Gun laying
Gun laying or gun training is the process of aiming an artillery piece or turret, such as a gun, howitzer, or mortar, on land, at sea, or in air, against surface or aerial targets. It may be laying for either direct fire, where the gun is aimed directly at a target within the line-of-sight of the user, or by indirect fire, where the gun is not aimed directly at a target within the line-of-sight of the user. Indirect fire is determined from the information or data that is collected, calculated, and applied to physical coordinates to identify the location of the target by the user. The term includes automated aiming using, for example, radar-derived target data and computer-controlled guns.
Description
Gun laying is a set of actions to align the axis of a gun barrel so that it points in the required direction. Such may be for direct fire, where the target is visible for the gun, or indirect fire, where the target is not be visible from the gun. This alignment is divided into in the horizontal and vertical planes, called:- elevation, sometimes laying – alignment in the vertical plane to range it to the target
- traverse, sometimes training – alignment in the horizontal plane to align it with the target
Laying in the vertical plane uses data derived from trials or empirical experience. For any given gun and projectile types, it reflects the distance to the target and the size of the propellant charge. It also incorporates any differences in height between gun and target. With indirect fire, it may allow for other variables as well.
With direct fire, laying in the horizontal plane is merely the line of sight to the target, although the layer may make allowance for the wind, and with rifled guns the sights may compensate for projectile "drift". With indirect fire the horizontal angle is relative to something, typically the gun's aiming point, although with modern electronic sights it may be a north-seeking gyro.
Depending on the gun mount, there is usually a choice of two trajectories that will result in the shot landing in the same spot. The dividing angle between the trajectories is about 45 degrees, it varies slightly due to gun dependent factors. Below 45 degrees the trajectory is called "low angle", above 45 degrees is "high angle". The differences are that low angle fire has a shorter time of flight, a lower apex, and flatter angle of descent.
All guns have carriages or mountings that support the barrel assembly. Early guns could only be traversed by moving their entire carriage or mounting, and this lasted with heavy artillery into World War II. Mountings could be fitted into traversing turrets on ships, coast defences or tanks. From circa 1900 field artillery carriages provided traverse without moving the wheels and trail.
The carriage, or mounting, also enabled the barrel to be set at the required elevation angle. With some gun mounts it is possible to depress the gun, i.e., move it in the vertical plane to point it below the horizon. Some guns require a near-horizontal elevation for loading. An essential capability for any elevation mechanism is to prevent the weight of the barrel forcing its heavier end downward. This is greatly helped by having trunnions at the centre of gravity, although a counterbalance mechanism can be used. It also means the elevation gear has to be strong enough to resist considerable downward pressure but still be easy for the gun layer to use.
Until recoil systems were invented in the late 19th century and integrated into the gun carriage or mount, guns moved substantially backwards when they fired, and had to be moved forward before they could be laid. However, mortars, where the recoil forces were transferred directly into the ground, did not always require such movement. With the adoption of recoil systems for field artillery, it became normal to pivot the saddle on the lower carriage, initially this "top traverse" was only a few degrees but soon offered a full circle, particularly for anti-aircraft guns. The introduction of recoil systems was an important milestone.
History
Background
The earliest guns were loaded from the muzzle. They were typically little more than bare barrels moved in wagons and placed on the ground for firing, then wooden frames and beds were introduced. Horizontal alignment with the target was by eye, while vertical laying was done by raising the muzzle with timber or digging a hole for the closed end.Gun carriages were introduced in the 15th century. Two large-diameter wheels, axle-tree and a trail became the standard pattern for field use. The barrel was mounted in a wooden cradle with trunnions to mount it on the carriage. As technology improved, the trunnions became part of the barrel and the cradle was abandoned. Nevertheless, they were relatively large and heavy.
Horizontal alignment was a matter of moving the trail. To achieve the required elevation angle, various arrangements were used. At the simplest, it was wedges or quoins between the breech and the trail, but wooden quadrants, or simple scaffolds mounted on the trail, were also used to support the breech and provided larger choice of elevation angle. Screw elevation devices were also used as early as the 16th century.
Image:Grand Turk.jpg|thumb|left|240px|A naval cannon mounted on its gun carriage. The breech rope is visible.
However, naval and some fortress carriages and mounting evolved differently. Field mobility was not required, so large wheels and trails were irrelevant. Headspace below decks was often low. This led to compact carriages, mostly on four small wheels. Obviously, large horizontal traverses were more difficult, but such things were unnecessary when shooting broadside. However, in fortresses wider traverse was required. One solution was platform and slide mountings. Wide traverse was also useful on some shipmounted guns.
Laying required sights. At its simplest, this means nothing more than aiming the guns in the right direction. However, various aids emerged. Horizontal aiming involved sighting along the barrel, this was enhanced by a notch made in the ring around the barrel at the breech end and an 'acorn' on the ring around the muzzle. This was still used in the 19th century in some instances.
The range with a flat trajectory was called 'point blank' range. However, while point blank may have been enough for some purposes, field artillery and guns in fortresses needed longer range. This required ways to measure elevation angles and know the relationship between the elevation angle and the range.
Early mechanical gunnery aids
The first recorded device to measure an elevation angle was Niccolò Tartaglia's invention of a gunners' quadrant circa 1545. This device had two arms at right angles connected by an arc marked with angular graduations. One arm was placed in the muzzle, and a plumb bob suspended against the arc showed the elevation angle. This led to many calculations relating elevation angle to range.The problem was that these calculations assumed what today is called an "in vacuo" trajectory – they made no allowance for air resistance against the projectile. What was needed were range and accuracy trials to determine the actual relationship between range and elevation angle. The practical approach was conducted by William Eldred, Master Gunner at Dover Castle, in gunnery trials in 1613, 1617 and 1622. He used a wide variety of guns, including the culverin, demiculverin, falconet and Saker. From the results of these trials, he produced range tables for elevations up to 10 degrees for each type with a standard propelling charge weight.
A problem affecting gun laying, was the tapered external barrel shape. This affected elevation when the gun was aimed by sighting along the top of the barrel. In the early 17th century, 'dispart sights' compensated for this. This was a piece of metal placed on the muzzle to make the line of sight parallel to the axis of the bore. Another technique involved measuring the depth of the barrel through the touchhole and at the muzzle, the difference being the wedge size needed to compensate for the tapered barrel.
Image:Ballistic pendulum.svg|thumb|300px|left|Ballistic pendulum, invented by Benjamin Robins to calculate muzzle velocity.
The ballistic pendulum was invented in 1742 by English mathematician Benjamin Robins, and published in his book New Principles of Gunnery, which revolutionized the science of ballistics, as it provided the first way to accurately measure the velocity of a bullet.
Robins used the ballistic pendulum to measure projectile velocity in two ways. The first was to attach the gun to the pendulum, and measure the recoil. Since the momentum of the gun is equal to the momentum of the ejecta, and since the projectile was the large majority of the mass of the ejecta, the velocity of the bullet could be approximated. The second, and more accurate method, was to directly measure the bullet momentum by firing it into the pendulum. Robins experimented with musket balls of around one ounce in mass, while other contemporaries used his methods with cannon shot of.
The first system to supplant ballistic pendulums with direct measures of projectile speed was invented in 1808, during the Napoleonic Wars and used a rapidly rotating shaft of known speed with two paper disks on it; the bullet was fired through the disks, parallel to the shaft, and the angular difference in the points of impact provided an elapsed time over the distance between the disks. A direct electromechanical clockwork measure appeared in 1840, with a spring-driven clock started and stopped by electromagnets, whose current was interrupted by the bullet passing through two meshes of fine wires, again providing the time to traverse the given distance.
Tangent sights were introduced in the 19th century. These provided the rear sight used with an 'acorn' or similar foresight at the muzzle. The tangent sight was mounted in a bracket beside or behind the breech, the eyepiece was atop a vertical bar that moved up and down in the bracket. The bar was marked in yards or degrees. This direct-fire sight was aimed at the target by moving the trail horizontally and elevating or depressing the barrel. By the late 19th century the simple open tangent sights were being replaced by optical telescopes on mounts with an elevation scale and screw aligned to the axis of the bore.