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In accordance with the U.S. Vision for Space Exploration, NASA has been tasked to send human beings to the moon, Mars, and beyond. The first stage of NASA's new Ares I crew launch vehicle (Figure 1), which will loft the Orion crew exploration vehicle into low-Earth orbit early next decade, will consist of a Space Shuttle-derived fivesegment Reusable Solid Rocket Booster (RSRB); a pair of similar RSRBs also will be used on the Ares V cargo launch vehicle's core stage propulsion system. This paper will discuss the basis for choosing this particular propulsion system; describe the activities the Exploration Launch Projects (ELP) Office is engaged in at present to develop the first stage; and offer a preview of future development activities related to the first Ares l integrated test flight, which is planned for 2009. As part of the U.S. Vision for Space Exploration, NASA commissioned the Exploration Systems Architecture Study (ESAS) to provide recommendations for fulfilling the U.S. goals of providing human transportation to the International Space Station, returning to the Moon, and traveling on to Mars. The ESAS committee recommended using a two-vehicle approach for these missions, separating crew from cargo for added safety . 2 The Ares V cargo launch vehicle will go into orbit first, carrying the Lunar Surface Access Module in the Earth departure stage. Once the Ares V is in orbit, the Ares I crew launch vehicle will loft the Orion crew exploration vehicle into orbit to rendezvous with the Earth departure stage, which then ignites for the trans-lunar injection burn.The original configuration for Ares I would have used a four-segment RSRB for the first stage and a Space Shuttle Main Engine (SSME) for the upper stage, while the Ares V would have used five SSMEs and two five-segment RSRBs for its core stage, followed by a Saturn-derived J-2X engine for the Earth departure stage . 3 After further engineering and business studies showed it would be more expensive to redesign the SSME to ignite in the upper atmosphere, the Constellation Program accepted the ELP team's recommendation to use a derivative of the J-2 engine that powered the Saturn V third stage to the Moon. However, because the J-2X produced less thrust than the SSME, the first stage needed to be upgraded to provide additional thrust. This change proved valuable for two reasons: The five-segment RSRB still uses Shuttle-derived hardware, allowing NASA to draw upon existing institutional knowledge and infrastructure. The five-segment RSRB is also part of the Ares V core stage, so ELP can apply test data, hardware, and lessons learned by experienced personnel from Ares 1 to Ares V development.ELP will benefit from the Space Operations Mission Directorate's long experience operating the four-segment Shuttle booster, as the five-segment unit will use the same casing, propellant, thrust vectoring system, and a similar nozzle design. ELP will also gain valuable assistance from the Shuttle Program in developing the five-segment motor, as both directorate...