Atlas III
Despite embracing quite different design choices, the developers of both the Delta 4 and the Atlas 5 are using the same approach to maintain the experience gained by previous launch vehicles. Both Delta 4 and Atlas 5 have been preceded by intermediate vehicles serving as transitions between them and their proven ancestors. These "bridge" vehicles are the Atlas 3 and the Delta 3, both of which have a large degree of commonality with the older Delta and Atlas designs while pioneering various innovations for the follow-on Delta 4 and Atlas 5.
The Atlas 3 is an initial effort to reduce vehicle complexity while increasing vehicle perform-ance. It uses improved first-stage fuel tank construction and simplified components, while replacing the original Atlas's stage-and-a-half staging concept with a more conventional single stage. It also replaces the original design's three Rocketdyne engines with a single, more powerful, NPO Energomash/Pratt & Whitney RD-180 engine. As a result, the Atlas 3's first-stage thrust section undergoes only one staging event and has only seven fluid interfaces, as opposed to previous Atlas models with six staging events and 17 fluid interfaces.
The Atlas 3 family also introduces two improved versions of the Centaur upper stage: the Atlas 3A uses a single-engine Centaur, removing one RL10A-4-1 engine and centering the other along the Centaur's axis, while the Atlas 3B uses a lengthened version of the improved Centaur with two RL10A-4-2 engines. The improved Centaur engines include upgrades, such as chiller modifications and a health monitoring system designed to increase reliability and operational standards. Both the single- and dual-engine Centaurs will continue to be used on the Atlas 5 series after the Atlas 3 is retired.
