Fire Direction in U.S. Army Chemical Mortar Battalions

by Bruce Elliott for Rodney Young, 27 February 2015


First a bit on CMB organization in order to better understand fire direction and related operations. A CMB consisted of a Headquarters (HQ) Co and three line companies (A, B, and C). This triangular organization was adopted by the Army in the latter part of WWII, when it dropped the old rectangular organization. The change applied to all CMBs and all other elements of the Army. In the old rectangular organization, a CMB had four line companies, with a Co D being included.


A line company (aka mortar company) had an HQ section and three mortar platoons. The HQ section included the company commander, executive officer, first sergeant, company clerk, and people to run the supply room, motor pool, kitchen, communications, and ammunition re-supply.


A mortar platoon had four mortar squads, plus a platoon leader and his executive (officers), platoon sergeant, communi­cations men, and a few drivers to, among other things, drive the platoon’s ¾ ton truck used mainly for ammunition re-supply. A squad, authorized 8 men but rarely at full strength, was the crew for one 4.2" mortar; each had 2 jeeps (each one with a trailer), one for 4 men and the mortar, one for 4 men and the ammunition. Thus, the unit was highly mobile.


The basic mission of 4.2 mortar units was to provide close fire support to infantry units. A battalion, originally designed to mass its fires as a unit, actually never did so. Line companies routinely operated separately from their battalion, commonly attached to an infantry division or similar unit, often to reinforce that unit’s own artillery. There were times, thankfully few of them, when a company’s platoons were separated and attached to different infantry units. This made things real tough because a company was the smallest completely-contained unit, and platoons simply were not organized or equipped to independently support themselves.


When a company was about to go into action in support of a particular infantry objective, one of the first things its commander would do is conduct RSOP (pronounced arsop). That equals Army lingo for Reconnaissance, Selection and Occupation of Position. In other words, find a good place for the mortars to set up and perform their mission, and then have the platoons occupy those positions and prepare for action. At the same time, he would set up his command post (CP) and the Fire Direction Center (FDC), send out Forward Observers (FOs), and have wire communications laid between these elements and the platoons. Portable radio was used whenever wire wasn't working. A heavy duty vehicle-mounted radio was used for contact with the unit being supported and other distant units.


The company’s FDC had wire and/or radio contact with each platoon and each FO. In combat operations, it was the nerve center and ran the show under the leadership of an officer, who operated as the company’s field executive. His help included a senior sergeant as his assistant, plus a sergeant and assistant for each platoon, whose duties were to take requests from FOs or instructions from the field exec, calculate firing instructions and send them to their platoons, and keep records of all firing missions.


The FDC was often set up in a tent or, if lucky, in a commandeered house or other structure. The operators used portable folding tables, whatever maps they could obtain, special slide rules to make firing calculations, and of course their phones and radios and mission records. The maps and large slide rules allowed operators to calculate direction (azimuth) for aiming the guns at the target, and the proper firing charge for the range to target. They were originally supposed to include tube elevation but most of us soon simplified that problem by using a standard elevation of 900 mils for all missions. CMBs and other artillery used the mil system rather than the degree one. As there are 6400 mils in a circle, 900 is a little higher than 45 degrees, a good angle for clearing obstructions ahead such as hills (one of the advantages of a mortar vs a howitzer).


Before firing a mission from a new position, each platoon would zero in to establish accurate terrain relationships. The platoon’s fire would be directed to a readily identified point in the expected target area, one round at a time and using bracketing as necessary (guided by the FO), until the target was hit or very nearly so. The platoon plotters in the FDC would then make the necessary notes on their map overlays. FS rounds (a mixture of sulfur trioxide and chlorosulfonic acid), producing white smoke, were usually used for zeroing in because they are so much more visible than HE (high explosive).


FS rounds were also used to produce smoke screens, which the infantry often asked for from CMBs. But, when firing for effect, HE and WP (white phosphorus) were used. WP was particularly useful. It could not only be used for spotting and smoke screens but was devastating when used against enemy in foxholes, bunkers or caves. It was very effective against Germans and Japanese, who definitely did not like it.


Each platoon had its own FO on the front line and often fired separate missions from the other platoons in the company. But a massed company - 12 mortars - could dump a lot of explosives on a target in a very short time. The chemical mortar, with a tube having an internal diameter of 4.2" (106.7mm), is sometimes compared to a 105mm howitzer, the artillery’s workhorse. The latter has a firing range about 3 times that of the 4.2, but it cannot fire at an equally high trajectory, or fire as fast as the 4.2 in its first minute, or fire shells with nearly as much payload in them. Chemical mortars can each fire 20 rounds in the first minute, so a massed battalion can deposit nearly a ton of HE on a target in one minute. Not bad for ground troops, but when that's not good enough we call on our USAF friends with the heavy bombers.


Here is how a typical mission might proceed. When the field exec (FDC boss) gets an order from his company commander or the CO of the supported unit, he decides which platoon or platoons will fire the mission, requires the one or more FOs to give him the azimuth and range to the target, passes that data to the FDC’s platoon plotters along with the nature of the target, tells them what type of ammo is to be used, how many rounds are to be fired and at what frequency, and when to commence firing. Of course, there are standard and quite succinct commands for all this. Obviously there are also variations and other aspects to such a procedure as outlined here, but this one is typical.


During combat operations, the company commander may be with one of his FOs on the front line, or he may be with the CO of the supported unit, or off doing something else necessary to keep the company operating and in good shape. The administrative and support elements of a company will then be well behind the lines in what is called the company rear. The elements there will be led by the company executive officer and include the first sergeant, company clerk, general supply section, motor pool, kitchen, communications, and ammunition re-supply, together with their ¾ ton and 2 ½ ton trucks.


As for meteorology or meteorologists, CMBs had no use for it or them unless the units were to conduct gas warfare, the first use of which was against our national policy, and none was employed in WWII.


[Thanks for Bruce Elliott’s thorough answer to my inquiry about how it all worked.]