by Gene Slover
Updated 06 July 2000
I just love the word "ponder."
When you apply it to Ships, Guns, and Fire Control, I feel that it is at it's zenith. Everything that I have written on this board from day one is hardly a scratch of what you need to know in order to understand gunnery and fire control. For example, ponder the thought that, as a ship rolls, the gears in the director take up slack in one direction, the gears in the computer take up the slack, and so on through the stable vertical, the gun turret or mount and on to the guns themselves.
Then the ship rolls the other way.
All of the gearing then takes up the slack on the other side of the gear teeth, through all of the above mechanisms, and there are hundreds of these mechanisms. The ship also bends, bows and vibrates. If you fire a round, everything gets worse. If you fire a salvo, then it gets a LOT worse.
And you expect this thing to hit anything?
Surprisingly, you can hit when the fire control equipment is used correctly, and you can hit very accurately.
Simple Truth: For every reason why you can hit a target there is an almost unimaginable amount of reasons why you cannot hit a target.
One reason that is hard to overcome is gun movement at the moment of firing. When you mount a gun on a platform that floats on the ocean, you have then created a system whereby the gun is at the whim of the ocean and is constantly in motion. Any movement of the gun at the moment of firing is transmitted to and superimposed on the flight of the projectile. Therefore, if the gun is moving up, or any other direction or combinations of directions, this movement is passed on to the projectile. Upward movement causes the projectile to move upward through its entire flight. This of course increases the range of the projectile.
About the first attempt to overcome this phenomena was to tie a cannon ball on a line and suspend it from a yardarm on a mast. The cannon ball would always hang straight down while the ship rolled, pitched and yawed. As gunners learned how to use this system, they soon found out that when the ship was rolling upwards on the side they were firing on, the guns all had a longer range due to the imparted upward motion of the gun. Somewhere along here the term, "Fire on the roll" was born, meaning to fire when the guns were moving upwards.
Another problem is determining the line of fire necessary for the projectiles to strike their target. The Line of Fire of the gun or guns is represented by the direction the gun is pointed. It is not the same as the line of sight to the target.
One of the things necessary to know about the gun is the tilt of the trunnions across the beam of the gun and the tilt of the bore of the gun with relation to the true horizontal plane. Movement of the ship due to roll pitch and yaw tilt the trunnions, which then causes the gun to be incorrectly pointed in the line of fire. The Stable Vertical and Stable Element are instruments that effectively measures the trunnion tilt and provides corrections for the tilt. Since we now have the gyro in the vertical position we now have to have some more parts in order to use the gyro.
When the Stable Verticals and Stable Elements were born they had a provision for firing on the roll. There are firing triggers or handles on the side, one for each gun mount or turret, and then a master that can fire all guns. On the top of the Stable Verticals and Stable Elements are the dials that indicate the roll and pitch of the ship. They have inner and outer dials. The inner dials, as I recall, show the position of the ship against a stationary mark so that you can visually see the degree of roll and pitch of the ship. Of course, I could be backwards on orientation of the dials.
The firing angle can be set to any position that you chose and if the firing trigger is engaged when the ship rolls through the degree you have set, a switch makes, sending the firing signal to the gun or guns selected.
There was no provision in my time for computating the rate of roll and
subtracting it from the gun position. All we had were the Stable
Verticals and Stable Elements that provided a true horizontal signal that
drove into the Fire Control computers. Any gun that was in automatic
in train and elevation then was repositioned by the computer to compensate
for the roll and pitch of the ship keeping the gun pointed in the direction
of the target. The Fire control computers did not have a rate of
roll computing section in them.
The Stable Vertical
Stable Verticals are only found on Cruisers and Battleships.
The Stable Vertical was a companion to the Mark 1 computer. As revisions were made to the Mark 1 that eventually led to the Mark 8 Range Keeper the Stable Vertical went right along and was also its companion.
The Stable Vertical is about 36” square and was about the same height as a Mark 1, about 48 inches. The gyro is a little smaller than the gyro in the Sperry Fire Control Compass, which weighs about 72 pounds on the outer rim, when up to full speed. Full speed of the Sperry Fire Control Compass is about 12,000 RPM and requires a full 4 hours to get up to this speed for maximum accuracy.
The Stable Vertical Gyro weighs about 30 pounds on the outer rim, when up to full speed, and can come up to its full speed of about 12,000 RPM in about 30 minutes to an hour. Both gyros are powered from a ship's generator, which produces 400-cycle 3-phase power. Depending on the size or class of the ship, the number of backup generators available may vary.
The Stable Vertical Gyro has enough speed to come to the vertical in 5 minutes or less so that it can provide an accurate enough true horizontal plane to commence firing from.
When the Stable Vertical Gyro is in the vertical position, the shaft of the gyro is straight up, or vertical or perpendicular to the earth’s surface, and the outer rim or diameter of the rotor is parallel to the earth’s surface. When the gyro has come to this position it is said to be "At the Vertical." Once the gyro is "At the Vertical," accurate firing can commence.
The Stable Vertical Gyro is mounted in a set of gimbals that allow movement in any direction as the ship rolls and pitches, so that the gyro is undisturbed in its vertical position by the ship movement. When the gyro is not powered up the gyro is lying at an angle of about 45 degrees.
What brings the gyro to the vertical is a very simple mechanism. When powered up the entire gimbal mechanism containing the gyro rotates at about 10 or 15 RPM. On each side of the gyro housing, there is housing around the gyro rotor, which contains a vacuum; there is a container that has about 4 cubic inches of space in it. A pipe or tube connects these two containers together from their bottom side. These containers are for Mercury. There is enough Mercury to fill one container when the gyro is at rest or not powered up and leaning to one side. When the gyro is powered up the gimbal mechanism containing the gyro rotates. Since the gyro is tilted, as it rotates the Mercury flows through the pipe or tube to the container which is the lowest due to gravity. Thus as the rotation continues the heavier container is always on the low side, as the gimbal mechanism rotates, the heavier container is brought uphill, which causes precession to occur in the gyro, causing it to come to the vertical a little more with each rotation of the gimbal mechanism. Of course the gyro has to have some speed before precession can begin to occur, but in about one minute after powering up, the gyro has enough speed for precession to begin to occur.
When the gyro is at the vertical both containers have the same weight of Mercury in them and no precession of the gyro is caused. Any time that the gyro gets slightly out of the vertical position there is an unbalance caused in the weight of the mercury of the two containers. This then causes precession to occur, bringing the gyro back to the vertical position.
A little about precession control of a gyro, when viewed from the top of the gyro: When weight is applied to the outer rim of a gyro, a point 90 degrees away from the point the weight is applied and in the direction of rotation the gyro will tilt in the direction the weight was applied. The gyro will continue to tilt in this direction until the weight is removed. This is called precessing the gyro.
So the two containers, the Mercury, and the effects of gravity, rotating
with the gyro constantly and continually check and precess the gyro as
necessary to keep it in the vertical position.
The “Bonnet”, or “Umbrella”.
The stable vertical resides at the end of the fire control computer, separated by about 18 inches. Mechanical shafting drives directly into the bottom of the stable vertical from the FC computer the computed relative bearing, relative to the ship centerline fore and aft, of the line of fire of the gun turret or mount. This relative bearing information then is transmitted up in the stable vertical to the bonnet. The bonnet is then rotated by this relative bearing information so that the X-axis hour glass coil is at 90 degrees to the computed line of fire, and the y-axis hour glass coil is in line with or parallel to the computed line of fire.
Above the gyro there is a part called a “Bonnet”, some call it an “Umbrella”, as it is curved to fit the arc of the swing of the gyro, so it sort of looks like an umbrella. The bonnet is mechanically fixed so that it is supposed to be, or should be horizontal with the training rings of the guns and directors, which are all, supposed to be parallel to each other. Thus as the ship rolls, pitches, and yaws, the bonnet remains parallel to the training rings at all times.
Imbedded in the bonnet are two coils of wire. Each coil is sort of in the shape of an hourglass. The two coils lie at 90 degrees to each other with the center or small part of the hourglass shape crossing each other in the center. One of the coils is the X-axis; the other coil is the Y-axis. On the top of the gyro housing is an electromagnetic, powered by 115 volts AC.
When the gyro is "At the Vertical" and the "Bonnet" is horizontal, the electromagnet on top of the gyro centers the two hour-glass shaped coils of wire. In this position equal current flow is generated in the two coils of wire, so their output is saying that the bonnet is square with the gyro and the world.
As the ship rolls, pitches, and yaws, the bonnet moves to follow the ship position. The gyro remains vertical, keeping the electromagnetic stationary in the vertical position. As bonnet moves across or around the stationary electromagnetic, different currents and voltages are generated in the two hour glass shaped coils. This then becomes an electrical signal that can be used in showing the angle of the bonnet, and the training rings, with respect to the true horizontal plane represented by the gyro.
Thus then the hour glass coils in the bonnet effectively assume the position of the gun trunnions of the gun that is being controlled by the fire control computer. So now as the ship rolls, pitches, and yaws, the stable vertical is able to measure the amount of tilt this causes in the gun trunnions no matter what relative position the gun is trained to, or in, with relation to the fore and aft axis of the ship. The electro-magnet located on top of the stable vertical gyro, generates an electrical current and voltage in the X and Y axis hour glass coils that is in proportion to the amount of tilt, or angle, that the bonnet is out of the true horizontal position with respect to the earth’s surface. The electrical signals so generated then serve to drive servomotors in the bottom of the stable vertical. The output from these servomotors is then transmitted by shafting up to the top of the stable vertical to position the dials you can see in the top of the stable vertical. Shafting into the fire control computer also transmits the output from these servomotors. Through the use of mechanical differentials, this information is then added to or subtracted from the mechanical inputs to the synchro transmitters that are transmitting the computed line of fire to the guns, in bearing and elevation.
This then is the measurement of gun trunnion tilt, as measured by the stable vertical, and is the correction of gun trunnion tilt that is provided by the stable vertical.
Of course what you actually see if you are watching the gun turrets
or gun mounts is that they are constantly training, and or, elevating or
depressing, to maintain their position of the line of fire they are in.
The Stable Element
Stable Elements are found on any ship with a Mark 1A computer.
The Stable Element was born to be a companion to the Mark 1A computer. As revisions were made to the Mark 1 that eventually led to the Mark 1A Computer which could compute the fire control problem for aircraft, a gyro that could come to the vertical faster was needed. Thus the Stable Element was born to be its companion.
The gyro in the stable element is a little smaller that the gyro in the Stable Vertical. The Stable Element Gyro weighs about 10 pounds on the outer rim when up to full speed, and can come up to its full speed of about 12,000 RPM in about 30 minutes.
The Stable Element Gyro has enough speed however to come to the vertical in 1 minute or less so that it can provide an accurate enough true horizontal plane to commence firing from. The Stable Element does everything the stable vertical does except it can come to the vertical faster because of its lighter rotor weigh, which allows you to commence accurate gun fire faster and quicker to answer an attack.
As I have previously pointed out the Mark 1A computer was developed as a dual purpose computer that could handle air as well as surface targets, controlling the gun fire of the 5”/38 gun. All three items were operational by 1938.
In my opinion, the stable element is just as accurate as the stable
My memory tells me that the Coriolis Force theory was proven about 1923. It has always fascinated me that the originator of a theory in many cases has been dead for a long time before his theory is actually proven.
As it relates to gunnery, the “Effect” of Coriolis Force causes right hand drift in the northern hemisphere and left hand drift in the southern hemisphere of a projectile, or anything else that is thrown through the air. I would also point out that on the equator there is no drift of a projectile.
In the Stable Vertical you find the adjustment to correct the “Effect” of Coriolis Force. You do not correct for “Coriolis Force” but rather for the “Effect” of Coriolis Force. The correction is done by shifting the Bonnet in the X-Axis only, and this is a manual adjustment.
There is a bar on the top of the Bonnet in the Stable Vertical which reads zero in the center and on each end I believe, it reads about 70 or 80. This is degrees of latitude. This is a manual adjustment and as you change latitude you must adjust the bonnet to fit the degree of north or south latitude that the ship is in.
When the adjustment is set to zero, you should be on the equator, and you want the bonnet at this point to be level with the world. We are going to assume that the ship is in the level position at this point in this discussion.
The electro-magnetic on top of the gyro is centered in the X and Y hour glass shaped coils and the output of the coils reflects this by producing an equal output of current and voltage. As you move the adjustment to the degree of north or south latitude you are in, the X axis coil center is moved off center of the electro-magnetic on top of the gyro, producing an unequal current in the X axis hour glass coil. This makes the stable vertical think that the ship has rolled slightly and that the gun trunnions have tilted slightly due to roll.
Remember that the X-Axis is at 90 degrees to, or across the Line of Fire.
Of course, at this point, the output of the X-Axis coil causes the gun mount or turret to train to the right or left as necessary to find itself at a level position and overcome the ship's apparent roll.
But in our case here there was no roll of the ship and the gun trunnions were level with the earth’s true horizontal plane. So by tilting the true horizontal plane the stable vertical is transmitting or out-putting, the gun is repositioned to compensate for the drift of the projectile caused by the “Effect” of Coriolis Force.
It is of interest to note that if a gun is at zero degrees of elevation, tilting the trunnions in the X-Axis does not change the line of fire of the gun. On the other hand when the gun is elevated to 90 degrees, every degree the gun trunnion is tilted, changes the line of fire by the same degree. So range to the target - which sets gun elevation - tempers the resultant training of the gun mount or turret, due to transmitting a slightly tilted true horizontal plane by the stable vertical.
Since the adjustment moves the bonnet in the X-axis only, what it does
is tilt the true horizontal plane, being transmitted to the guns, in the
X-Axis only and leaves the Y-Axis at the true horizontal. This then
tells the gun that it must train right or left, as the case may be, to
correct itself and bring itself back to what it is being told is the true
How the Stable Element and Stable Vertical are used
To save my typing finger, assume that I am also saying Stable Element as well as Stable Vertical in all of the following description.
Information: Gun elevation has no consideration in the true horizontal plane. It is derived from range and has nothing to do with the true horizontal plane, although it is based on and from the true horizontal plane.
The Stable Vertical also transmits the true horizontal plane to the Mark 37 Fire Control Director electrically with differential synchros. The differential synchros are located in the fire control computer. The rotors of the differential synchros are driven by the shafting from the Stable Vertical transmitting the X and Y values of the true horizontal plane for the gun trunnions into the computer. The differential synchros are wired in between the synchro transmitters in the handwheels of the trainer and pointer in the Mark 37 gun director and the synchro receivers, that are receiving the signals created by turning the hand wheels.
Even though the horizontal plane information being transmitted to the Mark 37 Director is in or from the Line of Fire rather than the Line of Sight, the data provided is good enough to be of great assistance to the Mk 37 Director pointer and trainer.
That’s shorthand for saying that they don’t have to work nearly as hard as they would without this assistance.
If the target is coming directly at the firing ship, then there is no error in the information as the line of sight and the line of fire in the horizontal plane are the same. If the target is at any angle to the ship up to 90 degrees or paralleling the ship, and depending on speed and distance, then the amount of lead angle is the amount of error.
It really does not make any difference and was not worth the cost of another Stable Vertical because the pointer and trainer have to keep their cross hairs on the target anyway. This signal just makes it easier as they do not have to constantly overcome roll, pitch and yaw.
If the director is equipped with tracking radar, then the trainer and pointer have the capability to put the handwheels in automatic and then the radar provides the train and elevation signals as well as range data needed to stay on target.
When in automatic mode the Information from the Stable Vertical helps the Radar position the director and more easily stays on target as with manual pointing and training (Editor's note: This is "closed loop control").
The Rangefinder in the director receives X-Axis information only and
stays level due to this information.