GERMAN LIGHT TRENCH MORTAR MODEL 1916
Leichtes MinenWerfer 16
This is my initial version of the light trench mortar. Any additional information and comments would be welcome.
– : Jalkaväkitykit (Kevyt miinanheittäjä 16 ja jalkaväkikanuunat 14 ja 15) – rakenne, ammukset ja tulenjohtovälineet. Yliesikunnan julkaisuja.
Kustannusosakeyhtiö Otava, Helsinki 1926.
– : Jalkaväkitykki-harjoitusohjesaantö (J. T. H. O.) I. Kustannusosakeyhtiö Otava, Helsinki 1926.
– : Jalkaväkitykki-harjoitusohjesaantö (J. T. H. O.) II – Taistelu – Ehdotus. Kustannusosakeyhtiö Otava, Helsinki 1926.
– : Kenttätykistön ampumaohjesääntö (K.T.A.O). Kustannusosakeyhtiö Otava, Helsinki 1937.
Sihvo, Kustaa: Kevyt miinanheittäjä. Kustannusosakeyhtiö Otava, Helsinki 1922.
In addition the author is grateful to colonel Jyri Paulaharju for his answers to a few specific questions on trench mortars.
[box] TECHNICAL DATA
Length of rifling: 38.5 cm
Total length with direct-firing carriage: 210 cm (82.7 in.) estimated
Height: 100 cm (39.3 in.) estimated
Carriage wheel gauge: 98.8 cm
Tube : 23 kg
Recoil system : 30.5 kg
Baseplate : 68 kg
Indirect-fire carriage: 26.5 kg
Direct-fire carriage : 68 kg
Limbers : 14 kg
Toolbox : 20 kg
Travel weight with direct-fire carriage: 270 kg
Travel weight with indirect-fire carriage: 200 kg
Total weight in firing position with direct-fire carriage: 185 kg
Total weight in firing position with indirect-fire carriage: 117.5 kg
Direct fire angle: 0 to +27 degrees
Indirect fire angle: +45 to +74.25 degrees
Muzzle velocity: 120 m/s
Maximum range: 1300 m
Minimum range for direct fire with wheels attached to carriage: 600 m
Weight 4.6 kg
Propelling charge II : 11 g
Propelling charges III to IV: 3.5 g[/box]
LIGHT TRENCH MORTAR MODEL 1916
Trench mortars made their first appearance in the Russo-Japanese war and were subsequently used by all belligerents in the First World War. It was found that the new, deadlier battlefield now demanded that all infantry troops be assisted by support weapons. The advantage of the trench mortar was that it was under the command of an infantry commander, not some artillery commander potentially a long way from the battle. In addition they were relatively light and thus could follow infantry on the battlefield.
The German trench mortar model 1916 was a representative weapon of its time. Unlike modern mortars of similar caliber it was mounted on a light wheeled carriage and was in appearance rather like a miniature cannon of the era but the wheels of the carriage were mainly intended for transportation only and were to be removed when firing indirect fire. The wheels could withstand direct firing at angles of more than 12 degrees however but the gun then had to be fitted with a limber with a support for direct firing. There were two different sizes of wheels available for the carriage. A box of tools was attached to the carriage.
Unlike later mortars, these early trench models were rifled, the model 16 had six grooves with an ascent of seven degrees. The mortar was muzzle loaded in order to save weight and cost. The arrangement also made the gun faster to load in a trench. Since the mortar was muzzle loaded the bombs had grooves which fit the rifling, but this in turn made the fit of the bombs and tube less than perfect, causing increased barrel wear and also increased spread when firing. The combination of muzzle loading and rifling also made the mortar more difficult to clean than an ordinary gun.
The mortar\’s tube was attached to a hydraulic recoil mechanism, rather like that of a normal field artillery piece. The liquid in the mechanism was a mixture of water and glycerine.
Accessories were provided with the gun. These included a locked, metal toolbox (with various tools, spare parts, a can of glycerine and a logbook for recording the number of shots fired), a short (one meter) and a long (ten meters) lanyard, leather tow lines, carrying equipment (steel drawbars which could be attached to the carriage for carrying purposes), a tarpaulin and some cleaning equipment.
The crew of a trench mortar consisted of a commander (a junior sergeant or a sergeant, with the key to the toolbox), a gunlayer, a loader, a gunlayer\’s assistant, two loaders and four additional men, one of whom would act as the tube commander\’s messenger when necessary. A Finnish Army mortar platoon was composed of two mortars and an unspecified number of observers. A company in turn consisted of three platoons. Mortar companies were often directly under the command of rifle divisions or regiments. Only men who were in good physical health and \”not weakened by veneral disease\” were to be selected for trench mortar units.
Finnish manuals state that the mortars could be pulled behind regular horse carts. For marches longer than four kilometers the weapons were to be lifted onto the carts in order to prevent them from suffering from the shaking. The crews were to march in good order behind their mortar while the officers were on horseback.
On the battlefield the mortars were often transported by the crews themselves. When fitted with the regular carriage the mortar required five men to move it, three pulling on the limbers and two pushing the carriage and tube. When fitted with the direct fire carriage the number of men moving the piece was increased to six and the direction of movement was also reversed. Now four men pulled the weapon tube first using leather thongs. Two additional men pushed from the limbers.
The mortar could also be disassembled into four loads: the base plate (borne by two men), the carriage without wheels, the tube (borne by two men), and the tool box. The carriage wheels and ammunition were to be transported by additional men.
On command the mortar was pushed into firing position and the wheels removed. Next, the carriage was rotated back and forth in place so that it dug itself into the earth. The firing sight was attached and a spirit level gauge was used to ascertain that the mortar was standing level. Then the tube commander checked the weapon\’s north-south orientation: two sticks were placed on the ground so that a string drawn between ran exactly north to south. The mortar was rotated so that its direction finder wheel\’s angle 320 pointed straight towards the magnetic north.
Meanwhile, the crew dug dirt and sand onto the carriage pounding it flat so that it would support the carriage. A protective wall was constructed between the mortar and the ammunition and crew. Three or four boxes of ammunition – twelve or sixteen bombs in all – were carried into position by the additional men.
The toolbox was placed behind the protective wall and opened by the gun commander. The gunlayer removed the mortar\’s sight from the toolbox and screwed it into place. The loader took the loading and fuse setting tools from the tool box and hung them about his neck. The gunlayer\’s assistant took the bomb opening and charge setting wrenches from the toolbox and gave the latter to one of the loaders. The gun commander oversaw all of this and then grabbed the longer lanyard from the toolbox.
Once everything was ready the commander, gunlayer and loader stayed with the weapon and the rest of the crew took cover. The gunlayer took position at knee on the left and the loader on the right of the tube. A metal hook at the end of the longer lanyard was attached to the weapon\’s bolt and the other end tightened around a limber which was struck into the ground on the other side of the protective wall. At this point the tube commander was to notify the platoon commander that his mortar was now ready to fire.
In an emergency it was permissible to operate the mortar without removing the wheels but this was not recommended. In such a situation the crew was to dig holes for the wheels or to place rocks or lengths of wood behind the wheels in order to prevent the carriage from moving due to recoil.
For direct fire, the mortar was fitted with the direct-firing carriage, a meter-long extension of the regular carriage with a spade in the end which was to be dug into the ground in order to keep the mortar in place while firing. It was recommended that a semi-circular groove be dug for the carriage\’s spade so that the mortar could be rotated in place.
For indirect fire the direction towards which the mortar would be pointed was determined using the mortar\’s sight, a special aiming compass (Kleine Markscheidegerät) or by using an aiming periscope. Alternatively one could also point the mortar straight towards the north and use a map to figure out the compass direction the tube should point at.
The mortar\’s sight was a simple device which also incorporated the mortar\’s firing tables, which were etched onto the metal. The distance reading gave required propellant charges, firing angles, fuse settings and corrections due to effects of rifling in ten-meter increments.
The aiming compass fit onto the direction finder wheel (artillery director) and the gunlayer would use both devices and make some calculations in order to arrive at the aiming direction.
The aiming periscope stood on a tripod which had extensible legs and thus could be easily carried by an observer. The telescope came in a wooden box with an instruction booklet of its own and had 4x magnification lense which projected one meter above the sight, and a 17-degree viewing angle meaning that at a range of 1 km the observer saw a section of 300 meters. The telescope could be used for determining direction and location of enemy forces. The aiming compass could also be fitted onto the same tripod. Both the telescope and aiming compass could also be used without the tripod.
It is mentioned in the 1922 manual that the aiming would often be performed from a forward position with instructions being relayed by field telephone.
For direct fire, the mortar was aimed using a sight on the left side of the barrel. This sight listed fuse lengths and rifling corrections relative to range.
Once the mortar commander was given the order to fire he would give his crew the firing command, for example: \”Attention\” – at this point the gunlayer and loader were either to stand to attention or to remain at knee in good order – \”Distance 700, angle 540, charge three, fuse 20\”. The gunlayer and the loader both repeated their part of the order.
The commander then gave the order \”Sight ! Load !\” The gunlayer was then to turn the mortar as ordered and to set the direction wheel to the correct angle and the charge and distance dials to correct settings. After double checking everything he would call out \”Sighted !\” The mortar commander would then in turn double check the settings and then give the gunlayer permission to move beyond the protective wall.
Meanwhile on the other side of the protective wall, one of the crew would be preparing a bomb of the appropriate type. In order to prevent problems the bomb\’s caliber would be measured using a special tool. Then the man was to carry the bomb over the wall and to hand it to the loader. Using the loading tool hanging about his neck the loader would open the rear plate of the bomb, place in the ordered propelling charges and then show the rear of the bomb to the mortar commander. After approval the loader would screw the rear plate back in. Next, he would remove the bomb\’s safety and drop it into the tube. He would then call out \”Loaded\” and walk over the wall, immediately taking cover.
The tube commander was then to attach the lanyard\’s free end to the mortar\’s bolt, take cover and announce: \”Attention !\” The gunlayer was to tighten the lanyard and prepare to fire. On the command \”Fire !\” the gunlayer tugged at the lanyard, immediately releasing it.
After the shot the commander, gunlayer and loader would return to the mortar. The commander would ascertain that the recoil mechanism had returned the tube to its original position, and re-attach the lanyard, which had been released by the recoil, onto the breech of the mortar. Meanwhile, the gunlayer and loader were to make sure that the mortar was still standing firmly.
It was also possible to shoot \”rapid fire\”. In this firing mode the mortar was operated much like a modern mortar. The gunlayer and loader remained with the mortar and the former would use the shorter lanyard to fire. Alternatively, the firing pin could be fixed in place, producing a detonation immediately when a bomb reached the bottom of the tube. In this firing mode sufficient ammunition was to be placed next to the mortar, not on the other side of the makeshift wall. It was possible to reach a rate of 20 to 25 bombs per minute when shooting \”rapid fire\”. The mortar could fire 120 to 150 bombs per hour without overheating but rapid fire would quickly increase tube and recoil dampener temperatures beyond the recommended.
The mortar was fired by pulling a lanyard. Two lengths of lanyard, classified as short 1 meter) and long (10 meters), were provided with the mortar.
Using indirect fire, the trench mortar could engage targets at ranges from 300 to 1300 meters.
At 12 degrees elevation, the mortar\’s range was 600 meters with the number 1 and 2 charges. The tube was rifled and this caused the bomb to drift about one meter to the right for every 100 meters of range.
Wind also affected the trajectory of mortar bombs. Side wind could cause the bombs to drift up to fifty and head or tail wind by thirty meters. Differences in air humidity could add or substract up to 15 percent from the mortar\’s range.
All bombs from a properly emplaced mortar would fall in a rectangle area 90 meters long by 30 meters wide. Fifty percent of the bombs would fall in a 30 by 10 meter area in the middle of the rectangle. Because of this considerable deviation, crews were instructed to not adjust fire until two to four bombs had been shot. In addition, the manual suggested that whenever possible the mortars should be positioned on the flanks of the enemy.
Direct fire was naturally more accurate than indirect fire.
The Germans used as many as 33 different types of munitions with this trench mortar. Some bomb types were experimental and used only briefly but it seems that the Finnish Army inherited a wide and confusing selection of bombs which in the absence of official nomenclature were generally called by their German nicknames.
The mortar\’s bombs were much like artillery shells in shape but there was no separate shell casing. The charges which sent the bombs on their way were attached onto the bomb\’s rear. Propellant charges I and II (a total of 11 grams of \”nitroglycerine powder #13\”) were always integral parts of the shell. Other charges were added as needed.
The Light Explosive Bomb was a steel-encased HE fragmentation munition and the primary ammunition type for the mortar. It was 28 cm long and weighed 4.6 kg of which 0.56 to 0.58 kg was explosives (perdite aka picric acid or pertite, or donarite aka ammon-gelite). Several variations of the HE bomb existed – the Lotte, Erna-Liese and Anna-Liese (L, EL and AL, respectively) – the differences seem to have been minor construction details.
It was often equipped with two fuses, an impact fuse and a time fuse, because the bombs often tumbled in the air and did not land impact fuse first. The time fuse could be set between seven and 24 seconds in 0.2 second intervals, which made timed airbursts possible. The 1922 manual outlines a procedure to set the fuse so that the bomb would explode about eight meters above and slightly in front of the target. Basically the fuses would first be set too long and then shortened first by two, then by one and finally one half a second per shot until the desired result was obtained. Bombs with impact fuses only were also very common.
The HE fragmentation bomb was said to be approximately as lethal as an artillery round of comparable caliber but to lack the artillery round\’s speed so that it had trouble penetrating field fortifications.
HE bombs were shipped in wooden crates containing four or six bombs, the former crate model was equipped with straps so that it could be carried on a man\’s back. Inside the crate, the bombs were packed in a tin box sealed with pitch. A special tool for cutting open the tin box was provided in every crate.
The Armor-Piercing Bomb \”Olga\” was introduced in 1917 after the introduction of tanks. The tip of the armor-piercing bomb was of high-grade steel and equipped with a slow impact fuse. The 24.75 cm long shell\’s walls were thicker but the explosive charge correspondingly smaller. Under favorable conditions it could pierce up to 15mm of steel. The armor-piercing munition was only used with direct fire using propelling charge V.
The Light Gas Bomb contained 0.69 kg of liquid gas – at first bromacetone, later phosgen and finally perstoff – which evaporated when the shell was broken. The heavy gas would sink into bunkers and trenches and either kill the enemy or force them out into the open where they would be annihilated by more conventional weapons. The gas bomb was both impact and time detonated. The bomb\’s explosive charge was too small to cause a shrapnel effect.
The Light Smoke Bomb \”R\” contained a 100 gram black powder and tar charge which gave off smoke when ignited. The smoke poured out from eight paper-covered holes. It was used for ranging shots and at war games, the manual makes no mention of using smoke rounds to conceal assault or withdrawal by friendly troops. The black powder charges were kept in separate boxes containing 150 such charges and placed into the shells only shortly prior to use. The bomb had a special fuse which were likewise kept in airtight tin boxes before use, fifty fuses to a box.
The Light Practice Bomb was a dummy bomb used for gunnery practice and gunnery drills. For public demonstrations there existed a practice bomb with a 10-gram charge which exploded when the trigger was pulled, creating a realistic sound effect. The bomb itself remained in the tube however and had to be extracted using special pliers.
The Light Message Bomb was an empty shell which could be filled with newspapers, leaflets or such intended for either freindly or enemy troops. A small illumination charge indicated where the bomb fell. There were two types of these mines, the \”Nora\” and \”Nora-Alma\”, the former having a range of 1300 meters. The Nora-Alma mine weighed only 4 kg and was provided with an additional propelling charge which increased its range to 1775 meters and demanded new firing tables. The message bombs came in boxes which included a ten-page notebook on which any messages could be written, a carbon copy sheet, and tools for opening and closing the bombs.
The Light Illumination Bomb \”Bertha\” could be used for signaling or to illuminate a large area for 20 seconds. The 1926 manual includes the following firing table:
Charge Angle Time_Fuse_Setting Diameter_Illuminated
III 65 14 500 m
IV 45 14 750 m
V 45 14.6 1000 m
V 75 11 For signaling only
A small black powder charge broke open the bomb and set the illumination charge on fire. The bomb descended with the aid of a parachute and the manual warns of accidentally illuminating friendly positions on a windy day. Upon landing, the burning illumination bomb was a fire hazard.
Munitions of the era left much to be desired as far as reliability was concerned. If the propelling charges were moist they would at times not explode with their full force, causing the armed bomb to fly only a few meters. Of course HE bombs were often also time-detonated so the crew was then in a terrible hurry to act according to the field manual, which was to \”throw the bomb further and assume full cover\”. At other times the mere act of removing the safety of the impact fuse set off the time fuse, after which the loader had between seven and twenty-four seconds to either fire off the round or fling it as far away as he could…
Great care was to be taken while handling boxes of bombs as, according to the manual, a box might explode if dropped. If gas bombs were found to leak chemicals the crews were instructed to handle them wearing gas masks and to remain upwind of them during transport. If a gas bomb broke in firing position it was to be dug into the ground and the fire mission continued with the crew now wearing gas masks.
In static trench warfare the trench mortar was to be used as a harrassment weapon. In defense, mortars concentrated on machine guns and avenues of enemy attack, forcing the enemy to halt its advance. Direct fire was used against machine guns and tanks – the APHE bomb could penetrate a WWI tank at ranges up to 500 meters. Trench mortars were to be used in force in order to deliver the maximum shock effect.
In the chain of command trench mortar companies could be subordinated to divisional or regimental commanders. There was also the possibility that the mortars be subordinated to an artillery forward observer. It was recommended that the mortars not be divided into smaller units than a platoon.
USE IN FINLAND
Finland probably received her trench mortars from Germany during WWI. It is likely they were used in the Finnish War of Independence in 1918. The last field manuals concerning these mortars were written in 1926 but by the time the Winter War begun they were no longer in service and there was no ammunition for them, production in Germany having been stopped years earlier. In around 1928 Finland had received her first modern mortars, the 81mm model 1926 which was based on the British and French Stokes and Brandt design of 1924-1925. The new type of mortar was easier to carry, had a longer range and more powerful and reliable ammunition. By the early Thirties similar mortars were being produced in Finland and the trench mortar faded into obscurity.