Rangekeepers were electromechanical fire control

Fire-control system

A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, 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...

 computers used primarily during the early part of the 20th century. They were sophisticated analog computers whose development reached its zenith following World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, specifically the Computer Mk 47 in the Mk 68 Gun Fire Control system. During World War II, rangekeepers directed gunfire on land, sea, and in the air. While rangekeepers were widely deployed, the most sophisticated rangekeepers were mounted on warships to direct the fire of long-range guns.

These warship-based computing devices needed to be sophisticated because the problem of calculating gun angles in a naval engagement is very complex. In a naval engagement, both the ship firing the gun and the target are moving with respect to each other. In addition, the ship firing its gun is not a stable platform because ships roll, pitch, and yaw

Flight dynamics

Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...

 due to wave action, ship change of direction, and effect of board firing. The rangekeeper also performed the required ballistics

Ballistics

Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...

 calculations associated with firing a gun. This article will focus on US Navy shipboard rangekeepers, but the basic principles of operation are applicable to all rangekeepers regardless of where they are deployed.

A rangekeeper is defined as an analog fire control system that performed three functions:

• Target tracking

The rangekeeper continuously computed the current target bearing. This is a difficult task because both the target and the ship firing (generally referred to as "own ship") are moving. This requires knowing the target's range, course, and speed accurately. It also requires accurately knowing the own ship's course and speed.

• Target position prediction

When a gun is fired, it takes time for the projectile to arrive at the target. The rangekeeper must predict where the target will be at the time of projectile arrival. This is the point at which the guns are aimed.

• Gunfire correction

Directing the fire of a long-range weapon to deliver a projectile to a specific location requires many calculations. The projectile point of impact is a function of many variables, including: gun azimuth, gun elevation

Elevation

The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

, wind speed and direction, air resistance, gravity, latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

, gun/sight parallax

Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

, barrel

Gun barrel

A gun barrel is the tube, usually metal, through which a controlled explosion or rapid expansion of gases are released in order to propel a projectile out of the end at a high velocity....

 wear, powder

Gunpowder

Gunpowder, also known since in the late 19th century as black powder, was the first chemical explosive and the only one known until the mid 1800s. It is a mixture of sulfur, charcoal, and potassium nitrate - with the sulfur and charcoal acting as fuels, while the saltpeter works as an oxidizer...

 load, and projectile

Projectile

A projectile is any object projected into space by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more commonly refers to a weapon....

 type.

During WWII, all the major warring powers developed rangekeepers to different levels.
Rangekeepers were only one member of a class of electromechanical computers

Analog computer

An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...

 used for fire control during World War II. Related analog computing hardware used by the United States included:

• Norden bombsight
  Norden bombsight
  The Norden bombsight was a tachometric bombsight used by the United States Army Air Forces and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars to aid the crew of bomber aircraft in dropping bombs accurately...
  
  

US bombers used the Norden bombsight, which used similar technology to the rangekeeper for predicting bomb impact points.

• Torpedo Data Computer
  Torpedo Data Computer
  The Torpedo Data Computer was an early electromechanical analog computer used for torpedo fire-control on American submarines during World War II . Britain, Germany, and Japan also developed automated torpedo fire control equipment, but none were as advanced as US Navy's TDC...
  
   (TDC)

US submarines used the TDC

Torpedo Data Computer

The Torpedo Data Computer was an early electromechanical analog computer used for torpedo fire-control on American submarines during World War II . Britain, Germany, and Japan also developed automated torpedo fire control equipment, but none were as advanced as US Navy's TDC...

 to compute torpedo launch angles. This device also had a rangekeeping function that was referred to as "position keeping." This was the only submarine-based fire control computer during World War II that performed target tracking. Because space within a submarine hull is limited, the TDC designers overcame significant packaging challenges in order to mount the TDC within the allocated volume.

• M-9/SCR-584 Anti-Aircraft System
  SCR-584 radar
  The SCR-584 was a microwave radar developed by the MIT Radiation Laboratory during World War II. It replaced the earlier and much more complex SCR-268 as the US Army's primary anti-aircraft gun laying system as quickly as they could be produced...
  
  

This equipment was used to direct air defense artillery. It made a particularly good account of itself against the V-1 flying bombs.

During World War II, rangekeeper capabilities were expanded to the point where the name rangekeeper was deemed to be inadequate. The name computer, which had been reserved for human calculators, then began to be applied to the rangekeeper equipment. After World War II, digital computers began to replace rangekeepers. However, components of the analog rangekeeper system continued in service with the US Navy until the 1990s.
The performance of these analog computers was impressive. The battleship

Battleship

A battleship is a large armored warship with a main battery consisting of heavy caliber guns. Battleships were larger, better armed and armored than cruisers and destroyers. As the largest armed ships in a fleet, battleships were used to attain command of the sea and represented the apex of a...

 North Carolina

USS North Carolina (BB-55)

USS North Carolina was the lead ship of her class of battleship and the fourth in the United States Navy to be named in honor of this U.S. state. She was the first new-construction U.S. battleship to enter service during World War II, participating in every major naval offensive in the Pacific...

 during a 1945 test was able to maintain an accurate firing solution on a target during a series of high-speed turns.
It is a major advantage for a warship to be able to maneuver while engaging a target.

Night naval engagements at long range became feasible when radar data could be input to the rangekeeper. The effectiveness of this combination was demonstrated in November 1942 at the Third Battle of Savo Island

Naval Battle of Guadalcanal

The Naval Battle of Guadalcanal, sometimes referred to as the Third and Fourth Battles of Savo Island, the Battle of the Solomons, The Battle of Friday the 13th, or, in Japanese sources, as the , took place from 12–15 November 1942, and was the decisive engagement in a series of naval battles...

 when the USS Washington

USS Washington (BB-56)

USS Washington , the second of two battleships in the North Carolina class, was the third ship of the United States Navy named in honor of the 42nd state. Her keel was laid down on 14 June 1938 at the Philadelphia Naval Shipyard. Launched on 1 June 1940, Washington went through fitting-out before...

 engaged the Japanese

Imperial Japanese Navy

The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...

 battlecruiser

Battlecruiser

Battlecruisers were large capital ships built in the first half of the 20th century. They were developed in the first decade of the century as the successor to the armoured cruiser, but their evolution was more closely linked to that of the dreadnought battleship...

 Kirishima

Japanese battleship Kirishima

was a warship of the Imperial Japanese Navy during World War I and World War II. Designed by British naval engineer George Thurston, she was the third launched of the four Kongō-class battlecruisers, among the most heavily armed ships in any navy when built...

 at a range of 8400 yards (7.7 km) at night. The Kirishima was set aflame, suffered a number of explosions, and was scuttled by her crew. She had been hit by nine 16 inches (406.4 mm) rounds out of 75 fired (12% hit rate).
The wreck of the Kirishima was discovered in 1992 and showed that the entire bow section of the ship was missing.
The Japanese during World War II did not develop radar or automated fire control to the level of the US Navy and were at a significant disadvantage.
Even the British did not adopt gyroscopic stabilization of their guns until quite late in the history of rangekeepers.

Rangekeepers were very large, and the ship designs needed to make provisions to accommodate them. For example, the Ford Mk 1A Computer weighed 3150 pounds (1,428.8 kg)

The Mk. 1/1A's mechanism support plates, some an inch (25 mm) thick, were made of aluminum alloy, but nevertheless, the computer is very heavy. On at least one museum ship, the destroyer USS Cassin Young (now in Boston), the computer and Stable Element more than likely still are below decks, because they are so difficult to remove.

The rangekeepers also required a large number of electrical signal cables for synchro data transmission links over which they received information from the various sensors (e.g. gun director, Pitometer, rangefinder, gyrocompass) and sent commands to the guns.

History

The early history of naval fire control was dominated by the engagement of targets within visual range (also referred to as direct fire

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct fire...

). In fact, most naval engagements before 1800 were conducted at ranges of 20 to 50 yd (18.3 to 45.7 m).
Even during the American Civil War, the famous engagement between the USS Monitor

USS Monitor

USS Monitor was the first ironclad warship commissioned by the United States Navy during the American Civil War. She is most famous for her participation in the Battle of Hampton Roads on March 9, 1862, the first-ever battle fought between two ironclads...

 and the CSS Virginia

CSS Virginia

CSS Virginia was the first steam-powered ironclad warship of the Confederate States Navy, built during the first year of the American Civil War; she was constructed as a casemate ironclad using the raised and cut down original lower hull and steam engines of the scuttled . Virginia was one of the...

 was often conducted at less than 100 yards (91.4 m) range.
With time, naval guns became larger and had greater range. At first, the guns were aimed using the technique of artillery spotting. Artillery spotting involved firing a gun at the target, observing the projectile's point of impact, and correcting the aim based on where the shell was observed to land, which became more and more difficult as the range of the gun increased.

Between the American Civil War and 1905, numerous small improvements, such as telescopic sights and optical rangefinders, were made in fire control. There were also procedural improvements, like the use of plotting boards to manually predict the position of a ship during an engagement. Around 1905, mechanical fire control aids began to become available, such as the Dreyer Table

Frederic Charles Dreyer

Admiral Sir Frederic Charles Dreyer, GBE, KCB was an officer of the Royal Navy who developed a fire control system for British warships...

, Dumaresq

Dumaresq

The Dumaresq is a mechanical calculating device invented around 1902 by Lieutenant John Dumaresq of the Royal Navy.The dumaresq is an analog computer which relates vital variables of the fire control problem to the movement of one's own ship and that of a target ship...

 (which was also part of the Dreyer Table), and Argo Clock, but these devices took a number of years to become widely deployed. These devices were early forms of rangekeepers.

The issue of directing long-range gunfire came into sharp focus during World War I with the Battle of Jutland

Battle of Jutland

The Battle of Jutland was a naval battle between the British Royal Navy's Grand Fleet and the Imperial German Navy's High Seas Fleet during the First World War. The battle was fought on 31 May and 1 June 1916 in the North Sea near Jutland, Denmark. It was the largest naval battle and the only...

. While the British were thought by some to have the finest fire control system in the world at that time, during the Battle of Jutland only 3% of their shots actually struck their targets. At that time, the British primarily used a manual fire control system. The one British ship in the battle that had a mechanical fire control system turned in the best shooting results. This experience contributed to rangekeepers becoming standard issue.

The US Navy's first deployment of a rangekeeper was on the USS Texas (BB-35)

USS Texas (BB-35)

USS Texas , the second ship of the United States Navy named in honor of the U.S. state of Texas, is a . The ship was launched on 18 May 1912 and commissioned on 12 March 1914....

 in 1916. Because of the limitations of the technology at that time, the initial rangekeepers were crude. For example, during World War I the rangekeepers would generate the necessary angles automatically but sailors had to manually follow the directions of the rangekeepers (a task called "pointer following" or "follow the pointer"). Pointer following could be accurate, but the crews tended to make inadvertent errors when they became fatigued during extended battles. During World War II, servomechanisms (called "power drives" in the U.S. Navy) were developed that allowed the guns to automatically steer to the rangekeeper's commands with no manual intervention. The Mk. 1 and Mk. 1A computers contained approx. 20 servomechanisms, mostly position servos, to minimize torque load on the computing mechanisms.
During their long service life, rangekeepers were updated often as technology advanced, and by World War II they were a critical part of an integrated fire control system. The incorporation of radar into the fire control system early in World War II provided ships the ability to conduct effective gunfire operations at long range in poor weather and at night.

The rangekeeper's target position prediction characteristics could be used to defeat the rangekeeper. For example, many captains under long-range gun attack would make violent maneuvers to "chase salvos." A ship that is chasing salvos is maneuvering to the position of the last salvo splashes. Because the rangekeepers are constantly predicting new positions for the target, it is unlikely that subsequent salvos will strike the position of the previous salvo. Practical rangekeepers had to assume that targets were moving in a straight-line path at a constant speed, to keep complexity to acceptable limits. A sonar rangekeeper was built to include a target circling at a constant radius of turn, but that function had been disabled.

The last combat action for the analog rangekeepers, at least for the US Navy, was in the 1991 Persian Gulf War

Gulf War

The Persian Gulf War , commonly referred to as simply the Gulf War, was a war waged by a U.N.-authorized coalition force from 34 nations led by the United States, against Iraq in response to Iraq's invasion and annexation of Kuwait.The war is also known under other names, such as the First Gulf...

 when the rangekeepers on the Iowa class battleships directed their last rounds in combat.

The problem of rangekeeping

long-range gunnery is a complex combination of art, science, and mathematics. There are numerous factors that affect the ultimate placement of a projectile and many of these factors are difficult to model accurately. As such, the accuracy of battleship guns was ~1% of range (sometimes better, sometimes worse). Shell-to-shell repeatability was ~0.4% of range.

Accurate long-range gunnery requires that a number of factors be taken into account:

• Target course and speed
• Own ship course and speed
• Gravity
• Coriolis effect
  Coriolis effect
  In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
  
  : Because the Earth is rotating, there is an apparent force acting on the projectile.
• Internal ballistics
  Ballistics
  Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...
  
  : Guns do wear, and this aging must be taken into account. There are also shot-to-shot variations due to barrel temperature and interference between guns firing simultaneously.
• External ballistics
  External ballistics
  External ballistics is the part of the science of ballistics that deals with the behaviour of a non-powered projectile in flight. External ballistics is frequently associated with firearms, and deals with the behaviour of the bullet after it exits the barrel and before it hits the target.-Forces...
  
  : Different projectiles have different ballistic characteristics. Also, air conditions have an effect as well (temperature, wind, air pressure).
• Parallax
  Parallax
  Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
  
   correction: In general, the position of the gun and target spotting equipment (radar
  Radar
  Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
  
  , mounted on the gun director, pelorus
  Pelorus (instrument)
  In appearance and use, a pelorus resembles a compass or compass repeater, with sighting vanes or a sighting telescope attached, but it has no directive properties. That is, it remains at any relative direction to which it is set. It is generally used by setting 000° at the lubber's line. Relative...
  
  , etc) are in different locations on a ship. This creates a parallax error for which corrections must be made.
• projectile characteristics (e.g. ballistic coefficient
  Ballistic coefficient
  In ballistics, the ballistic coefficient of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration—a high number indicates a low negative acceleration. BC is a function of mass, diameter, and drag coefficient...
  
  )
• powder charge weight and temperature

These issues are so complicated and need to be performed so quickly that the need arose for an automated way of performing these corrections. Part of the complexity came from the amount of information that must be integrated from many different sources. For example, information from the following sensors, calculators, and visual aids must be integrated to generate a solution:

• Gyrocompass
  Gyrocompass
  A gyrocompass­ is a type of non-magnetic compass which bases on a fast-spinning disc and rotation of our planet to automatically find geographical direction...
  
  : This device provided an accurate true north
  True north
  True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid north...
  
   own ship course.
• Rangefinder
  Rangefinder
  A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure ; others measure distance using trigonometry...
  
  s: Optical devices for determining the range to a target.
• Pitometer Logs
  Pitometer log
  Pitometer logs are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines...
  
  : These devices provided an accurate measurement of the own ship's speed.
• Range clocks: These devices provided a prediction of the target's range at the time of projectile impact if the gun was fired now. This function could be considered "range keeping".
• Angle clocks: This device provided a prediction of the target's bearing at the time of projectile impact if the gun was fired now.
• Plotting board
  Plotting board
  A plotting board was a mechanical device used by the U.S. Coast Artillery to track the observed course of a target , project its future position, and derive the uncorrected data on azimuth and range needed to direct the fire of the guns of a battery to hit that target...
  
  : A map of the gunnery platform and target that allowed predictions to be made as to the future position of a target. (The compartment ("room") where the Mk.1 and Mk.1A computers was located was called "Plot" for historical reasons.)
• Various slide rule
  Slide rule
  The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...
  
  s: These devices performed the various calculations required to determine the required gun azimuth and elevation
  Elevation
  The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
  
  .
• Meteorological sensors: Temperature
  Temperature
  Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
  
  , wind speed
  Wind speed
  Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
  
  , and humidity
  Humidity
  Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
  
   all have an effect on the ballistics of a projectile. U.S. Navy rangekeepers and analog computers did not consider different wind speeds at differing altitudes.

To illustrate the complexity, Table 1 lists the types of input for the Ford Mk 1 Rangekeeper (ca 1931).

{| class="wikitable" style="background:#FFF8DC"

|+ Table 1: Manual Inputs Into Pre-WWII Rangekeeper
|-
| style="background:#DEB887; color:#800000; width:50pt"|Variable
| style="background:#DEB887; color:#800000; width:200pt"|Data Source
|-
| Range
| Phoned from range finder
|-
|Own ship course
|Gyrocompass repeater
|-
|Own ship speed
|Pitometer log
|-
|Target course
|Initial estimates for rate control
|-
|Target speed
|Initial estimates for rate control
|-
|Target bearing
|Automatically from director
|-
|Spotting data
|Spotter, by telephone
|}

An integrated solution was needed and the first rangekeepers were developed. The ultimate solution also included the automated steering of the guns to the proper azimuth and elevation through the use of servomechanism

Servomechanism

thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...

s. The first rangekeepers were being deployed during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. During World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, many types of rangekeepers were in use on many types of warships.

Implementations

The implementation methods used in analog computers were many and varied. The fire control equations implemented during World War II on analog rangekeepers are the same equations implemented later on digital computers. The key difference is that the rangekeepers solved the equations mechanically. While mathematical functions are not often implemented mechanically today, mechanical methods exist to implement all the common mathematical operations. Some examples include:

• Addition
  Addition
  Addition is a mathematical operation that represents combining collections of objects together into a larger collection. It is signified by the plus sign . For example, in the picture on the right, there are 3 + 2 apples—meaning three apples and two other apples—which is the same as five apples....
  
   and Subtraction
  Subtraction
  In arithmetic, subtraction is one of the four basic binary operations; it is the inverse of addition, meaning that if we start with any number and add any number and then subtract the same number we added, we return to the number we started with...
  
  

Differential gears, usually referred to by technicians simply as "differentials", were often used to perform addition and subtraction operations. The Mk. 1A contained apporoximately 160 of them. The history of this gearing for computing dates to antiquity (see Antikythera mechanism

Antikythera mechanism

The Antikythera mechanism is an ancient mechanical computer designed to calculate astronomical positions. It was recovered in 1900–1901 from the Antikythera wreck. Its significance and complexity were not understood until decades later. Its time of construction is now estimated between 150 and 100...

).

• Multiplication
  Multiplication
  Multiplication is the mathematical operation of scaling one number by another. It is one of the four basic operations in elementary arithmetic ....
  
   by a Constant

Gear ratios were very extensively used to multiply a value by a constant.

• Multiplication of two variables

The Mk. 1 and Mk.1A computer multipliers were based on the geometry of similar triangles.

• Sine/cosine generation

These mechanisms would be called resolvers, today; they were called "component solvers" in the mechanical era. In most instances, they resolved an angle and magnitude (radius) into sine and cosine components, with a mechanism based on the Scotch yoke in steam-engine technology, but with a variable crankpin radius, so to speak.

• Integration

Disk and ball integrators (or its variants) performed the integration operation. As well, four small Ventosa integrators in the Mk. 1 and Mk. 1A computers scaled rate-control corrections according to angles.

• Differentiation
  Derivative
  In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
  
  

Differentiation was performed by using an integrator

Integrator

An integrator is a device to perform the mathematical operation known as integration, a fundamental operation in calculus.The integration function is often part of engineering, physics, mechanical, chemical and scientific calculations....

 in a feedback loop.

• Evaluation of Functions

Rangekeepers used a a number of cams to generate function values. For surface fire control (The Mk. 8 Range Keeper), a single flat cam was sufficient to define ballistics, but in the Mk. 1 and Mk 1A computers, four three-dimensional cams were needed. Many face cams (flat discs with wide spiral grooves) were used in both rangekeepers.

A note on the servomechanisms used on the Mk.1 and Mk.1A computers: These were electromechanical, using reversible two-phase capacitor-run induction motors and tungsten contacts. They were stabilized primarily by rotary magnetic drag (eddy-current) slip clutches, like high-torque versions of classical rotating-magnet speedometers. One part of the drag was geared to the motor, and the other was constrained by a fairly stiff spring. The latter part offset the null position of the contacts by an amount proportional to motor speed, thus providing velocity feedback. Flywheels mounted on the motor shafts, but coupled by magnetic drags, prevented contact chattering when the motor was at rest. Unfortunately, they also must have slowed down the servos somewhat.
A more-elaborate scheme, which placed a rather-large flywheel and differential between the motor and the magnetic drag, removed velocity error for critical data, such as gun orders.

The Mk. 1 and Mk. 1A computers used a motor with its speed regulated by a clock escapement, cam-operated contacts, and a jeweled-bearing spur-gear differential to drive the integrator discs. Although its speed cycled slightly, the total inertia made it effectively a constant-speed motor. At each tick, contacts switched on motor power, then the motor opened the contacts again. It was in effect slow pulse-width modulation of motor power according to load. When running, the computer had a unique sound as motor power was switched on and off at each tick—dozens of gear meshes inside the cast-metal computer housing spread out the ticking into a "chunk-chunk" sound.

Some notes on the computing mechanisms

These computers had to be formidably rugged, partly to withstand the shocks created by firing the ship's own guns, and also to withstand the effects of hostile enemy hits to other parts of the ship. They not only needed to continue functioning, but also stay accurate.

The Mk. 1/1A mechanism was mounted into a pair of approximately-cubical large castings with very wide openings, the latter covered by gasketed castings. (See the Ford Instrument Company museum site, listed below under "See Also".) Individual mechanisms were mounted onto thick aluminum-alloy plates, and along with interconnecting shafts, were progressively installed into the housing. Progressive assembly meant that future access to much of the computer required progressive disassembly.

A Navy Ordnance Pamphlet (OP) (OP number[s] needed!), actually a two-volume book with several hundred pages and several hundred excellent photographs, described in great detail how to dismantle and reassemble. (That OP was a treasure!) When reassembling, shaft connections between mechanisms had to be loosened and the mechanisms mechanically moved so that an output of one mechanism was at the same numerical setting (such as zero) as the input to the other. OP 1140, cited below, gives specific procedures, but these perhaps were superseded. Most fortunately, these computers were especially well-made, and very reliable.

The mechanisms were interconnected by rotating shafts mounted in ball bearings fitted into brackets fastened to the support plates. Just about every corner was a right angle, and nearly all were done by miter gears (1:1 ratio). Contrast this with the ease of running a wire carrying data, or having a copper trace on a circuit board.

The Mk 47 computer was a radical improvement in accessibility. It was more akin to a tall, wide storage cabinet in shape, with most or all dials on the front vertical surface. Its mechanism was built in six sections, each mounted on very heavy-duty pull-out slides. Behind the panel were typically a horizontal and a vertical mounting plate, arranged in a tee. (Of course, the computer was mounted so the slides moved fore and aft! They were heavy, and a ship rolls much more off the vertical than it pitches.)

There were rotating shafts to interconnect the six sections, by way of shafts inside the back of the cabinet. However, it was not necessary to adjust the connection as described above for the Mk. 1/1A. Shrewd design meant that the data carried by these shafts had no boundaries. Only their movement was what mattered. One such sort of data could be the aided-tracking output from an integrator roller. When a section was put back into normal position, the shaft couplings apparently mated as soon as the shafts rotated.

Typical mechanisms in the Mk. 1/1A were lots of miter-gear differentials, a group of four 3-D cams, some disk-ball-roller integrators, and servo motors with their associated mechanism; all of these had bulky shapes. However, most of the computing mechanisms were thin stacks of wide plates of various shapes and functions. A given mechanism might be an inch (25 mm) thick, possibly less, and more than a few were maybe 14 inches (35.6 cm) across. Thinness meant that they took up less space, while width permitted a total range of movement much greater than slight looseness in sliding parts; that width enhanced accuracy.

The Mk. 47 had gears and shafts, differentials (although fewer), totally-enclosed disk-ball-roller integrators, but no mechanical multipliers or resolvers ("component solvers"); they were electrical. (Precision potentiometers did the multiplying.) It was an hybrid, doing some computing electrically, and the rest mechanically.

In the Mk. 1/1A, however, except for the electrical (not electronic) servos, all computing was mechanical. For a truly excellent and possibly very interesting set of illustrations and explanations, see Chapter 2 of the Navy manual OP 1140, cited below under "See Also".

The integrators had rotating discs and a full-width roller mounted in a hinged casting (!), pulled down toward the disc by two strong springs. Twin balls permitted free movement of the radius input with the disk stopped, something done at least daily for static tests. Integrators were made with discs of 3, 4 and 5 inch (7.6, 10 and 12.5 cm) diameters, the larger being more accurate. Ford Instrument Company integrators had a clever mechanism for minimizing wear when the ball-carrier carriage was in one position for extended periods.

Resolvers, called "component solvers" back then, did polar-to-rectangular conversion. One input was an angle, and the other, the magnitude, expressed as a radius.

Steam enthusiasts know of the Scotch yoke, and a common type of this resolver mechanism could be described as crossed Scotch yokes at 90 degrees, with a variable-radius crankpin.

Component integrators were essentially Ventosa integrators. all enclosed. Think of a traditional heavy-ball computer mouse and its pickoff rollers at right angles to each other. Underneath the ball is a roller that turns to rotate the mouse ball. However, the shaft of that roller can be set to any angle you want. In the Mk. 1/1A, a rate-control correction (keeping the sights on target) rotated the ball, and the two pickoff rollers at the sides distributed the movement appropriately according to angle. That angle depended upon the geometry of the moment, such as which way the target was heading.

Three-dimensional cams for ballistic computation rotated on their axis for one input. The other input moved a ball follower along the length of the cam.

The four cams in the Mk. 1/1A computer provided mechanical time fuse setting, time of flight (this time is from firing to bursting at or near the target), time of flight divided by predicted range, and superelevation combined with vertical parallax correction. (Superelevation is essentially the amount the gun barrel needs to be raised to compensate for gravity drop.)

Of course, inside the computer were many synchros, both to receive and to send data.

Some minor corrections and other notes:

The specifications stated (on the order of 100 A) for current consumption of the Mk. 1 are vastly more than what's required for normal computer operation. It's likely that the servos and time motor could be powered by a 15-ampere 115 V 60 Hz circuit. Apparently, the stated figure was worst-case for planning purposes, and probably referred to current drawn by all synchro transmitters with their receiver rotors locked in worst-case misalignment, or their stator leads short-circuited. (Computer power was the same as ordinary utility power in the USA, 115 V 60 Hz.)

Another item of confusion is that the discs in the integrators are not nearly as thick as one inch (25 mm), or even a half inch (12.5 mm); they are more like 1/4 in thick. (They are also removable and reversible should they become worn).

Some National Archives photos of the insides of the Mk. 1 are rotated a quarter turn; the top of the computer is to the left, on at least one. The flywheels for the servos, and those with minimal velocity error are easy to see (when you know what to look for!).

See also

• Director (military)
  Director (military)
  A director, also called an auxiliary predictor, is a mechanical or electronic computer that continuously calculates trigonometric firing solutions for use against a moving target, and transmits targeting data to direct the weapon firing crew....
  
  
• Gun data computer
  Gun Data Computer
  The gun data computer is a series of artillery computers used by the U.S. Army, for coastal artillery, field artillery, and antiaircraft artillery applications...
  
  
• Fire-control system
  Fire-control system
  A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, 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...
  
  
• Kerrison Predictor
  Kerrison Predictor
  The Kerrison Predictor was one of the first fully automated anti-aircraft fire-control systems. The predictor could aim a gun at an aircraft based on simple inputs like the observed speed and the angle to the target...
  
  

External links

• USN Report on IJN Technology
• Excellent article on the performance of long-range gunnery between the World Wars.
• British fire control
• British fire control expert
  Frederic Charles Dreyer
  Admiral Sir Frederic Charles Dreyer, GBE, KCB was an officer of the Royal Navy who developed a fire control system for British warships...
  
  
• Ford Instrument Company museum site. Ford built rangekeepers for the US Navy during World Wars I and II
• OP1140, a superb Navy manual. Chapter 2 has many fine illustrations and clearly-written text.