SLS will use a new expendable (and presumably cheaper) version of the liquid-oxygen/liquid-hydrogen Space Shuttle Main Engine (SSME), designated RS-25D/E. Its upper stage will also use oxygen and hydrogen—and include a throwback to the Apollo era: a modern version of the venerable J-2 upper-stage engine from the 1960s Saturn V, fittingly called J-2X. The diameter of the first and second stages will be the same as the shuttle external tank (about 27 feet), and probably built with the same tooling in Michoud, La. In its initial form, SLS will be able to deliver at least 70 metric tons to low-Earth orbit, as the legislation specifies, and could grow to 130 metric tons. That would make it the biggest rocket ever built, surpassing the Apollo Saturn V.