Captain Swoop's Virtual ICBM Tour presents...
Missile Parts: The Missile Guidance Set (MGS)

MGCS COMPONENTS
1. Permutation Plug ("P-Plug")
2. Battery Power Supply
3. Missile Guidance Set Control
4. Gyro Stabilized Platform
5. Liquid Coolant Passages 		    6. Umbilical Connection
7. RFI Shield/ Body Shell
8. Digital Computer Unit
9. R/S Connectors
10. Amplifier Assembly

 

The Missile Guidance Set (MGS), or "can" as it is called, is probably a good benchmark of the evolution that the Minuteman Missile has gone through. The original Missile Guidance-Control System (MGCS) units aboard the older Minuteman I and II missiles were much bulkier, far less reliable and accurate, and had a shorter lifetime before gyro or computer failure, to boot. They had many quality problems (at one time a major scandal occurred when it was discovered that contractors were obtaining low grade parts from Radio Shack and surplus suppliers to install in the MGCS's when milspec parts ran short). For a maintenance standpoint they were a major headache, as the frequent failures meant major manpower expendatures (10 or more personnel required to overwrite and reload targeting data, plus de-mate and reinstall the warhead - all aside from swapping out the "can"). Programming the Target sets into a Minuteman II MGCS was a chore in itself: theodolites had to be set up and aligned on various platforms and monuments to allow the MGCS to optically sight the North Star (Polaris) as a reference point. The job also involves calculating times, gyro angles, and lots of other fun. All this changed with the latest versions of the MGS and software. The "Polaris Tube" used for sighting still remains on the silos today - but its only (occasional) use now is to drop sandwiches to workers below ground. Boeing is now beginning yet another upgrade of the MGS for Minuteman III, which will improve accuracy and reliability even more, and allow the aging ICBM's to remain servicable until at least 2020.

Primary components of the G model MGS consist of the Digital Computer Unit, on-board Batteries, Control & Amplifier Assemblies, and the Gyro Platform.

  • The Guidance Computer (DCU) actual stores the missile targeting, warhead fusing data, and also generates fault alarms for the missile, while still sitting in the Launch Tube. Data and commands from the Launch Control Center travel to the MGS computer via a large "Upper Umbilical" which also contains coolant lines for the computer as well. The computer is cooled constantly by a chiller unit mounted in the Lower Equipment Rooms, using a toxic Sodium Chromate solution. When launch is commanded, the umbilical disconnects and the computer relies on an internal methods for cooling until its' target is reached. One of the odd features of the outdated software it uses is the fact that the missile has no exact "starting point" reference; it never knows "where" it is! Instead, it uses its' target location as a reference point, and calculates "where it's not"!

  • MGS batteries provide in-flight power to the missile. They come on-line when the missile enters "terminal countdown"; until then all power for the missile computer and gyro platform is provided by power supply racks in the Lower Equipment Rooms.

  • The Missile Guidance Control Set and Amplifier are the "muscle" in the missile, so to speak, actually connecting the "brain" (DCU) to the nozzle control units and the Gyro , to use the data it has to send the missile to its' target.

  • The Gyro Stabilized Platform is used to maintain the missile's reference points before and during flight; because of this it is extremely movement sensitive. Before any maintenance near the missile can take place, the Launch Crew must command the guidance computer into a "PIGA Leveling" mode, which basically tells it to ignore any data sets from the gyro. Accidentally bumping the missile can start the gyro slewing and cause the computer to drop "off-alert" for a short time as it goes into a safe mode; seismic disturbances (earthquakes) will do the same thing, and minor ones have actually sent entire squadrons of missiles into safe mode. The system was designed this way so that the ground shocks from nuclear weapons hitting nearby would not damage the gyro platform or confuse the guidance computer's reference points.
Another interesting feature is the "P-Plug", or Permutation Plug. This is a device installed by the MMT team which contains 1/2 of the launch code; the guidance computer uses data from it combined with a code storage device in the first stage to verify a launch command sent to the missile. This stops an outside agency from tapping into a communications line and pretending to be a launch control center, to launch the missile. They still would have to know the correct codes, even if they were able to do this, undetected.

 

2 members of a Missile Maintenance Team (MMT) work to mate cabling during a Minuteman III "can" install. One is standing in a "workcage", a 2-man box with rollers that is moved about in the Launch Tube by a hoist motor that the team installs for maintenance. This cage is the only method for accessing the entire length of the missile and the bottom of the launch tube. the other MMT member is standing on the "diving board", a section of launch tube wall that can be swung horizontally into the tube to allow workcage installation and some missile maintenance. The gold colored head of the "Upper Umbilical" can be seen at the lower left; it has been demated from the MGS connector nearby. Note how dark it is - the only lights in the launch tube are located on the workcage, on an operational missile site. The lighting from the camera would normally not be there to give this nice of a view.

 

For comparison, here is a Minuteman II Missile Guidance-Control System (MGCS) section. Notice the much bulkier size than the Minuteman III MGS. The optical sight(3) formerly used in targeting can be seen, as well.

1. Gyro Stabilized Platform

2. RFI Shield

3. Optical Alignment Window

4. Liquid Coolant Passages

5. Umbilical Connection

6. Body Section

7. R/V Connector

8. Battery

9. Missile Guidance Set Control

10. Missile Guidance-Control Computer

11. Amplifier Assembly

12. Permutation Plug ("P-Plug")