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.