Gun laying

Gun laying is the process of aiming an artillery piece, such as a gun, howitzer or mortar on land, or at sea, against surface or air targets. It may be laying for direct fire, where the gun is aimed similarly to a rifle, or indirect fire, where firing data is calculated and applied to the sights. The term includes automated aiming using, for example, radar-derived target data and computer-controlled guns.

Gun laying means moving the axis of the bore of the barrel in two planes, horizontal and vertical. A gun is traversed –  rotated in a horizontal plane – to align it with the target, and elevated – moved in the vertical plane – to range it to the target.

Description
Gun laying is a set of actions to align the axis of a gun barrel so that it points in the required direction. This alignment is in the horizontal and vertical planes. Gun laying may be for direct fire, where the layer sees the target, or indirect fire, where the target may not be visible from the gun. Gun laying has sometimes been called "training the gun".

Laying in the vertical plane (elevation angle) 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. 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" (or lower register), above is "high angle" (or upper register). The differences are that low angle fire has a shorter time of flight, a lower vertex and flatter angle of descent.

All guns have carriages or mountings that support the barrel assembly (called the ordnance in some countries). 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 (around which the elevating mass rotates vertically) 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 (or water, if mounted on a ship), 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.

Before recoil systems
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.

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. However, various arrangements were used to achieve the required elevation angle. 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.

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. An early and significant example was the two-gun turret, containing smoothbore muzzle loaders mounted on the foredeck of USS Monitor launched in 1862.

Laying requires 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 (whether mobile or static) 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.

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.

An example of the practical approach are the trials conducted by William Eldred, Master Gunner at Dover Castle, in 1613, 1617 and 1622 with various types of guns (Whole culverin, Demy culverin, Saker & Falcon). From this he produced range tables for elevations up to 10 degrees for each type with a standard propelling charge weight.

A gun laying issue was the tapered external barrel shape, which 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. 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.

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 bedside or behind the breech, the eyepiece (a hole or notch) 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.

Technical advances in gun design and field gunnery
Breech loading, and sometimes rifled, guns have obvious attractions over muzzle loaders. There were many developed from the 15th century onwards; some proved modestly successful, such as early-17th-century Russian guns. They suffered from the problem of fouling created when gunpowder burns. William Armstrong's rifled breech-loading design in the 1850s held promise, and starting in 1860, the British horse and field artillery was almost completely equipped with such guns, but changed to rifled muzzle loaders in 1870. None of this had any impact on gun laying.

However, in 1872 a Russian engineer, Vladimir Stepanovich Baranovsky, invented a 2.5-inch rapid-firing gun. It had a screw breech, a self-cocking firing mechanism and fired a fixed round (shell and cartridge case together). Most notably, it had a simple recoil mechanism where the barrel recoil was absorbed by hydraulic cylinders and then the barrel was returned to its firing position by a spring that had stored some of the recoil energy. This meant the gun did not have to be repositioned after each time it was fired. It also saw the return of the cradle after some 400 years. The cradle now contained the recoil mechanism and was fitted with trunnions. However, the Russian development appears not seem to have been well known, and it was some 20 years later when the French 75mm appeared that recoil systems started to become normal. By this time smokeless powder was replacing gunpowder as the standard propellant.

Direct-fire sighting arrangements also evolved. Rocking-bar sights replaced tangent sights, and telescopes mounted on the trunnion (introduced with the new breech loading guns in the 1880s). Early in the 20th century rocking-bar (or 'bar and drum') sights were introduced. These had an elevation scale, could mount a telescope as well as the open sight, and provided a small amount of horizontal deflection. These provided 'independent line of sight' because they enabled data to be set on the mount and the telescope (or open sight) aimed at the target independent of the barrel elevation. A related problem, particularly for large and longer range guns such as the British 60-pr, was that the wheels could be at different heights due to the slope of the ground. This caused inaccuracy, so 'oscillating' sight mounts were introduced. These mounts could be cross-leveled, which removed the need for the gun commander to calculate a deflection correction for uneven wheels. Cross-leveling introduced the third axis into laying.

The most significant advance in gun laying also originated in Russia. In 1882 Lieutenant Colonel Karl Georgiyevich Guk wrote a book Indirect Fire for Field Artillery. Among other important details this described the modern geometry for indirect fire. This involved orienting the guns of a battery so that their barrels were parallel in a known azimuth and recording the angle from each gun to an aiming point(s). By determining the angle between a target and the known azimuth of the guns they could be laid in the horizontal plane; elevation laying was, in principle, unchanged once the distance from gun to target had been determined.

It should be noted that indirect fire was not entirely new. Positioning guns out of sight of their target, then placing an aiming post in the line between gun and target and aiming horizontally at the aiming post with an elevation to reach the target was a longstanding but not widespread practice. It was used by the Russians at Paltsig in 1759. and possibly by the Burgundians in 1534.

Limited indirect fire was possible with late-19th-century direct-fire telescopes, because their sight mounts provided a few degrees of deflection, and this could be used with an aiming post. This limited deflection could be extended by using a gun-arc to provide a foresight. This was adopted by British howitzers for indirect fire in the Second Anglo-Boer War 1899–1902.

However, using an aiming post or point in front of the gun limited indirect-fire capability; it needed a horizontal plane sight that could have its aimpoint in any direction. This was invented in the early 1890s in Germany – the Richtfläche or 'lining plane', these were open sights and had a scale in degrees or grads (360, 400 or 600 to a circle). Optical sights appeared in the first years of the 20th century, and the German Goerz panoramic sight became the pattern for the rest of the 20th century. They were graduated in degrees and 5 minute intervals, decigrads or mils (4320, 4000 or 6000/6300/6400 to a circle).

More precise horizontal angles and the gun that did not move (helped by spade(s) that dug into the ground) meant that moving the trail was too slow for small movements and insufficiently accurate. Therefore gun design changed to provide 'top-traverse'. Instead of the cradle trunnions fitting to the fixed carriage they were mounted in a rotating saddle fitted to the trail. The layer had a hand wheel connected to gearing that traversed the saddle. Another hand wheel and gearing elevated the cradle.

A feature of 20th-century laying was the use of one- or two-man laying. The US was notable for using two-man laying, horizontal on one side of the gun, elevation on the other. Most other nations mostly used one-man laying. The laying drill, dealing with all three axes, typically adopted this sequence: "roughly for line, roughly for elevation, cross-level, accurately for line, accurately for elevation".

The other main difference in sighting arrangements was the use of an elevation angle or alternatively the range. This issue became more complicated in World War I when the effects of barrel wear in changing muzzle velocity were fully recognised. This meant that different guns needed a different elevation angle for the same range. This led many armies to use an elevation angle calculated in a battery command post. However, in the 1930s the British adopted calibrating sights in which range was set on the sight, which automatically compensated for the difference of muzzle velocity from standard. An alternative to this was a 'gun rule' at each gun; in this case the range was set on the rule and an elevation angle read and given to the layer to set on the sight. The issue was finally resolved by the introduction of digital computers in the battery command post that calculated the correct elevation angle for the range and muzzle velocity accurately and quickly.

Apart from calibrating sights, there was no significant difference in field artillery laying arrangements for most of the 20th century. However, in the 1990s new or modified guns started adopting digital sights, following their successful use in the multi-launch rocket system developed in the 1970s. In these the azimuth and elevation were entered manually or automatically into a layers computer, then guided the layer's use of horizontal and elevation controls until the barrel was in the required horizontal and vertical alignment. This computed a correction for the cross level of the gun and used feedback from electro-mechanical devices (e.g. gyroscope and electronic clinometers) aligned to the axis of the bore. These electro-mechanical devices were subsequently replaced by ring laser gyros.

Anti-aircraft gun laying
The need to engage balloons and airships, from both the ground and ships, was recognised at the beginning of the 20th century. Aircraft were soon added to the list and the others fell from significance. Anti-aircraft was direct fire, the layer aiming at the aircraft. However, the target is moving in three dimensions and this makes it a difficult target. The basic issue is that either the layer aims at the target and some mechanism aligns the gun at the future (time of flight) position of the target or the layer aims at the future position of the aircraft. In either case the problem is determining the target's height, speed and direction and being able to 'aim-off' (sometimes called deflection laying) for the anti-aircraft projectile time of flight.

During World War 1 instruments were introduced to provide laying data; typically Height and Range Finders (HRF) were optical rangefinders of the coincident type, for example the Barr & Stroud 2 metre UB2, that also measured the elevation angle and hence produced height. Deflection was found by entering the range into tachymetric devices that tracked the target in range and elevation to determine the rate of change and hence the deflection. The French Brocq instrument produced both and provided a remote display at the guns. The British Wilson-Dalby was two instruments (vertical and horizontal), and its data was plotted in the command post before being ordered to the guns.

Anti-aircraft guns always had a recoil system. They needed relatively rapid traversing over very wide arcs of fire with very high maximum elevation angles and relatively high rates of fire. This meant they had to have a very stable mounting.

Broadly, anti-aircraft aiming developed along two paths, by World War 2 the situation was:
 * For targets up to a few thousand yards away, a smaller-calibre automatic gun was used, with simple sights that enabled a layer to judge the lead based on estimates of target range and speed; projectiles had tracers, allowing the layer to observe their flight path.
 * For longer-range targets, manually controlled predictors were used to track the target, taking inputs from optical or radar rangefinders, and calculating firing data for the guns, including allowance for wind and temperature. Electrical signals transferred aiming data to the guns, and the gun layers' task was to keep pointers representing the actual gun alignment and required gun alignment together.  Usually there were two layers (horizontal and vertical planes), and they did not look at the target.

Increasing automation, particularly on ships, eventually eliminated the need for layers on the guns, and this happened at much the same time as missiles started to replace heavier anti-aircraft guns. After World War II predictors changed from being electro-mechanical analogue computers to digital computers, but by this time heavy anti-aircraft guns had been replaced by missiles, but electronics enabled smaller guns to adopt fully automated laying.

Coast artillery gun laying
Most coast artillery was in fixed defences, "fortresses" in some form. Their targets moved in two dimensions, and, like anti-aircraft laying, the gun had to be aimed at the target's future position. Again like anti-aircraft, some guns were relatively small calibre and dealt with relatively close targets, others were much larger for long-range targets. Since the targets were far better protected than aircraft, the coast guns were larger.

Coast artillery employed direct fire, and until the late 19th century laying had changed little, apart from gaining telescopic sights, over the centuries. Nineteenth-century improvements in gun design and ammunition greatly extended their effective range.

In 1879 Major HS Watkins of the Royal Garrison Artillery invented the position-range finder and associated fire control. He had already invented the depression range-finder. His description explains its essence: "The position-finder traces the course of the ship, and when the guns are ready to lay, predicts the position the ship will occupy half a minute or more in advance. The dials on the gun floor automatically indicate the range and training to hit the predicted position.  When the guns are laid an electric tube (i.e. primer) is inserted and the signal goes up to the observing station that all is ready for firing.  The non-commissioned officer in charge of the position-finder watches for the appearance of the ship in the field of view of his telescope, and when she arrives at the cross wires presses a button, and the guns are fired."

It took almost 20 years to get it to full effectiveness, but its general principle became the norm for heavy coast artillery fire control and laying. Shorter-range guns retained conventional direct-fire laying with telescopes for much longer. In the 20th century, coast artillery, like field and the larger anti-aircraft guns, included corrections for non-standard conditions such as wind and temperature in their calculations. Coast artillery passed into history in most countries in the 1950s.

Naval gun laying


The introduction of breech-loading guns, then recoil systems and smokeless powder, completed the change in warship armament from hull-mounted to turreted guns or guns on pedestal mounts fitted with shields. Main armament in capital ships soon adopted laying arrangements broadly similar to Major Watkins' coast artillery pattern, and, as with coast artillery, shorter-range guns retained conventional direct-fire sights until after World War II. Anti-aircraft guns on ships also used arrangements similar to those used on land service pattern.

However, ships had a complication compared to land based guns: they were firing from a moving platform. This meant that their laying calculations had to predict the future position of both ship and target.

By World War II, ships, turret traverse, gun elevation and recoil were managed using hydraulic power. Ballistic calculations were performed by analog computers called predictors or rangekeepers. The guns were fired electrically, and a fire control system is arranged to fire the guns in sequence, firing each gun as the roll of the ship brought the gun to bear on its target. By the 1950s gun turrets were increasingly unmanned, with gun laying controlled remotely from the ship's control centre using inputs from radar and other sources.

Tank gun laying
Tanks (and anti-tank artillery) use direct-fire laying to engage moving targets. Telescopic sights were adopted before World War II, and these sights usually had a means of aiming off for target movement and graticules marked for different ranges. Tank sights were of two general types. Either the sight was in fixed alignment with the axis of the bore with ranges marked in the sight, and the gunner laid the range mark on the target. Or during laying the gunner physically set the range to offset the axis of the bore from the axis of the sight by the correct amount and laid using the centre mark in the sight.

Some sights had a means of estimating the range, for example using a stadiametric method. Other tanks used an optical coincident range-finder or after World War II, a ranging machine gun. From the 1970s these were replaced by laser range finders.

However, tank guns could not be fired accurately while moving until gun stabilisation was introduced. This appeared at the end of World War II. Some were hydraulic, while others used electrical servos.

During the 1970s tanks started being fitted with digital computers. These took the range and corrected it for conditions, basically the same calculations field artillery had been doing since World War I. In tanks this data was injected into the sight as an electronic aiming mark. The sight could also display other dynamic information, such as the range to the target. Sights were also provided with both day- and night-viewing capabilities.