Liquid oxygen
The Atlas II first stage is a in diameter and , the stage is powered by 3 RS-56 rocket engines burning of both RP-1 and Liquid oxygen. The Atlas II was the last Atlas rocket to use the "1.5 staging" technique, with this technique Atlas II ignite its 3 RS-56 engines at liftoff and then jettison the 2 RS-56-OBA side engines during ascent to only use the RS-56-OSA as this engine is more efficient in high altitude.