Indirect artillery fire required three components working in concert to deliver 105mm rounds where and when they were needed. Accurate fire began with an observer radioing our battery FDC requesting 105mm artillery shells at a specific location. Then the battery fire direction control center computed the settings necessary for the guns to deliver them and those settings were telephoned from FDC to the Chief of Battery and the guns. The gun crews used the settings to prepare a specific artillery shell, shoved it into the gun tube, set guns to the correct bearing and fired the round when ordered.
The observer had to accurately locate the target on a grid map the observer and FDC were using. If the observer was under fire he was likely making an educated guess. The other part of his job was observing the landing artillery and giving directions to adjust fire.
Helicopters were close to everywhere in Vietnam. Within a couple of minutes one arrived near the target and someone inside took over as forward observer. Altitude allowed the helicopter FO a better view and the helicopter could be flown near the gun/target line and eliminate the need for FDC correcting the adjustments of the observer. A ground observer could not orient on the gun target line, so FDC had a grid on a circular paper template that let us convert the ground observers direction of observation to the gun target line. FDC needed this to accurately compute the requested changes.
If the observer was lost but had a “rough idea” of his location on the map, FDC could compute a 600-meter air burst of white phosphorus over a point we selected. When the observer saw the white smoke he gave FDC his location relative to it and voila, we had a place to start.
Using the FO’s information FDC computed data settings and instructions for the howitzer crews. A large flat table, called a plotting board had a map divided into numbered grids covering the table top. Our battery was designated by a pin in the center of the map. The observers grid coordinates were designated by another pin. A large metal straight edge with 2 intersecting arms and an arc in between, had its butt end hooked to the pin that represented us, and this straight edge was moved against the pin representing the target to yield the “deflection”, or left/right setting for the gun . (A 360 degree circle was 6400 mils under this system) If the observer was not on the gun/target line, we stuck the center of the circle marked in grids under the pin that represented the target and oriented its grid to the observers point of view. A cheat sheet that worked. On every fire mission a backup plotting table operator did the same computations, just to be sure.
A slide rule, designed to compute artillery instructions, books of firing tables, the above charting tables and a new FADAC digital computer were our weapons of war. A FADAC computer is pictured on my homepage, I am there also. Radio communication linked us to the outside world and a telephone system linked us to the guns. Air and ground clearances to fire came via the radio. Getting permission to fire was often slow when we needed American ground clearance, American air clearance, and Vietnamese ground clearance from separate commands.
The settings from FDC included the type of shell, the proper fuse and time setting (if the fuse required it), the number of powder bags to leave in the brass casing and settings of deflection and elevation for the gun. In theory a round or rounds fired should hit the grid coordinates sent by the observer. The reality was many things were not exactly as theory said but on most fire missions we got the third round on, or very close to, the requested coordinates.
The guns were called to fire and given the computed settings from FDC. Game on! In most cases gun number 3 (base piece) shot to register on the requested location; five other guns copied base piece settings, and all six guns fired rounds. The first task of the gun crew was to get the rounds ready with the proper fuse, time set and the required number of powder bags. Seven powder bags were in the shell casing connected by a few threads of string; smallest on the bottom (bag 1) and largest at the top (bag 7). Unneeded bags were hung outside the lip of the brass casing, and the connecting string was cut when the projectile was added. It looked like a very large 22 rifle shell. The shell was inserted in the howitzer and its breach block closed.
The deflection setting at the gun was set by aligning two red and white aiming stakes in a viewer that looked like a small periscope. Several sets of the stakes had been precisely set so the gun could could be aligned, no matter which direction the guns were aimed. In most wars, artillery covered a half circle but we were prepared to shoot around a full circle. In most fire missions, a gun, or guns, would fire over the top of others, and the fired over crew caught the full concussion and noise of the other guns. Deflection points the gun at the target and the elevation setting moves the gun barrel up (measured from level). The guns are ready for the command, “FIRE!” After they fire this shell, and the spent brass casing is ejected from the breach, another is ready to be inserted.
Then, the observer radios correction observations to FDC and the cycle begins again.
When our fire support base was attacked, we were authorized to fire without being commanded, but we tried to establish the observer-FDC-gun line of command as soon as possible. 105mm howitzers have two main ways to blunt an attack. Really, three because, “let the infantry do it” was an option at times. Killer Junior was a high explosive round (our normal shell in indirect fire) with a split second time on the fuse. The howitzer barrel was nearly level. It was, in theory, meant to explode at 10 meters. Most times, it was about that height, but could deviate. The excitement of being attacked speeds up normal reaction time and, in the heat of battle, close becomes good enough
The “Beehive” shell was our second option. It contained nearly 8,000 small darts and more powder than other rounds. Those boys cleared a path for many tens of meters. In theory, the barrel should be replaced after shooting Beehive rounds, but that did not often happen, if ever. Removing and replacing a barrel is a serious job, requiring the availability of serious tools, and the hands of competent repair engineers. Replacements were typically done in base camp, and the gun was not in our battery for days. Barrels needed to be replaced after firing a fixed number of rounds. In theory.