangefinder telescopes on a separate mounting measured distance. The Fire Control Table converted these readings into bearings and elevations for the guns. In the turrets, gunlayers matched their elevation to a transmitted indicator, a turret layer did the same for bearing, and when everything lined up the guns fired centrally.

  Director-controlled firing, working with the fire control computer, moved gun laying out of the individual turrets and into a central position. Turrets kept a local control option, a simpler version the Royal Navy called turret tables, for moments when battle damage cut off director information. Guns could then fire in planned salvos, each barrel given a slightly different trajectory to spread the dispersion caused by gun-to-gun and shell-to-shell differences. Perched topmost on the superstructure, the gun directors gave their crews a better view than any turret sight, and kept them away from the sound and shock of the guns. The protruding ends of their optical rangefinders gave them a distinctive look.

  That height came at a cost. Directors were largely unprotected, because heavy armour was hard to carry so high on a ship. Even armour that stopped a shot would let the impact knock the delicate instruments out of alignment. The British favored coincidence rangefinders, while the Germans preferred the stereoscopic type, a trade between an instrument easier on the operator and one better at ranging an indistinct target.

• 5. Radar In The Dark

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  November 1942, at the Third Battle of Savo Island, proved what radar could do for naval gunnery. An American warship engaged the Japanese battleship Kirishima at a range of 8400 yards at night. Kirishima caught fire, suffered a series of explosions, and was scuttled by her own crew. She had taken at least nine 16-inch rounds out of 75 fired, a 12% hit rate. When her wreck was discovered in 1992, the entire bow section was gone.

  Feeding radar data into the rangekeeper made long-range night engagements feasible for the first time. The incorporation of radar early in World War II let ships fire effectively at long range in poor weather and after dark. The Japanese never developed radar or automated fire control to the level of the US Navy, leaving them at a significant disadvantage.

  The analogue computer's performance could be remarkable. During a 1945 test, a battleship held an accurate firing solution on a target through a series of high-speed turns. Maneuvering while engaging is a major advantage for a warship.

  Submarines carried fire control computers too, and their problem was sharper still. A torpedo might take one to two minutes to reach its target, which made calculating the proper lead between two moving vessels very difficult. Torpedo data computers were added to speed those calculations dramatically. After the war, gun turrets grew increasingly unmanned through the 1950s, laid remotely from the ship's control centre using radar and other inputs. The last combat action for the US Navy's analogue rangekeepers came in the 1991 Persian Gulf War.

• 6. Sights In The Sky

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  The Norden bombsight was the best known American device of an early airborne use of fire control. Computing bombsights took in altitude and airspeed, then predicted and displayed where a bomb released at that instant would strike. Late in World War II, simpler lead computing sights arrived as gyro gunsights. A gyroscope measured turn rates and shifted the aim-point through a reflector sight, leaving the gunner only one manual input, the target distance, set by dialing in the target's wing span at a known range. Small post-war radar units began to automate even that, though they took time to satisfy pilots. The B-29 carried the first centralized fire control system in a production aircraft.

  By the start of the Vietnam War, the Low Altitude Bombing System, or LABS, was being built into aircraft equipped to carry nuclear armaments. This computer gave the release command itself rather than leaving it to the pilot. The pilot designated the target with radar or another system, consented to release, and the computer let the weapon go at a calculated release point some seconds later. Earlier systems, even computerized ones, only showed an impact point for release at that moment. The key advantage was accurate release even while the plane maneuvered.

  LABS was first designed for toss bombing, a tactic meant to keep the aircraft clear of a weapon's blast radius. The release-point principle then spread into the fire control computers of later bombers and strike aircraft, enabling level, dive, and toss bombing alike. As these computers merged with ordnance systems, they could fold in the flight characteristics of the weapon being launched.

• 7. Guns On The Ground

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  The Kerrison Predictor solved laying in real time. An operator simply pointed the director at the target, then aimed the gun at a pointer it directed. It was deliberately built small and light so it could move alongside the guns it served. By the start of World War II, aircraft flew high enough that anti-aircraft guns faced the same predictive trouble as ships, and increasingly carried fire-control computers of their own. The difference from naval systems was size and speed.

  Britain's early High Angle Control System, or HACS, predicted on the assumption that a target's speed, direction, and altitude held constant through the prediction cycle, the time to fuze the shell plus its flight time. The US Navy's Mk 37 made similar assumptions but could allow for a constant rate of altitude change.

  The radar-based M-9/SCR-584 system directed air defense artillery from 1943. The MIT Radiation Lab's SCR-584 was the first radar with automatic following, and Bell Laboratory's M-9 was an electronic analogue computer that replaced hard-to-build mechanical ones like the Sperry M-7. Paired with the VT proximity fuze, the system shot down V-1 cruise missiles with fewer than 100 shells per plane, where earlier anti-aircraft systems needed thousands. It defended both London and Antwerp against the V-1.

  In the United States Army Coast Artillery Corps, fire control developed from the end of the 19th century through World War II. Multiple observation or base end stations found and tracked targets attacking American harbors, then passed data to plotting rooms where analogue devices like the plotting board estimated positions and derived firing data. The forts bristled with armament, from 12-inch coast defense mortars and 3-inch and 6-inch mid-range guns to 10-inch and 12-inch barbette and disappearing carriage guns, 14-inch railroad artillery, and 16-inch cannon. A system of time interval bells rang across each harbor defense to keep the data flowing on a fixed schedule. Only later in the war did electro-mechanical gun data computers tied to coast defense radars begin replacing optical observation, with manual methods kept as a backup through the end of the war.

• 8. The Digital Pipper

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  A pipper projected on a heads-up display now shows a pilot exactly where the target must sit, relative to the aircraft, to score a hit. Modern fire-control computers are digital, like all high-performance computers, and that power lets almost any input join the calculation, from air density and wind to barrel wear and heat distortion. These effects matter for any gun, and the computers have shrunk onto ever smaller platforms.

  Tanks were an early home for automated gun laying, drawing on a laser rangefinder and a barrel-distortion meter. Fire-control computers now aim machine guns, small cannons, guided missiles, rifles, grenades, and rockets, any weapon whose firing parameters can vary. One sits in the grenade launcher built for the Fabrique Nationale F2000 bullpup assault rifle. These computers passed through the same eras as all computers, from analogue designs to vacuum tubes to transistors.

  Sensors feed the modern system so operators enter less by hand. Sonar, radar, infra-red search and track, laser range-finders, anemometers, wind vanes, thermometers, and barometers all supply direction, distance, or environmental data. For very long-range rockets, that environmental data may need gathering at high altitudes or between launch and target, sometimes by satellites or balloons. Once a firing solution is set, many systems aim and fire the weapons themselves, freeing a pilot or gunner to fly or track at the same time. When a pilot maneuvers the aircraft until target and pipper are superimposed, the weapon fires, on some aircraft automatically, to beat the delay of the human hand. For a missile launch, the computer can tell the pilot whether the target is in range and how likely a hit is, so the pilot waits for a satisfactorily high probability before launching.