The storied "stage-and-a-half" system used on all Atlas rockets from Atlas B to Atlas II, where three engines are lit on the ground and two of them are dropped away during flight, was replaced by a single Russian RD-180 engine, boasting higher thrust and efficiency than previous engines. Unlike Atlas II and the later Atlas V, there was no option for solid rocket motors to be added to the first stage.
The "stage-and-a-half" system, used on all Atlas rockets from Atlas B to Atlas II, where three engines were ignited at launch and two were jettisoned during flight, was replaced by a single Russian RD-180 engine, which provided more thrust and efficiency than previous engines. Unlike the Atlas II and later Atlas V rockets, solid rocket motors could not be added to the first stage.