Indirect fire

• 1. Introduction

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  Indirect fire is the art of hitting a target you cannot see. A gunner points a weapon not by looking at the enemy, but by calculating angles, reading instruments, and trusting mathematics. It sounds abstract, yet this technique has shaped warfare from ancient siege engines to modern artillery that can reach targets more than 24 kilometers away.

  The questions worth asking are: how do you aim at something invisible? Who worked out the geometry? And when did armies actually start doing this reliably in battle? The answers stretch from a Song dynasty city siege to a morning in 1899 in South Africa, and they carry some genuine surprises.

• 2. The Geometry of Unseen Targets

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  NATO defines indirect fire as fire delivered at a target which cannot be seen by the aimer. That one sentence hides a great deal of complexity. Aiming requires solving two problems simultaneously: where to point the barrel left and right, and how high to tilt it.

  Azimuth handles the horizontal plane. Elevation handles the vertical, and elevation is governed not just by a chosen angle but by the energy of the propelling charge and the weight of the projectile. Longer range demands a higher trajectory; the theoretical sweet spot for maximum range sits at an elevation angle of 45 degrees.

  Atmospheric conditions complicate every calculation. Drag, crosswind, altitude differences between the gun and the target, and variations in propellant all push the projectile off course. Because the flight time of an indirect-fire shell is much longer than a direct-fire bullet, those deflectional forces have far more time to act. The result is a wider, more scattered shot grouping than direct fire produces.

  That scatter carries serious consequences. Explosive ordnance and shrapnel have a significant blast radius, so unpredictable accuracy raises the risk of collateral damage and friendly fire, especially when shooting what the military calls danger-close, meaning near friendly troops.

• 3. Ancient Beginnings

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  Archers were doing something like this long before gunpowder existed. Ancient armies routinely used indirect arrow fire in both open battles and sieges, loosing volleys on high arcs toward formations they could not see directly.

  Catapults joined the practice during sieges. In China, a governor named Chen Gui left a detailed account of how he used traction trebuchets to defend De'an city from 1127 to 1130. His book, City Defense Records, explained that the catapults should be placed inside the wall rather than on top of it, keeping them hidden from the enemy while still reaching targets outside.

  Chen Gui's instructions specified a systematic correction method. A designated person stood on the wall to watch where each stone landed and signal the crew. Small adjustments came from shifting the aimer's stand; large ones required moving the catapult rack. He wrote that if a shot went too long, the crew should reduce the number of pullers, and if it fell too short, they should add pullers. He concluded that with two or three shots, the catapult could hit the target.

  For several centuries after that, Coehorn mortars were also fired indirectly. Their elevation was fixed, so crews controlled range entirely by varying the amount of propelling powder. When these mortars were used from inside fortifications, their targets were by definition invisible to the crews.

• 4. Quadrants, Geometry, and Burgundians

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  Niccolò Tartaglia's invention of the gunner's quadrant in the 16th century opened a new chapter. The device was essentially a carpenter's set square fitted with a graduated arc and a plumb-bob; a gunner placed it in the muzzle to measure elevation without looking at the target at all. That instrument-based approach to elevation met one of the two defining conditions for true indirect fire.

  A French account from 1561, published in a work called Livre de Canonerie and later reproduced in a 1908 journal, suggests the Burgundians were already using indirect fire by that century. The Russians appear to have employed something similar at Paltzig in 1759, firing over trees, and their military instructions of the time indicate this was treated as normal practice. These early methods probably relied on placing an aiming point somewhere in line with the target.

  The earliest documented use of observer-adjusted indirect fire comes from the Battle of Waterloo. At the defence of Hougoumont, a battery of the Royal Horse Artillery fired an indirect shrapnel barrage against advancing French troops. The corrections came not from the gun crews themselves but from the commander of an adjacent battery who had a direct line of sight and relayed adjustments to the firing battery.

• 5. The Russian Solution and the Lining-Plane

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  Modern indirect fire has a clear point of origin. In 1882, a Russian officer, Lieutenant Colonel K. G. Guk, published a work titled Field Artillery Fire from Covered Positions. His contribution was a better geometry: instead of placing an aiming point directly between the gun and the target, he described using angles to aiming points that could sit in any direction relative to the target. That flexibility transformed what was possible.

  Guk's approach had one critical dependency: an accurate azimuth instrument. Clinometers for measuring elevation already existed, but nothing equivalent existed for azimuth. The Germans solved the problem around 1890 by inventing the lining-plane, a gun-mounted rotatable open sight aligned with the bore and capable of measuring large angles from it. Similar designs spread widely over the following decade. By the early 1900s the open sight was sometimes replaced by a telescope, and English speakers began calling the instrument a goniometer.

  The first incontrovertible, documented use of Guk's method in actual warfare came on the 26th of October 1899, during the Second Boer War. British gunners used the technique, though without lining-plane sights. Both sides demonstrated early ability with indirect fire, yet British commanders still sometimes ordered artillery to move forward and engage directly, responding to infantry concerns that the guns were abandoning them.

  The early goniometric sights had an ergonomic flaw: the gun aimer had to physically move around the weapon to look through the sight, which became very awkward when the aiming point was off to one side, especially on larger guns.

• 6. The Panoramic Sight

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  The solution to that awkward problem was the periscopic panoramic sight. It placed the eyepiece at the rear and put the rotatable top of the sight above the level of the aimer's head. The aimer could stay in one position regardless of which direction the aiming point lay.

  A design by the German firm Goertz, produced in 1906, was selected independently by both the British and the Russians. The British called it the Dial Sight. The Americans called it the Panoramic Telescope. The Russians kept the maker's name and called it the Goertz Panorama.

  These arrangements, combining a panoramic sight for azimuth with a clinometer for elevation, remained the standard approach for most of the 20th century. The clinometer used a spirit level to measure a vertical angle from the horizontal plane and went by many names across different armies, including gunner's level, range scale, elevation drum, and gunner's quadrant.

  Gyroscopes eventually replaced the need for external aiming points entirely. Robust, reliable, and cost-effectively accurate gyros, using sensors in all three axes, allowed a gun or launcher to be pointed in any required azimuth and elevation without reference to anything outside the weapon itself.

• 7. Command, Control, and World War I

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  The Boer War demonstrated that indirect fire worked. World War I showed what it was actually for. As artillery ranges increased, guns moved to covered positions farther from the front. A battery could no longer see its own targets, and it could not correct its own fire.

  The solution required organization as much as technology. Observers on the ground, in aircraft, and in observation balloons identified targets and reported where shells fell. The development of electrical communication made that reporting practical across large distances and allowed many dispersed batteries to concentrate fire on a single target. An important task for aircraft in the First World War, whether heavier-than-air or balloons, was artillery spotting.

  Naval use introduced an additional complication. Multiple ships might fire at the same target, making it difficult for spotters to determine which ship's shells were landing where. Different-colored dyes were often mixed into the charges of different ships so spotters could match splash color to a particular vessel.

  Fire could be adjusted, meaning an observer corrected it shot by shot, or predicted, a technique originally called map shooting, introduced during World War I. Predicted fire required the precise location of the target relative to the gun and incorporated corrections for prevailing atmospheric conditions into the initial calculations. Adjusted and predicted fire were not mutually exclusive; a battery might begin with predicted data and then adjust as the engagement developed.

  By the end of the 20th century, the typical maximum range for the most common guns had grown to about 24 to 30 km, compared with roughly 8 km at the time of the First World War.