SINS Mk2 Mod 6
   SINS Mk2 Mod6 (2) 0.75

Model number: Mk2 Mod 6

Date manufactured: 1950's (early versions) up to 2006

Description: The MK 2 SINS was designed specifically for long-term seaboard use aboard Fleet Ballistic Missile submarines. It’s modifications and improvements evolved from MOD 1 to MOD 7. The MOD 7 in current use is almost identical to the MOD 6; it is the production version for the navigation core of the Trident I Fleet Ballistic Missile system.


Item History: In late 1955, the Secretary of Defense authorized a sea-based Intercontinental Ballistic Missile (ICBM) to help negate the Union of Soviet Socialist Republic’s nuclear threat. In December of the same year, the Navy established the Special Projects Office to rapidly pursue a program to develop a submarinelaunched missile capability, known as the Polaris submarine. Admiral Arleigh Burke, Chief of Naval Operations, appointed Rear Admiral William F. Raborn to become the first director.

A ship’s inertial navigation system (SINS) was deemed crucial for the submarine’s navigation and its primary role of initializing the missile’s guidance system. Navigation was seen as a critical aspect of the Fleet Ballistic Missile’s (FBM) feasibility because of the effect that it had on overall system accuracy, and because of interservice rivalry, which prompted the Air Force to doubt the Navy’s ability to successfully launch an ICBM at sea. Minimizing the time between exposures for radionavigation and satellite fixes for “resetting” the SINS was a critical requirement to ensure stealth for survivability.

Inertial Navigators
Building upon World War II German rocketry experience, inertial navigators had been developed at the MIT Instrumentation Laboratory, under Dr. Charles Draper, and at the Autonetics Division of North American Aviation, for use in bomber aircraft and longrange cruise missiles. The Instrumentation Laboratory had also worked for the Navy on applying SINS technology to submarines even before the Polaris program. For the Polaris program, the Laboratory’s work was executed by its industrial partner the Sperry Gyroscope Corporation, pioneers of gyrocompass technology, and a firm with numerous Navy contracts. The MIT Laboratory/Sperry SINS was implemented as a space stabilized inertial navigator. In this configuration, the inertial sensors (gyroscopes and accelerometers) remain fixed in orientation with respect to the stars.

The major breakthroughs of gyro design were the improvement in rotor bearings, the air-lubricated bearings of the output axis, and the symmetrical rotor drive motors. These changes resulted in great improvement in mass shift under acceleration, and an inertial system; the XN1 was flown as early as 1950. The required accuracy of this system was maintained for 1 hour. Additional improvements were made for the XN2 through XN5 systems.

Designed for Guidance
The XN6 was designed specifically for guidance of the Navajo Missile. It contained a full digital-type computer with the bold innovation of a large magnetic memory rotor supported on a flat-faced gas bearing. State-of-the-art computer electronic components at the time were cats-whisker diodes and discrete transistor circuitry. The gas bearing was an extension of the research work then in progress on gas spin bearings for gyro rotors.

Although the Navajo Missile was cancelled, the XN6 became the production N6A, and went on to fame in 1958 by navigating the USS Nautilus and the USS Skate on historic transpolar missions to the North Pole. It also became the basis for the N7, which bore the designation MK 2 SINS, and used the highly accurate G7A gyros with gas spin bearings.

The MK 2 SINS was designed specifically for long-term seaboard use aboard Fleet Ballistic Missile submarines. It’s modifications and improvements evolved from MOD 1 to MOD 7. For the MOD 3, an improved G7B gyro was produced which had a higher angular momentum, and consequently a higher accuracy. An additional gimbal was added to the platform to support a 4th Gyro as a monitor to limit the errors of the other gyros. The 4 gyro configuration was identified as the MOD 6. The MOD 7 in current use is almost identical to the MOD 6; it is the production version for the navigation core of the Trident I Fleet Ballistic Missile system.

Although many improvements were made since the original N7, the embedded Mardan computer is of the same obsolete 1950’s technology of cat’s whisker diodes and discrete transistors. Its performance enhanced by using both sides of the memory rotor disk to increase its memory size. The Mardan is still in use on the few remaining operating MK2 SINS.

The Fleet Ballistic Missile submarines from the Polaris through Trident 1 have had versions of the MK2 SINS from 1959 to the present time. Although MK2 SINS were retained on retired SSBN submarines that were converted to other programs, the last two Trident 1 submarines, the USS Henry M. Jackson (SSBN 730) and the USS Alaska (SSBN 731), are currently being converted to SSGN tactical weapon systems. With them, the FBM MK2 SINS, one of the spectacular technological breakthroughs of the FBM programs will be relegated to history.



Additional Photos:
SINS Mk2 Mod6 Electronic Computer Suite
SINS Mk2 Mod6 (1)
SINS Mk2 Mod6 (2)
SINS Mk2 Mod6 (3)

For More Information, Contact:
Marvin May
mbm16@psu.edu

Submission authored by:
Marvin May



mbm16@psu.edu