Ship Gun Fire-control System - US Navy Systems - MK 68 Gun Fire Control System (GFCS)

MK 68 Gun Fire Control System (GFCS)

Introduced in the early 1950s, the MK 68 was an upgrade from the MK 37 effective against air and surface targets. It combined a manned topside director, a conical scan acquisition and tracking radar, an analog computer to compute ballistics solutions, and a gyro stabilization unit. The gun director was mounted in a large yoke, and the whole director was stabilized in crosslevel (the yoke's pivot axis). That axis was in a vertical plane that included the line of sight.

At least in 1958, the computer was the Mk. 47, an hybrid electronic/electromechanical system. Somewhat akin to the Mk. 1A, it had electrical high-precision resolvers instead of the mechanical one of earlier machines, and multiplied with precision linear potentiometers. However, it still had disc/roller integrators as well as shafting to interconnect the mechanical elements. Whereas access to much of the Mk. 1A required time-consuming and careful disassembly (think days in some instances, and possibly a week to gain access to deeply buried mechanisms), the Mark 47 was built on thick support plates mounted behind the front panels on slides that permitted its six major sections to be pulled out of its housing for easy access to any of its parts. (The sections, when pulled out, moved fore and aft; they were heavy, not counterbalanced. Typically, a ship rolls through a much larger angle than it pitches.) The Mk. 47 probably had 3-D cams for ballistics, but information on it appears very difficult to obtain.

Mechanical connections between major sections were via shafts in the extreme rear, with couplings permitting disconnection without any attention, and probably relief springs to aid re-engagement. One might think that rotating an output shaft by hand in a pulled-out section would misalign the computer, but the type of data transmission of all such shafts did not represent magnitude; only the incremental rotation of such shafts conveyed data, and it was summed by differentials at the receiving end. One such kind of quantity is the output from the roller of a mechanical integrator; the position of the roller at any given time is immaterial; it is only the incrementing and decrementing that counts.

Whereas the Mk. 1/1A computations for the stabilizing component of gun orders had to be approximations, they were theoretically exact in the Mk. 47 computer, computed by an electrical resolver chain.

The design of the computer was based on a re-thinking of the fire control problem; it was regarded quite differently.

Production of this system lasted for over 25 years. A digital upgrade was available from 1975 to 1985, and it was in service into the 2000s. The digital upgrade was evolved for use in the Arleigh Burke-class destroyers.

The AN/SPG-53 was a United States Navy gun fire-control radar used in conjunction with the Mark 68 gun fire-control system. It was used with the 5"/54 caliber Mark 42 gun system aboard Belknap-class cruisers, Mitscher-class destroyers, Forrest Sherman-class destroyers, Farragut-class destroyers, Charles F. Adams-class destroyers, Knox-class frigates as well as others.