Development
The U.S. Air Force assembled its initial blue print for the EELV in 1994, following many years of government funded studies into an improved system and architecture, which was intended to replace most if not all existing “legacy” spacelifters (e.g. Delta II, Atlas II/Centaur, Titan IV, etc.). The architecture called for the spacelifter to be based on standardized fairings, liquid core vehicles, upper stages, and solid rockets. The Standard Payload Interface bus was also proposed as another way to save money and improve efficiency.
The initial bids came from four major defense contractors: Lockheed Martin, Boeing, McDonnell Douglas, and Alliant Techsystems. Each of the bids had a variety of different concepts. One of the contractors, Boeing, initially proposed utilizing the Space Shuttle Main Engines (SSME). McDonnell Douglas merged with Boeing in 1997 and used its Delta IV spacelifter for their EELV proposal.
Read more about this topic: Evolved Expendable Launch Vehicle
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