President George W. Bush had announced the Vision for Space Exploration in January 2004, and NASA under Sean O'Keefe had solicited plans for a Crew Exploration Vehicle from multiple bidders, with the plan for having two competing teams.
These plans were discarded by incoming administrator Michael Griffin, and on April 29, 2005, NASA chartered the Exploration Systems Architecture Study to accomplish specific goals:
determine the "top-level requirements and configurations for crew and cargo launch systems to support the lunar and Mars exploration programs"
assess the "CEV requirements and plans to enable the CEV to provide crew transport to the ISS"
"develop a reference lunar exploration architecture concept to support sustained human and robotic lunar exploration operations"
"identify key technologies required to enable and significantly enhance these reference exploration systems"
A Shuttle-derived launch architecture was selected by NASA for the Ares I. Originally, the crewed vehicle would have used a four-segment solid rocket booster (SRB) for the first stage, and a simplified Space Shuttle Main Engine (SSME) for the second stage.
An uncrewed version was to use a five-segment booster with the same second stage.
Shortly after the initial design was approved, additional tests revealed that the Orion spacecraft would be too heavy for the four-segment booster to lift, and in January 2006 NASA announced they would slightly reduce the size of the Orion spacecraft, add a fifth segment to the solid-rocket first stage, and replace the single SSME with the Apollo-derived J-2X motor.
While the change from a four-segment first stage to a five-segment version would allow NASA to construct virtually identical motors, the main reason for the change to the five-segment booster was the move to the J-2X.
The Exploration Systems Architecture Study concluded that the cost and safety of the Ares was superior to that of either of the Evolved Expendable Launch Vehicle (EELVs).
The cost estimates in the study were based on the assumption that new launch pads would be needed for human-rated EELVs.
The facilities for the current EELVs (LC-37 for Delta IV, LC-41 for Atlas V) are in place and could be modified, but this may not have been the most cost effective solution as LC-37 is a contractor owned and operated (COGO) facility and modifications for the Delta IV H were determined to be similar to those required for Ares I. The ESAS launch safety estimates for the Ares were based on the Space Shuttle, despite the differences, and included only launches after the post-Challenger Space Shuttle redesign.
The estimate counted each Shuttle launch as two safe launches of the Ares booster.
The safety of the Atlas V and Delta IV was estimated from the failure rates of all Delta II, Atlas-Centaur, and Titan launches since 1992, although they are not similar designs.

In May 2009 the previously withheld appendices to the 2006 ESAS study were leaked, revealing a number of apparent flaws in the study, which gave safety exemptions to the selected Ares I design while using a model which penalized the EELV-based designs.
President George W. Bush had announced the Vision for Space Exploration in January 2004, and NASA under Sean O'Keefe had solicited plans for a Crew Exploration Vehicle from multiple bidders, with the plan for having two competing teams. These plans were discarded by incoming administrator Michael Griffin, and on April 29, 2005, NASA chartered the Exploration Systems Architecture Study to accomplish specific goals:
determine the "top-level requirements and configurations for crew and cargo launch systems to support the lunar and Mars exploration programs"
assess the "CEV requirements and plans to enable the CEV to provide crew transport to the ISS"
"develop a reference lunar exploration architecture concept to support sustained human and robotic lunar exploration operations"
"identify key technologies required to enable and significantly enhance these reference exploration systems"
A Shuttle-derived launch architecture was selected by NASA for the Ares I. Originally, the crewed vehicle would have used a four-segment solid rocket booster (SRB) for the first stage, and a simplified Space Shuttle Main Engine (SSME) for the second stage. An uncrewed version was to use a five-segment booster with the same second stage. Shortly after the initial design was approved, additional tests revealed that the Orion spacecraft would be too heavy for the four-segment booster to lift, and in January 2006 NASA announced they would slightly reduce the size of the Orion spacecraft, add a fifth segment to the solid-rocket first stage, and replace the single SSME with the Apollo-derived J-2X motor. While the change from a four-segment first stage to a five-segment version would allow NASA to construct virtually identical motors, the main reason for the change to the five-segment booster was the move to the J-2X.
The Exploration Systems Architecture Study concluded that the cost and safety of the Ares was superior to that of either of the Evolved Expendable Launch Vehicle (EELVs). The cost estimates in the study were based on the assumption that new launch pads would be needed for human-rated EELVs. The facilities for the current EELVs (LC-37 for Delta IV, LC-41 for Atlas V) are in place and could be modified, but this may not have been the most cost effective solution as LC-37 is a contractor owned and operated (COGO) facility and modifications for the Delta IV H were determined to be similar to those required for Ares I. The ESAS launch safety estimates for the Ares were based on the Space Shuttle, despite the differences, and included only launches after the post-Challenger Space Shuttle redesign. The estimate counted each Shuttle launch as two safe launches of the Ares booster. The safety of the Atlas V and Delta IV was estimated from the failure rates of all Delta II, Atlas-Centaur, and Titan launches since 1992, although they are not similar designs.
In May 2009 the previously withheld appendices to the 2006 ESAS study were leaked, revealing a number of apparent flaws in the study, which gave safety exemptions to the selected Ares I design while using a model which penalized the EELV-based designs.
Ares I