The Minuteman I, formally known as the SM-80, was a second generation
intercontinental ballistic missile (ICBM). It used solid propellants rather than
liquid fuels, used on the earlier Atlas and Titan. It was smaller and easier to maintain than
its predecessors and
its use of solid propellants permitted it to be launched almost instantaneously.
It was designed to be maintained in, and launched from, hardened underground
silos where it would be virtually immune from an enemy nuclear attack.