page_title: LGM-30 Minuteman
section_title: Guidance system
hierarchical_section_title: LGM-30 Minuteman / History / Guidance system
caption_attribution_description: English: Autonetics D-17 guidance computer from a Minuteman I missile.
context_page_description: The LGM-30 Minuteman is a U.S. land-based intercontinental ballistic missile, in service with the Air Force Global Strike Command. As of 2020, the LGM-30G Minuteman III version is the only land-based ICBM in service in the United States and represents the land leg of the U.S. nuclear triad, along with the Trident submarine-launched ballistic missile and nuclear weapons carried by long-range strategic bombers.
  Development of the Minuteman began in the mid-1950s when basic research indicated that a solid fuel rocket motor could stand ready to launch for long periods of time, in contrast to liquid-fueled rockets that required fueling before launch and so might be destroyed in a surprise attack. The missile was named for the Colonial Minutemen of the American Revolutionary War, who could be ready to fight on short notice.
  The Minuteman entered service in 1962 as a deterrence weapon that could hit Soviet cities with a second strike and countervalue counterattack if the U.S. was attacked.
context_section_description: Previous long-range missiles used liquid fuels that could only be loaded just prior to firing. The loading process took from 30 to 60 minutes in typical designs. Although lengthy, this was not considered to be a problem at the time, because it took about the same amount of time to spin up the inertial guidance system, set the initial position, and program in the target coordinates.
  Minuteman was designed from the outset to be launched in minutes. While solid fuel eliminated the fueling delays, the delays in starting and aligning the guidance system remained. For the quick launch, the guidance system would have to be kept running and aligned at all times, which was a serious problem for the mechanical systems, especially the gyroscopes which used ball bearings.
  Autonetics had an experimental design using air bearings that they claimed had been running continually from 1952 to 1957. Autonetics further advanced the state of the art by building the platform in the form of a ball which could rotate in two directions. Conventional solutions used a shaft with ball bearings at either end that allowed it to rotate around a single axis only. Autonetics design meant that only two gyros would be needed for the inertial platform, instead of the typical three.
  The last major advance was to use a general-purpose digital computer in place of the analog or custom designed digital computers. Previous missile designs normally used two single-purpose computers; one ran the autopilot that kept the missile flying along a programmed course, and the second compared the information from the inertial platform to the target coordinates and sent any needed corrections to the autopilot. To reduce the total number of parts used in Minuteman, a single faster computer was used, running separate routines for these functions.
  Since the guidance computer would be inactive while the missile sat in the silo, the same computer was also used to run a program that monitored the various sensors and test equipment. With older designs this had been handled by external systems, requiring miles of extra wiring and many connectors. In order to store multiple programs, the computer, the D-17B, was built in the form of a drum machine but used a hard disk in place of the drum.
  Building a computer with the required performance, size and weight demanded the use of transistors, which were at that time very expensive and not very reliable. Earlier efforts to use transistorized computers for guidance, BINAC and the system on the SM-64 Navaho, had failed and were abandoned. The Air Force and Autonetics spent millions on a program to improve transistor and component reliability 100 times, leading to the "Minuteman high-rel parts" specifications. The techniques developed during this program were equally useful for improving all transistor construction, and greatly reduced the failure rate of transistor production lines in general. This improved yield, which had the effect of greatly lowering production costs, and had enormous spin-off effects in the electronics industry.
  Using a general-purpose computer also had long-lasting effects on the Minuteman program and the US's nuclear stance in general. With Minuteman, the targeting could be easily changed by loading new trajectory information into the computer's hard drive, a task that could be completed in a few hours. Earlier ICBMs' custom wired computers, on the other hand, could only have attacked a single target, whose precise trajectory information was hard-coded directly in the system's logic.
caption: Autonetics D-17 guidance computer from a Minuteman-I missile.
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