The RS-68 engine is being designed and developed by the Boeing Company, Rocketdyne Division. Rocketdyne is located in Canoga Park, CA. The engine is being developed as a part of the Boeing Delta IV launch vehicle for use by commercial customers as well as the United States Air Force Evolved Expendable Launch Vehicle (EELV) program. The first launch of the new EELV is scheduled in 2001. The Rocketdyne RS-68 engine is a high thrust, very low cost liquid oxygen / liquid hydrogen booster engine. Due to the RS-68 non-traditional design process of first developing heavier more robust engines, a technique was needed to estimate the weight and mass properties of the lighter flight configuration engines. This paper discusses several methodologies that use the Pro/ENGINEER solid models of the heavier development engines to simulate the flight engine mass properties.