Indirect Fire - History

History

For several centuries Coehorn mortars were fired indirectly because their fixed elevation meant range was determined by the amount of propelling powder. It's also reasonable conjecture that if these mortars were used from inside fortifications their targets may have been invisible to them and therefore met the definition of indirect fire.

It could also be argued that Niccolò Tartaglia's invention of the gunner's quadrant (see clinometer) in the 16th Century introduced indirect fire guns because it enabled gunlaying by instrument instead of line of sight. This instrument was basically a carpenter's set square with a graduated arc and plumb-bob placed in the muzzle to measure an elevation. There are suggestions, based on an account in Livre de Canonerie published in 1561 and reproduced in Revue d'Artillerie of March 1908, that indirect fire was used by the Burgundians in the 16th Century. The Russians seem to have used something similar at Paltsig in 1759 where they fired over trees, and their instructions of the time indicate this was a normal practice. These methods probably involved an aiming point positioned in line with the target. The earliest example of indirect fire adjusted by an observer seems to be during the defence of Hougoumont in the Battle of Waterloo where a battery of the Royal Horse Artillery fired an indirect Shrapnel barrage against advancing French troops using corrections given by the commander of an adjacent battery with a direct line of sight.

Modern indirect fire dates from the late 19th century. In 1882 a Russian, Lt Col KG Guk, published Field Artillery Fire from Covered Positions that described a better method of indirect laying (instead of aiming points in line with the target). In essence, this was the geometry of using angles to aiming points that could be in any direction relative to the target. The problem was the lack of an azimuth instrument to enable it; clinometers for elevation already existed. The Germans solved this problem by inventing the lining-plane in about 1890. This was a gun-mounted rotatable open sight, mounted in alignment with the bore, and able to measure large angles from it. Similar designs, usually able to measure angles in a full circle, were widely adopted over the following decade. By the early 1900s the open sight was sometimes replaced by a telescope and the term goniometer had replaced "lining-plane" in English.

The first incontrovertible, documented use of indirect fire in war using Guk's methods, albeit without lining-plane sights was on 26 October 1899 by British gunners during the Second Boer War. Although both sides demonstrated early on in the conflict that could use the technique effectively, in many subsequent battles, British commanders nonetheless ordered artillery to be "less timid" and to move forward to address troops' concerns about their guns abandoning them. The British used improvised gun arcs with howitzers; the sighting arrangements used by the Boers with their German and French guns is unclear.

The early goniometric devices suffered from the problem that the layer (gun aimer) had to move around to look through the sight. This was very unsatisfactory if the aiming point was not to the front, particularly on larger guns. The solution was a periscopic panoramic sight, with the eyepiece to the rear and the rotatable top of the sight above the height of the layer’s head. The German Goertz 1906 design was selected by both the British and the Russians. The British adopted the name "Dial Sight" for this instrument; the US used "Panoramic Telescope"; the Russia used "Goertz panorama".

Elevations were measured by a clinometer, a device using a spirit level to measure a vertical angle from the horizontal plane. These could be separate instruments placed on a surface parallel to the axis of the bore or physically integrated into some form of sight mount. Some guns had clinometers graduated in distances instead of angles. Clinometers had several other names including "gunner’s level", "range scale", "elevation drum" and "gunner’s quadrant" and several different configurations. Those graduated in ranges were specific to a type of gun.

These arrangements lasted for most of the 20th Century until robust, reliable and cost effectively accurate gyroscopes provided a means of pointing gun or launcher in any required azimuth and elevation, thereby enabling indirect fire without using external aiming points. These devices use gyros in all three axes to determine current elevation, azimuth and trunnion tilt.