Atlas V
A three-digit (XYZ) naming convention is used for the Atlas V configuration identification. An Atlas V XYZ will have a diameter payload fairing, Y solid rocket boosters (0-5), and Z RL-10's on the Centaur upper stage (1-2). As an example, an Atlas V 551 has a 5.4 m payload fairing, 5 SRBs, and 1 RL-10.