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Minuteman missile

The Minuteman I and II were United States nuclear missiles (ICBMs) in service from 1960 until 1997. The Minuteman III entered service in 1975 and is still in use. (USAF plans from 1998 intend to keep the Minuteman III until 2025.)

The Minuteman had two innovations that gave it a long practical service life: a solid rocket booster, and a digital flight computer.

The solid rocker 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.

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 program's flight computer, which had similar weight constraints.

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 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.

Guidance computer from a Minuteman missile, on display at the National Air and Space Museum in Washington, DC

Another gain that pursuaded 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.

A particular oddity of the Minuteman flight computer is that it used a rotating magnetic disk to store the computer program. This was inexpensive, and completely immune to radiation from nearby nuclear explosions. By contemporary standards (or even those of the 1980s), it was extremely slow and its capacity tiny (about 4KB), but neither drawback was particularly significant for its task. The contractors used special development software that optimized the placement of the instructions on the disk.

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