the atlas v has a special way of naming the different kinds. the atlas v nomenclature for numbering the variants goes like this: the first number says how wide the fairing is. a 400 series atlas v has a 4-meter wide fairing. a 500 series atlas v has a 5-meter wide fairing. an atlas v n22 have no fairing since it will be the launch configuration for the boeing's cst-100 starliner. second number says how many strap-on solid rocket boosters the rocket has. an atlas v 501 has no strap-on boosters, an atlas v 441 has 4 strap-on boosters, while the atlas v n22 has two strap-on boosters, which will be the boeing starliner launch configuration. the last number says how many engines the centaur second stage has. all of the atlas v rockets flown have had only 1 engine on the centaur, but in the future other atlas v rockets will have 2 engines on the centaur upper stage, such as the atlas v n22 that i frequently mentioned in this article. there is also the hlv which has two of the first stage (common core booster) strapped on like strap-on boosters, but it has never flown.