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 are lit on the ground and two are dropped away during flight, was replaced by a single Russian RD-180 engine, which offered greater 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.