Despite embracing quite different design choices, the developers of both the Delta IV and the Atlas V were using the same approach to maintain the experience gained by previous launch vehicles. Both Delta IV and Atlas V have been preceded by intermediate vehicles serving as transitions between them and their proven ancestors. These "bridge" vehicles are the Atlas III and the Delta III, 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 IV and Atlas V.
The Atlas III was an initial effort to reduce vehicle complexity while increasing vehicle performance. It used 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 replaced the original design's three Rocketdyne engines with a single, more powerful NPO Energomash/Pratt & Whitney RD-180 engine. As a result, the Atlas III'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 III family also introduced two improved versions of the Centaur upper stage: the Atlas IIIA uses a single-engine Centaur, removing one RL10A-4-1 engine and centering the other along the Centaur's axis, while the Atlas IIIB 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 V series after the Atlas III is retired.
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