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 following World War II, 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.Technically, it would be more accurate to use the term "rifle" for long-range ship-board cannon. However, the term "gun" is commonly used and that nomenclature is maintained here.
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 it will roll, pitch, and yaw due to wave action, ship change of direction, and board firing. The rangekeeper also performed the required ballistics calculations associated with firing a gun. This article focuses on US Navy shipboard rangekeepers, but the basic principles of operation are applicable to all rangekeepers regardless of where they were deployed.
Function 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, wind speed and direction, air resistance, gravity, latitude, gun/sight parallax, barrel wear, powder load, and projectile type.
History
Manual fire control
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 . Even during the American Civil War, the famous engagement between the and the was often conducted at less than range. The two ironclads continue circling and firing at ranges varying from 100 yards to a matter of feet. 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 (fall of shot), 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.The increasing range of the guns also forced ships to create very high observation points from which optical rangefinders and artillery spotters could see the battle. The need to spot artillery shells was one of the compelling reasons behind the development of naval aviation and early aircraft were used to spot the naval gunfire points of impact. In some cases, ships launched manned observation balloons as a way to artillery spot. Even today, artillery spotting is an important part of directing gunfire, though today the spotting is often done by unmanned aerial vehicles. For example, during Desert Storm, UAVs spotted fire for the Iowa-class battleships involved in shore bombardment.
Predecessor fire control tools and systems
Between the American Civil War and 1905, numerous small improvements were made in fire control, such as telescopic sights and optical rangefinders. 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, Dumaresq (which was also part of the Dreyer Table), and Argo Clock, but these devices took a number of years to become widely deployed.The reasons were for this slow deployment are complex. As in most bureaucratic environments, institutional inertia and the revolutionary nature of the change required caused the major navies to move slow in adopting the technology. 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 British fleet's performance at Jutland has been a subject of much analysis and there were many contributing factors. When compared to the long-range gunnery performance by the US Navy and Kriegsmarine, the British gunnery performance at Jutland is not that poor. In fact, long-range gunnery is notorious for having a low hit percentage. For example, during exercises in 1930 and 1931, US battleships had hit percentages in the 4-6% range (Jurens).
Power drives and Remote Power Control (RPC)
The US Navy's first deployment of a rangekeeper was on the in 1916. Because of the limitations of the technology at that time, the initial rangekeepers were crude. During World War I, the rangekeepers could 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 and RPC in the Royal 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 approximately 20 servomechanisms, mostly position servos, to minimize torque load on the computing mechanisms. The Royal Navy first installed RPC, experimentally, aboard HMS Champion in 1928. In the 1930s RPC was used for naval searchlight control and during WW2 it was progressively installed on pom-pom mounts and directors, 4-inch, 4.5-inch and 5.25-inch gun mounts.Friedman.
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 with the ability to conduct effective gunfire operations at long range in poor weather and at night.The degree of updating varied by country. For example, the US Navy used servomechanisms to automatically steer their guns in both azimuth and elevation. The Germans used servomechanisms to steer their guns only in elevation, and the British began to introduce Remote Power Control in elevation and deflection of 4-inch, 4.5-inch and 5.25-inch guns in 1942, according to Naval Weapons of WW2, by Campbell. For example s 5.25-inch guns had been upgraded to full RPC in time for her Pacific deployment.
Service in World War II
During World War II, rangekeeper capabilities were expanded to the extent that the name "rangekeeper" was deemed to be inadequate. The term "computer," which had been reserved for human calculators, came 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 during a 1945 test was able to maintain an accurate firing solutionThe rangekeeper in this exercise maintained a firing solution that was accurate within a few hundred yards (or meters), which is within the range needed for an effective rocking salvo. The rocking salvo was used by the US Navy to get the final corrections needed to hit the target. 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 when the engaged the Japanese battlecruiser at a range of at night. The Kirishima was set aflame, suffered a number of explosions, and was scuttled by her crew. She had been hit by nine 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. The Royal Navy began to introduce gyroscopic stabilization of their director gunsights in World War One and by the start of World War Two all warships fitted with director control had gyroscopically controlled gunsights.Appendix one, Classification of Director Instruments, see external links.
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 s directed their last rounds in combat.
Construction
Rangekeepers were very large, and the ship designs needed to make provisions to accommodate them. For example, the Ford Mk 1A Computer weighed
The Mk. 1/1A's mechanism support plates, some were up to thick, were made of aluminum alloy, but nevertheless, the computer is very heavy. On at least one refloated museum ship, the destroyer (now in Boston), the computer and Stable Element more than likely still are below decks, because they are so difficult to remove.
The rangekeepers 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.
These computers also 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 Ford Mark 1/1A mechanism was mounted into a pair of approximately cubical large castings with very wide openings, the latter covered by gasketed castings. 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.
The Mk 47 computer was a radical improvement in accessibility over the Mk 1/1A. 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.
Mechanisms The problem of rangekeeping