The research and development phase of the Atlas ICBM program produced three experimental models, Atlas A, B, and C. Three subsequent models reached operational status. The first, Atlas D, was a liquid-fueled one-and-a-half-stage missile equipped with radio-inertial guidance and a nuclear warhead. It was stored horizontally above ground in an unprotected launcher. Atlas E featured all-inertial guidance, stronger engines, and a larger payload capacity; it was also stored horizontally, but in a semihardened launcher. Atlas F also used all-inertial guidance, but these missiles were deployed in underground, blast-protected silos; stored vertically; and raised on elevators for launch. All Atlas models had a range of approximately 12,000 kilometers.