Mk 38 Gun Fire Control System

The Mk 38 Gun Fire Control System (GFCS) is the central nervous system of the 16” gun weapon system on the Iowa class battleships. It’s main components include the rangefinders, radars, computers, stable verticals, directors and switchboards. Each component works with the others to aim the sixteen inch guns to accurately put ordinance on target. I will attempt to explain in simple to understand terms how the fire control system works.

Before radars were invented, fighting ships were still quite able to precisely determine the distance to an enemy ship with the use of the “rangefinder.” A "RANGEFINDER" is an optical device that uses the principles of triangulation to provide very accurate distance information to the gun firecontrol system. A rangefinder is a long hollow tube that is mounted on the fire control radar director or right on the battleship turret. The longer the tube, the more accurate results that could be obtained. The hollow tube serves as the "base" or leg of the right triangle -- a known distance. The other leg of the triangle is the actual distance to the target. A right triangle is formed with the rangefinder optics and a fixed mirror on one side. The hypotenuse of the triangle is formed with the other side of the tube and a mirror with an adjustable angle.

How a Rangefinder worksThe rangefinder operator sits at the middle of the tube and peers into twin eyepieces. The light from the fixed mirror is fed into one eye and the light from the adjustable mirror goes to the other eye. The operator, using "stereoscopic vision," adjusts the range knob which moves the adjustable mirror, until the target merges into a 3-dimensional image. The rules of trigonometry allow computing the length of a side of a right triangle if the other side and opposite angle is known. Because both are known on a rangefinder, distance can be accurately computed. Known Length * tan(Known Angle) = Computed Length (distance to target).

If you look at the battleship turrets number two and three, you'll notice the big "ears" sticking out the sides-- those are the two ends of the rangefinder. Originally, turret one had a rangefinder as well, but on the Missouri, they found that it’s line of sight was too low to be effective so it was removed. You can also see rangefinders on all but one of the fire control directors as well. Once radar was installed on ships, operating the rangefinder took a much lesser role in solving the fire control problem. Rangefinders were installed on US Navy ships into the late '60's and possibly early '70's, but they are now obsolete. (the rate insignia worn on the uniform of a fire controlman is a rangefinder with lightning bolts.)

The Mk 8 “RANGEKEEPER” was the analog computer that actually aimed the guns. It is composed of gears and ballistic cams. It is basically a giant clockwork device. It ran on electricity, but actually had a hand crank that could keep it running if the power was knocked out.

All sorts of information was fed into the rangekeeper. Distance to target could be entered either by radar, by rangefinder, or even by hand.Zane Watts fires the guns from the Stable Vertical. Own ship's course and speed as well as target course and speed were entered as well. The natural rocking motion of the ship at sea is corrected by the “STABLE VERTICAL,” which is a form of gyroscope. While most input signals to the rangekeeper come in the form of analog electrical signals, the level and crosslevel corrections from the stable vertical were linked directly to the rangekeeper in a unique configuration of two rotating drive shafts. While the stable vertical quietly cranks away at correcting for the pitching and rolling of the ship almost unnoticed, it is often times the focal point of the fire control system for another, yet critical function. The sixteen inch gun firing keys are mounted on the stable vertical. The firing key operator pulls the salvo alarm buzzer key twice then on the third pull, he closes the firing key as well. Bzzz Bzzz Ka-BOOM! (I had the privilege of firing a nine gun broadside on my 22nd birthday on the Missouri!)

The Mk 48 “shore bombardment computer” was added to the USS Missouri for naval gun fire support in the Korean Conflict. This machine is a big box with a glass table on the top. A combat chart is placed on the glass and a light shines from below that can be used to show the current ship’s position as well as the target position. Target position information is entered via hand cranks. The ship’s position is established by having one of the fire control radars track a known position such as a lighthouse or other landmark. If there is no landmark available to track, it is possible to shoot from dead reckoning mode. The ship’s position is called down from the quartermasters, or entered from the satellite navigation fix. The computer monitors the ship’s course and speed inputs from the ships gyro and pitometer. It then continually estimates the ship’s position. Naturally, this is far less desirable than using a firecontrol radar fix. The more precise ship’s position is established, the more accuracy that can be expected from the guns. The output from the 48 computer is fed to the Mk 8 rangekeeper and then on to the turrets.

The Mk 13 “radar” is a very early fire control radar. It is actually closer to a search radar that of a modern fire control radar. It has the same banana shaped antenna of a search radar except instead of rotating completely around, it vibrates back and forth over a small arc. The antenna is belt driven and the thing sounds like an off balance washing machine when it’s running, but it works! The picture it paints on the scope is similar to a section of surface search radar’s display. You can’t track an individual object with the Mk 13, only distinct shoreline features. It has no auto track, but instead the radar man must crank the range knob by hand to keep the radar on target.

It may be worthwhile to mention at this point that all of the electronics in the Mk 38 GFCS operate with vaccum tube technology. Whether it’s an amplifier in the radar or the 48 computer, it’s all pre-transistor circuitry. This is notable because if our modern solid state electronic weapon systems are ever exposed to an EMF pulse from an overhead atomic burst, they will be rendered useless, but the Mk 38 system would be unaffected by such a burst.

Finally, the Mk 38 “switchboard.” This device is a large panel that runs the length of the plotting room that is covered with barrel switches. Every shipboard fire control system has a switchboard, but close examination of a battleship’s board will reveal what make’s the battleship unique. Here, you will see the incredible redundancies built into the system. With the switchboard, you can configure the guns to be controlled in any number of ways.

Since there are two plotting rooms and three turrets, it can be set up to have turrets one and two controlled from forward plot and have turret three fired by aft plot. This is the typical General Quarters configuration. During Desert Storm, we fired all three turrets from either forward or aft plot, usually forward plot because it’s a larger room. For most of the fire missions during Desert Storm, the ship was not at General Quarters.

We had a slightly unusual setup for naval gunfire support in Desert Storm in that we used a secondary battery radar director for a reference instead of our own main battery radar. This is due to the fact that the 5” gun radars could lock onto a landmark in auto-track. They tracked the same oil rig for hours and hours on end. Again, the switchboard was used to set this up.

Of course, modern naval guns are controlled by digital fire control systems, but somehow these lack the elegance of the forgotten mechanical marvels of yesteryear.

Plotting Room Layout

The information provided on this webpage is as I remember it. If you have corrections or additional details to add, please e-mail me at

© 1997 William E. Genereux -- All Rights Reserved