User:Ziggle/Draft of Rangekeeping Article

Rangekeepers were electromechanical fire control computers used primarily during the early part of the 20th century. They were sophisticated analog computers whose development reached its zenith during World War II. While rangekeeper technology was used on land, sea, and in the air, the most sophisticated rangekeepers where mounted on warships to direct the fire of long range guns. These computing devices needed to be sophisticated because the problem of calculating gun angles in a naval engagement is a very complex mathematical problem. 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 due to wave action. The rangekeeper also performed the required ballistics 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 performs 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 your 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
 * Firing a long range weapon requires many calculations. The point of projectile impact is a function of many variables, including: gun azimuth, gun elevation, air resistance, gravity, latitude, gun/sight parallax, barrel wear, powder load, and projectile type.

During WWII, all the major warring powers developed rangekeepers to different levels. Rangekeepers were only one member of a class of electromechanical computers used for fire control during World War II. Related analog computing hardware used by the United States included:
 * Norden bombsight
 * US bombers used the Norden bombsight, which used similar technology to the rangekeeper for predicting bomb impact points.


 * Torpedo Data Computer(TDC)
 * US submarines used the 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.

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, now begin 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.
 * M-9/SCR-584 Anti-Aircraft System
 * This equipment was used to direct air defense artillery. It made a particularly good account of itself against the V-1 flying bombs.

The performance of these analog computers was impressive. The battleship North Carolina 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. Warships without this capability were at a severe disadvantage.

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 when the USS Washington engaged the Japanese battlecruiser Kirishima at a range of 18,500 yards at night. The Kirishima was quickly sunk. 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. The wreck of the Kirishima was discovered in 1992 and showed that the entire bow section of the ship was missing. Rangekeepers were very large and the ship designs needed to make provisions to accommodate them. For example, the Ford Mk 1A Computer weighed 3150 lbs. The rangekeepers also required a large number of electric cables over which they received information from the various sensors (e.g. pitometer, rangefinder, gyrocompass) and send 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). In fact, most naval engagements before 1800 were conducted at ranges of 20 to 50 yards. Even during the American Civil War, the famous engagement between the USS Monitor and the CSS Virginia was often conducted at less than 100 yard 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 became available, such as the Dreyer Table, Dumaresq (which was also part of the Dreyer Table), Pollen Clock, and Arago 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. 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) 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 were developed that allowed the guns to automatically steer to the rangekeeper's commands with no manual intervention.

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 last combat action for the analog rangekeepers, at least for the US Navy, was in the 1991 Persian Gulf War 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. Accuracy of battleship guns was ~1 % 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
 * Earth's rotation: While the projectile is in the air, the earth moves underneath it.
 * Coriolis effect: Because the Earth is rotating, there is an apparent force acting on the projectile.
 * Interior ballistics: Guns do wear and this aging must be taken into account.
 * Exterior ballistics: Different projectiles have different ballistic characteristics. Also, air conditions have an effect as well (temperature, wind, air pressure).
 * Parallax correction: In general, the position of the gun and target spotting equipment (radar, pelorus, 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)
 * power charge weight

These issues are so complicated and need to be performed so quickly that the need arose for an automated way of performing these corrections. At first, separate pieces of equipment were developed to handle each problem. For example:


 * Gyrocompass: This device provided a accurate true north own ship course.
 * Rangefinders: Optical devices for determining a projection of the range to a target at the time of projectile impact.
 * Pitometer Logs: This device provide an accurate own ship speed.
 * Range clocks: This device provided an accurate projective time of flight.
 * Plotting board: A map of the gunnery platform and target that allowed predictions to be made as to the future position of a target.
 * Various slide rules: These devices performed the various calculations required to determine the required gun azimuth and elevation.

All these devices were in place prior to World War I. However, they made for a complex fire control situation because there is so much information to be input manually. 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
 * }
 * Initial estimates for rate control
 * Target bearing
 * Automatically from director
 * Spotting data
 * Spotter, by telephone
 * }
 * Spotter, by telephone
 * }

An integrated solution was needed and the first rangekeepers were developed. By World War I, the first rangekeepers were being deployed. During World War II, 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 mechanical mathematical functions are not used often today, mechanical methods exist to implement all the common mathematical operations. Some examples include:


 * addition and subtraction
 * Differential gears were often used to perform addition and subtraction operations. The history of this gearing for computing dates to antiquity (see Antikythera mechanism).
 * multiplication by a constant
 * Gear ratios can be used to multiple a value by a constant.
 * integration
 * Disk and ball integrators (or its variants) performed the integration operation.
 * differentiation
 * Differentiation was performed by using an integrator in a feedback loop.
 * Evaluation of functions
 * Rangekeepers used a large number of cams to generate function values.