The Mark 68 Linear Rate Gun Fire Contol System

The Mark 68 GFCS is a dual-purpose (for surface and air targets), linear rate system that is used to control the 5-inch 54 caliber gun. The system can automatically track and engage surface targets and aircraft as well as missiles that travel less than 2,000 knots (greater than Mach 3). The Mark 68 GFCS can provide continuos orders (gun train, gun elevation, fuze order, sight angle, sight deflection, and parallax) to the gun mount. The GFCS is a combination of elements:

  • Mark 68 Director
  • AN/SPG-53 Tracking Radar
  • Mark 47 Fire Control Computer
  • Mark 16 Stable Element
  • Mark 42 (Mod 10) 5-Inch 54-Caliber Gun

The Mark 68 Gun Director

The gun director determines the present target's position in bearing and elevation along the line of sight. It is always located high in the ship to increase the visual detection range and reduce interference from the ship's superstructure. The director may track the target either optically or with the fire control radar. The fire control radar antenna is located on top of the director and the axis of the radar beam is parallel to the optical line of sight. Optical sights in the director consist of the director officer's binoculars, the director tracker's telescope, and a stereoscopic range finder. Slip rings, an assembly of brushes and circular rings, enable the director to have unlimited train.

To stabilize the line of sight in cross-level (motion across the line of sight), the major part of the director is stabilized and pivots on the cross-level gears attached to the director supporting member. The radar antenna and director optics are stabilized in level, up and down motion in the line of sight. Level and cross-level are sent to the director from the stable element.

The AN/SPG-53 Tracking Radar

The gun fire control radar (AN/SPG-53) is used to search for, acquire, and track air or surface targets automatically or manually. When used with Radar Signal Processing Equipment (RSPE), the radar will automatically acquire and track a target. In addition, the RSPE improves radar operation in an electronic jamming environment. The radar provides target range and range-rate (the rate at which a target's range is changing) to the computer. Electrical positioning signals are sent from the radar to the director train and elevation power drives to kep the director line of sight on target in the automatic target tracking mode of operation. The parabolic radar antenna is mounted on top of the director and the radar beam axis is parallel to the director tracker's telescope. Thus radar ranges can be obtained when the target is tracked visually. The radar beam has two scan patterns - spiral (12 degrees wide) for search and acquisition and conical for tracking.

The Mark 47 Fire Control Computer

The Mark 47 Computer is a dual-purpose computer designed to solve the anti-aircraft and surface-shore fire control problems and calculates gun and fuze setting orders for 5-inch 54-caliber guns. This analog computer is fully transistorized and has operating controls on the front cover along with dials that show computer output and inputs.

Automatic and continuous inputs to the computer include the ship's speed from the pit log, the ship's course from the gyrocompass, target range and range-rate from the fire control radar, target bearing and elevation from the director, and level and cross-level from the stable element.

Manual inputs to the computer include Initial Velocity (IV), wind direction and speed, and spotting data.

The fire control radar and director measure present target position continuously. When the computer's generated present target position equals the actual present target position established by the director, the computer can predict a future target position. It is at this future target position that the projectile will intercept the target. When the ballistic variables (superelevation, drift, wind, and IV) have been added to or subtracted from future target position, the aiming point for the gun mount has been calculated. This aiming point will be sent to the gun mount as a gun train order and gun elevation order that will change continuously as the range, bearing, and elevation to the target's present position changes. When the computer solves the fire control problem, it uses true horizontal and tru vertical reference planes to develop gun orders. These references are provided by the gyroscopes of the stable element. Nevertheless, since the gun is located on the moving deck, these gun orders must be converted to account for gn platform motion in level, cross-level, and deck-plane (trunnion) tilt before being sent to the gun. Further corrections compensate for horizontal and vertical parallax, as well as roller path tilt to complete the positioning of the gun

The Mark 16 Stable Element

The stable element is located in the gun fire control room next to the Mark 47 Computer. In general, its primary function is to measure the angles of level and cross-level. The stable element does this by using a gyroscope to establish true vertical and true horizontal. When the ship pitches and rolls, she will cause the director line of sight to move off target when tracking optically, or induce deceptive errors when in full automatic radar track. To keep the director line of sight on target, the stable element continuously sends orders compenstaing for cross-level motion to the director. It also sends level and cross-level to the computer.

Mark 42 (Mod 10) 5-Inch 54-Caliber Gun

Mount Weight 139,000 lbs
Train Limits 720 Degrees
Elevation Limit +85 Degrees
Depression Limit -15 Degrees
Train Velocity 40 Degrees/Second
Elevation Velocity 25 Degrees/Second
Projectile Weight 70 lbs
Projectile Initial Velocity 2,650 Feet/Second
Rate of Fire 40 Rounds/Minute
Ready Service Rounds 40
Personnel Required 12
Mount Personnel 2
ABS Crew Protection Fair
Re-gunning Time 1 Hour
Horizontal Range 25,909 Yards
Target Capability Anti-Ship
Shore Bombardment