Project Constellation is cancelled by the Obama Administration and NASA must reevaluate its human spaceflight exploration program and consider most cost-effective program goals, launch vehicles, and spacecraft. NASA initiates trade studies for launch vehicle configurations, and the U.S. Senate proposes NASA move forward with the Space Launch System (SLS). NASA chose to employ the flight-proven RS-25 liquid propellant engines that powered the Shuttle Orbiter and the Solid Rocket Boosters for the SLS Core Stage propulsion system. An Interim Cryogenic Propulsion Stage (ICPS) is adapted from the Delta IV rocket for the SLS upper stage. The CEV continues development as the Multi-Purpose Crew Vehicle (MPCV). The SLS will have several configurations, called Blocks, as part of a long-term exploration mission program.