In the 1990's Rocketdyne was simultaneously developing the first two new large liquid-fueled rocket engines in the United States in more than 25 years. One of these - the RS-68 - would power the Delta IV evolved expendable launch vehicle (EELV) being developed by The Boeing Company. The bell nozzle RS-68 is a liquid hydrogen-liquid oxygen booster engine that develops 650,000 lb. of sea level thrust. The RS-68 utilizes a simplified design philosophy resulting in a drastic reduction in parts compared to current cryogenic engines. This design approach results in lower development and production costs. The engine can be throttled to 60% of full thrust level. Designed for booster applications. Gas generator, pump-fed.

Upgrade to basic RS-68 for Delta IV Heavy growth versions. Details not specificed, except that it would use a regeneratively-cooled expansion nozzle, presumably allowing it to run hotter, higher thrust and specific impulse.

Upgrade (details not specificed) to basic RS-68 for Delta IV Heavy growth versions.