How did defensive artillery fire work

We’ve all seen it in the movies, a thin line of men crouched in foxholes and trenches, valiantly trying to hold off the attack with their rifles and light machine guns, while one calls up artillery fire on the telephone. And then woosh, bang, the hordes of attacking infantry are consumed in a maelstrom of exploding shells, the attack beaten back with heavy loss. But how did it really work.

Well, no doubt the defensive lines have been built taking in the advantages of the terrain where they can, bunkers, trenches and foxholes dug, wire erected in front, and most importantly, telephone cable run and radio communications established with the artillery battery/regiment positioned some miles back.

But how do the artillerymen hit the right spot. Both they and the infantry holding the line have the same maps, with various co-ordinated points marked, and when called upon, the artillery know the area they are being asked to hit.

But you’re aiming a gun/howitzer at a specific point, where as the desire is to spread your fire across, say a 400-yard-wide front, and maybe 800 yards deep. So, it is fire three rounds, and the adjust, up a notch, or rotate left/right, a notch, and fire again. And what are the other guns in the battery doing, to help spread the fire? Or are we crudely just firing at a point, knowing the variation in shell fired will provide a spread anyway?

 

Answered to you questions here

How did defensive artillery fire work

 

I can answer your question. Spent active duty as an infantry Marine, later when I went in the US Army, I never used those fire direction skills, mainly due to my job and operating well outside friendly fire support. I was a school trained 0331 (Machine gunner) but when I reported to my first unit, they were overstrength in 0331's, so they made me a mortarman and OJT'd (on the job training) me there. I spent about 8 months with the mortars, went through Amphib Training School at Little Creek, Va. and a MCCRES (Marine Corps Combat Readiness Evaluation System) test out at 29 Palms with them and became very proficient in laying, plotting, aiming, and firing the mortars (which basically work like field artillery on a smaller scale). After the MCCRES we began workup for a deployment, Marines without sufficient time for the deployment were transferred out/sent to schools, which opened up 0331 slots and I was moved to my actual MOS for workup and deployment. Later, as an NCO I was required to have a working knowledge of calling in arty, air and naval gunfire. When I ETS'ed from active duty, I spent time in a Marine Corps Reserve artillery battery (an M109 SP 155 battery), went to school for a 2531 communications MOS and worked as the RTO (Radio/Telephone Operator) for The FO and for FDC, then as the Communications Section Chief, responsible for all radio/wire communications related to the battery.

(My comments are based on US TTPs/SOPs and may or may not be the same for other countries)

1.) You are correct "defensive lines have been built taking in the advantages of the terrain where they can". Guns are positioned to cover specific areas, overlap fields of fire where possible, identify probable avenues of approach, identify probable targets and using a compas and the T/E mechanism on the tripod record direction, elevation, and range to each target on a range card, and install left and right lateral limit stakes. Riflemen/auto riflemen are placed to fill out the lines between the guns and to cover dead spaces if possible.

2.) The range cards are used by the unit commander to visualize his defensive fire plan, he or another officer (usually the Weapons company officer) will identify and mark probable targets for the mortars, you have quickest response from them. Once the mortars are layed, deflection (direction) and elevation to the various targets are calculated (using compass azimuths and ranges supplied by the spotting personnel), given an identifier and recorded for potential later use. If time and ammo supply permits spotting rounds may be fired, generally smoke, easier to identify the "splash" of the round. Normally a company will have 3 x 60mm mortars. You can actually "hip shoot" the 60mm, line of sight if you're really good. A Marine Corps gunner actually hit a Japanese submarine at Guadalcanal with a "hip shot" from a mortar.

3.) Next level up is the battalion. They will have 81mm mortars for general support of the companies.

4.) A division might typically have an artillery regiment, typically three battalions of 105mm artillery, each with three battery's and a HQ battery. With the US triangular system of three regiments to a division, three infantry battalions to a regiment, and three infantry companies to a battalion, it was common to assign an artillery battalion in direct support of an infantry regiment, and a battery to each battalion. (batterys were typically 4-6 guns). The artillery regiment also normally had one or more general support battalions of 155mm caliber or greater. This is all general, battlefield conditions, terrain, probable enemy actions, location of units, support available, etc. would determine the allocation of fire assets and the designation of direct/general support and the recording of pre-planned fires, if any.

5.) "But how do the artillerymen hit the right spot." Your position is known, or the position of an identifiable terrain feature are called in as a 6, 8, 10 place grid, the azimuth and range from the known point to the target and a description of the target are called in, also the type of fire requested. These might be one round, smoke, in adjust; or enemy troops in open, battery 3 rounds, HE, in effect (for a four-gun battery that would be 12 rounds total on the impact area), or fire target point Baker, enemy vehicles at crossroads, battery 4 rounds, HE, in effect, etc. The receiving units Fire Direction Center will calculate the propellant charge, best fuze type (or the FO can specify a specific type), deflection and elevation necessary to hit the target, using the data provided and firing tables, they even account for weather, humidity, topographic elevation, and many other factors, in their calculations. If it's figured correctly, then known physical variables will place the round within the target area. Ballistics variances in the weapon type, tube wear, distance, propellant lots, etc. may cause the round(s) to impact outside the calculated point of impact, but within known variances, that are factored in for safety. But when you figure a 105mm HE round, time or VT fused for airburst has a casualty producing radius of 100 yards, it will generally hit within its effective radius.

6.) "Both they and the infantry holding the line have the same maps, with various co-ordinated points marked, and when called upon, the artillery know the area they are being asked to hit." Yes, most times they were working off the same topographical map. You can determine your location and then call-in fire based on a compass azimuth and range from your position. There are other methods if topo maps are not available, using known terrain features and some SWAG, but they are quite complicated.

7.) "you’re aiming a gun/howitzer at a specific point, where as the desire is to spread your fire across, say a 400-yard-wide front, and maybe 800 yards deep. So, it is fire three rounds, and the adjust, up a notch, or rotate left/right, a notch, and fire again. And what are the other guns in the battery doing, to help spread the fire? Or are we crudely just firing at a point, knowing the variation in shell fired will provide a spread anyway?"

Depends on if it's a preplanned fire or not. The normal procedure would be for the FO, wanting to do what you said, to call in one round in adjust. Depending upon the battlefield conditions it would probably be a PD (point detonating) fused HE round or a smoke round. When the round is fired the battery calls shot out. The FO when/if he can observe it will call adjustments and/or fire-for-effect.
Battery, troops in open, one round HE, fuze PD, in adjust (it's harder to determine the exact location of an air-burst). Shot over. Shot out. Battery drop 25, right 50, repeat. Roger, drop 25, right 50, repeat. Shot over. Shot out. (only one gun is used at this time) Battery 3 rounds, fuze VT, fire for effect. Roger, battery 3 rounds in effect, fuze VT. Shot over. Each of the four guns fires three rounds, a slightly different deflection can be fired from each gun, spreading the point of impacts, if necessary to cover the impact area. In your scenario probably not needed, the dispersion of the guns in their position should pretty much take care of things.
After firing 12 rounds (3 per tube) the battery calls, Rounds complete. The enemy attack has probably been stopped, at least severely thinned and is falling back. You've rained down death on anything in a 400 x 100-yard area. Now you have to adjust to what the enemy has done, they may have pushed through, more troops may have moved into the original area, they might all have gone to ground over the 800-yard-deep area. Your FO would then call-in adjustments, off the original mission, left, right, up, down or repeat.
A walking barrage like you're describing is normally an offensive tactic, used to suppress and provide cover for a friendly assault moving against a defined enemy position.

Hope this helps.

 

Wow thank you USMCPrice, for such a detailed reply, which must have taken some considerable time to compose. Yes that's extremely helpful, including your offering of the number of rounds that might be used. This is of great interest, because rarely do forces have enough ammunition, and there has to be some conservation. Artillery and logistics go hand in hand, and batteries can't always be located nice and close to a good road or rail link, especially if they are a mountain battery, so the logistical problems of delivering enough ammo is always a concern.

I'm also interested in the thought process regards how armies differentiate between using the 3-inch mortar, and the battery guns, which in my timeline, would be between 3 inch (25 pounder) and 4.5 inch calibre. The guns have greater range, meaning they can cover a larger defensive line, as well as providing greater lethality per shell, but the mortar is much quicker brought into action, and possibly more accurate. And logistically a mortar round is a lot lighter than a shell from the artillery battery. There is also the fact a single mortar barrel can fire off a lot more bombs in a given time than a howitzer/gun. Again the mortar teams must exercise fire conservationism, and their bombs are mostly delivered to the fire team by the unfortunate squaddie carrying a few at a time!.

 

No need for thanks, it didn't take much time and once it started flowing it was like opening the floodgates, all kinds of info I hadn't thought about in years started flowing. I can't believe I actually remember the nomenclature of a lot of the equipment all these years later. Anyway, I wish I had spent more time because I did forget to mention a number of things. Hopefully, I'll address some of them in replying to your latest post.

"so the logistical problems of delivering enough ammo is always a concern" Correct, but generally speaking the further back from the front lines the lesser the problem. Again, I'll use the US system because that's what I'm most familiar with. During WWII in 1944, the US in the ETO underwent a fairly severe shortage of artillery rounds. This was due to a number of factors; I'm mentioning it just to make you aware of it because I'm not going to focus on strategic and theater wide logistics but focus on the operational considerations towards the pointy end.
You need to be aware of several things, I'll use the WWII Marine Corps as an example, because it's easier to find their TOE's (Table of Organization and Equipment), SOP's (Standard Operating Procedures) and TTP's (Tactics, Techniques and Procedures) online. If you want to focus on another service, or another nations military, just look at the TOE/SOP and TTP's of that country and you can understand their capabilities. Early war US Marine and US Army TOEs were quite similar, with the US Army's having a more robust transportation/logistical element at higher levels because much of the Marines higher level logistical needs were served by the US Navy and the US Army was designed to operate over greater land distances.
Here's a link to the Series F, 1944 version of a Marine Corps Infantry TOE HyperWar: Marine Regiment TO&E (1944) (ibiblio.org)

Thank our forum owner, Otto, and OpanaPointer, Hyperwar manager and co-creator for providing us with the HyperWar site, an invaluable resource.

To explain how to use these, we'll start on the company level. A Series F Rifle Company TOE, rated one truck, 1/4 ton, 4x4 (jeep) and one trailer, 1/4 ton, two-wheel, cargo. That's it for organic transport, if it wasn't in the jeep or trailer it was humped by the men or pulled on a handcart (7 authorized). The company also rated, per TOE, six M-1919 (one platoon of three sections of two squads of one machine gun each) and six M-1917 water-cooled machine guns. Per SOP the machine gun section carried the M-1919's and the M-1917's were held in reserve in the Company HQ section. The air-cooled guns were more light and agile, while the water-cooled guns were better in the sustained fire/defensive role. All 12 guns could be utilized if required, but the point is, part of the organic transport was taken up by the reserve guns. Then you had water, rations, ammo, batteries, etc. so the company's organic transport capability, a jeep and its trailer, was not very robust.
Per TOE the company rated three 60mm mortars, broken down into three squads of one tube and six men each. They had to carry the tube, bipod, baseplate, sight and aiming stakes, plus the ammo. The M49A2 HE round weighed 2.73lb (1.24 kg), the M49A3 HE 3.07lb (1.39 kg), M302 WP 4.10lb (1.86kg) and M83 Illumination 4.15lbs (1.88kg) that averages out to 3.5lbs per round, then add in the 42lbs for the tube, bipod and baseplate. All ammo for the mortars was in addition to the mortar crewmen's personal weapon and its basic ammo load, water, rations, clothing, medical kit, rain gear, etc. so even 10 rounds per ammo man (35-45lbs, ten rounds M49A3 boxed, weighed 42lbs) was pushing their load carrying ability. The gunner and A-gunner had to carry the tube/bipod assembly 29.2lbs (12.8lbs barrel, M2, and 16.4lbs bipod, M5) or the baseplate M5 and sight M4, (12.8 and 1.16lbs for 13.96lbs) the man with the bipod/tube likely carried zero but no more than a couple rounds and baseplate guy maybe six. So, the six men likely had around 46-48 rounds carried on the team.
TTPs had the riflemen/leaders, in the company each humping a mortar round and belt of machinegun ammo to supplement the ammo carried by the gun/mortar teams (automatic riflemen and asst. auto riflemen carried additional ammo/magazines to keep the BAR up and running). A little math will show you they didn't have much ammo if heavily engaged. In a prepared position, battalion (with more organic transport) would force additional ammo forward to be stockpiled.

60mm/2.36in M2 mortar-this was the mortar used at the company level. It had the range of 1.1 miles (1.8kn) and deliver 18 rounds per minute, up to 30-35rpm. It fired HE, Smoke (WP) and Illumination.
Here two statistics need to be discussed: 1.) 70% of an infantry company's firepower is located in its crew served weapons (machine guns and mortars) so proper utilization of these assets are critical to a unit's success. 2.) 70-75% of combat casualties were caused by fragmentation weapons (and this has remained constant across numerous wars). Air delivered bombs, artillery, mortars, grenades (rifle/hand/launcher), rockets, and land mines all fall in this category. The platoon/company you were referencing to in the original post would see these figures skew more towards small arms/machine guns during an assault or defensive action, but across their entire time of employment in active combat operations it would fall into the 70-75% range I stated.
Your 60mm was quick and responsive for illumination (about 30 seconds per round) and HE to cover blind spots. Not so much for a sustained fight. You need to move further up the chain to find more robust fire support.

(more to follow)

 

The Japanese equivalence was the Type 89 Grenade launcher, or erroneously called the "knee mortar". I use that term loosely, because it was far less capable, but was used at platoon and company level, akin to the 60mm you describe. The Type 89 was very much and offensive weapon, where as I can see the 60mm being a more all round piece of ordinance. Nevertheless, the 60mm was small enough to be carried by advancing infantry, which is where the two converge.

However, would US Marines, dug in defensively, have deployed the 81mm or 3-inch mortar forward with the troops, to give that significant boost in defensive firepower, or would this remain at battalion level control?

 

Normally it would be kept at battalion, US Marines or US Army. The 81mm had a weight of 136lbs. vs 42lbs. for the 60mm so it's not nearly as mobile. The ammunition is also significantly heavier, but the greater organic transport at battalion allows for a substantial ammo load. The mortar platoon in the HQ Company had eight tubes, 5 jeeps (truck 1/4 ton), two trailers and a radio jeep.
The M2 60mm had an HE range of 1.1 miles (1.8km), the M1 81mm mortar had a range of 3,300 yds. (3017m/1.8miles) so they can be located 3/4's of a mile to the rear and still have overlap. Normally, if in a night defensive position, you'd call in illum from the 81's and engage and rapidly adjust with your 60's. Unless you were being hit by heavier formations, then the higher capacity of the 81mm round would dictate it be used against the primary avenues of approach.
The 81's have a wider variety of rounds and fuses. The HE rounds are 3-5 times the weight/capacity, the 60mm M49A3 HE weighs 3.07lbs, the 81mm M56 Heavy HE 15.01lbs. The heavier 81mm round is much more effective than the 60mm on the offensive vs fortifications and structures. The higher capacity WP provides a greater quantity and density, plus greater incendiary effects.
Battalion also has greater communication assets, so they run wire to the lower echelons, and provide/maintain radio nets.
The British/Commonwealth 3in mortar (81.5mm) was close to the weight of the M1 81mm (115.5 lbs vs 132lbs) and had a similar lack of mobility. Unfortunately, until improved propellants were developed in 1942, it had less range than the US 60mm, (1500m vs 1800m). Even with the new propellants it still only had 2/3ds the range of the US 81mm. Your tactics are dictated by your weapons capabilities, so they would have operated differently.

 

An additional weapons system most modern observers overlook but was actually an important asset is the rifle grenade. All major powers fielded them.
It was important enough that rifleman/grenadiers were specified in USA and USMC TOE's for the rifle squad. The M1903 Springfield with grenade launcher M1, was retained in the squad after they had transitioned to the rifle, M1, specifically to allow for the rifle grenades employment. The M1 carbine was fielded with the grenade launcher, M8. Once the grenade launcher, M7 was developed for the M1 rifle, and sufficient quantities produced, the M-1903 disappeared from the TOE. The grenade launchers allowed the infantryman to fire a grenade out to 250yds. (228m). They had an adapter M1 for the hand grenade, M2; an anti-tank grenade, M9; the fragmentation grenade M17, which allowed for a PD fuse vs the timed fuse of the Mk2 grenade with adapter; and the M22 smoke. There were others but these were the most common rounds. The rifle grenade was eventually evolved into the modern M-79, M-203 40mm grenade launchers and M-72 LAW (the M-31 HEAT rifle grenade, was a 66mm round that was adapted to be fired from a disposable launcher, M-72). The M-10 rifle grenade as developed was too heavy to be fired by the rifle and was modified into a rocket propelled projectile, and used the tube launcher, M1 or bazooka.