United Launch Alliance operates the Delta IV Heavy, the largest rocket in the American launch inventory. The first stage of the Delta IV Heavy includes three core stages, each one 134-feet-tall and 16.7 feet in diameter. An RS-68 engine is at the base of each core stage to give the rocket a combined thrust of about 2 million pounds. The stage holds super-cold liquid oxygen and liquid hydrogen propellants. The second stage of the Delta IV Heavy is powered by a single RL10B-2 engine that also uses a combination of liquid hydrogen and liquid oxygen. The Orion spacecraft is bolted to the top of the second stage.