The Gun

The 1/92nd Field Artillery
Association - Vietnam

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FDC
Fire Direction Center

By Tom Kanis

  The FDC was a Section within the Firing Battery that computed solutions to fire missions and relayed the resultant commands to the Howitzer Sections. The "Gunnery Problem" that was solved was that the cannoneers manning the howitzers could not always see the target. If they could, the howitzer could be fired with on-board fire control equipment (sights) much like a rifle. But since the targets were almost always out of sight or fired upon at night, a "Gunnery Team" approach was used to attack them.

  The Team consisted of Forward Observers who could see or otherwise sense the targets, Howitzers and crews to shoot the targets and FDC's to calculate charge deflections and elevation commands for the guns to use during the mission.

  The FDC was usually located centrally to the rest of the Firing Battery, and usually had a lot of overhead cover and shelter, since it was a priority target during mortar and sapper attacks. Communication was usually by wire to the Howitzer Sections and by radio to Forward Observers, Supported Units and Battalion Headquarters.

  Personnel usually included the Fire Direction Officer (FDO), a Horizontal Control Operator (HCO) a Vertical Control Operator (VCO) a Battery Computer and an RTO or Battery Recorder. The senior NCO was usually designated the Section Chief in addition to his other duties. The VCO had the responsibility of accounting for the correction due to the difference in altitude of the guns and the target. If we were higher than the target, we would shoot long, or short if we were lower than the target. The HCO kept track of locating the target in the horizontal plane and making shifts from the Forward Observer's corrections. To do that, he'd use a plotting wheel like the one shown at the right.

  There sometimes was a mechanical computer present as well. The M18 FADAC (know to us all as 'Freddy the FADAC') was used in several firebases. It had its own 1.5Kw 400 Hz Generator, was about 4 feet square and weighed several hundred pounds. In comparison to any modern computer it was very slow in calculating the solutions to ballistics problems. In most instances, the FDC crew could beat it with the manual calculations on the first round, but in 1966-71, it was the very best available. For a while, the A Btry FDC at FSB Kelly had two FADACs. Twice the fun. It is interesting to note that the Army replaced these , at least as an interim fix, with H-P TI-59 hand-held programmable calculators. FADAC data had to be checked against manual data before firing, per most SOPs.

  Fire missions came to the FDC from FO's, Intel Sources or Higher HQ's. Once in receipt of the mission, the HCO would plot on a firing chart and measure the range and angle from the Battery to the target. The VCO would locate the target on a map and calculate the difference between the altitude of the Battery and the target. Ballistic tables in the Tabular Firing Tables (TFT) would be consulted, or more often, slide-rule-like devices called Graphical Firing Tables (GFT) would be used to determine firing commands for the Howitzers. Corrections for all predictable non-standard conditions were made, to include power temperature winds aloft, humidity, Coriolis Effect, projectile weight. Data on the guns' individual differences were previously determined in a special mission called a "registration". If it sounds complicated, it was.

  The task was made more difficult by the fact that there was a considerable pressure to get the rounds out rapidly. The usual criteria was 90 seconds to include time of flight of the projectile. More time than that and an FDO would be moved to another job. The Battery recorder took data from the HCO, VCOn and Freddy operator and filled out the Record of Fire form (DA 6-1). The Form tracked commands to the guns, kept track of ammo expended and formed a permanent record to be checked if there was a firing incident. Firing incidents were to be avoided at all costs, since the least that could happen would be to waste ammo and the worst would be friendly casualties. The FDC would usually have at least two radios. One would be used to talk to supported units, the other to talk to higher HQ and area commands. One of the radios would usually be equipped with "secure voice" capability, usually with a KYB-6 or KY-38 unit that could be encoded to net only with similarly equipped radios. This system would be used to transmit sensitive data such as the night locations of friendly units in the area of operations. The AN-RC-292 antennas that were hooked to the radios made the FDC very identifiable.