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LGM-30 Minuteman
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LGM-30 Minuteman

The Minuteman I and II were United States nuclear missiles (ICBMss) in service from 1960 until 1997. The Minuteman III entered service in 1975 and is still in use (1998 USAF plans are to operate it until 2025). The name comes from the Revolutionary War's Minutemen.

Innovation and risk

The Minuteman had two innovations that gave it a long practical service life: a solid rocket booster, and a digital flight computer. This computer was one of the very first recognizably modern embedded systems.

The solid rocket booster made the Minuteman faster to launch than other ICBMs, which used liquid fuels. A crucial innovation in this area was to include a valve to release the booster pressure, and permit effective throttling of the booster.

A reprogrammable inertial guidance system was a major risk in the original program. When first proposed, no one had built a digital computer that would fit in a missile. One program, the SNARK supersonic cruise missile, had already failed to produce such a system.

A digital computer was essential to obtain the accuracy gains that kept this weapon effective throughout the cold war. As the Defense Mapping Agency more accurately mapped mass concentrations in the Earth, the inertial guidance software could be updated and loaded into the missiles to make them ever more accurate by having them compensate for these sources of gravity.

Another gain that persuaded program managers to accept the risk of the computer was that the computer could also be used to test the missile. This saved a large amount of weight in cables and connectors.

Minuteman I

The Minuteman I flight computer used a rotating magnetic disk for about 4KB of primary storage. Unlike modern computers, which use descendants of that technology for secondary storage on hard disk, this was the active computer memory. At the time, this was a small and inexpensive method to store data, although it was extremely slow by modern standards. The 5ms average access time is about 100,000 times slower than the PC133 SDRAM commonly used as main memory in home PCs around the year 2000.

The disk storage was considered completely immune to radiation from nearby nuclear explosions, making it an ideal storage medium, if a bit slow. To overcome speed problems, the contractor (the Autonetics Division of North American Aviation, which produced small commercial computers that used a disk for memory) had developed special software that optimized the placement of the instructions on the disk to give the system a speed boost (a technique employed on earlier magnetic drum computers).

Various upgrades and replacements of the computer over the years replaced this memory with magnetic core memory and eventually radiation-resistant semiconductor RAM.

Minuteman II

The Minuteman II program was economically crucial to the development of integrated circuits. It was the first mass-produced system to use a computer constructed from integrated circuits, and used most of the production of such circuits from 1962 through 1967. The other major customer of these circuits was the Apollo Guidance Computer, which had similar weight and ruggedness constraints.

Influences

The author Thomas Pynchon worked as a technical writer for the field support unit for the Minuteman missile, something that is probably reflected in the narrative of his novels The Crying of Lot 49 and Gravity's Rainbow.

Inventory

As of June 28, 2004, there are 517 Minuteman III missiles in active inventory.

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