NASA unveiled this week its plans to pay companies $1.6 billion to develop commercial services to carry astronauts to low-Earth orbit.

The space agency released a draft request for proposals (RFP) for companies to provide “a complete end-to-end design, including spacecraft, launch vehicles, launch services, ground and mission operations and recovery,” according to a statement.

The draft solicitation is for an Integrated Design Contract (IDC), for multiple companies with a value of up to $1.61 billion, running from July 2012 through April 2014.

“This IDC effort will bring us through the critical design phase to fully incorporate our human spaceflight safety requirements and NASA’s International Space Station mission needs,” Commercial Crew Program Manager Ed Mango said in a statement, adding he is hoping for a “strong U.S. industry response.”

NASA awarded four contracts worth a combined $269.3 million to the four firms in April as part of the second round of its Commercial Crew Development (CCDev) effort, or CCDev2. Those companies currently working on systems to carry astronauts into low-Earth orbit are Boeing [BA], SpaceX, Sierra Nevada Corp., and Blue Origin. They are charged with advancing concepts for commercial crew space transportation systems and maturing the design and development of launch vehicles and spacecraft for carrying up to seven people.

NASA Administrator Charles Bolden announced Monday, at the Air Force Association’s 2011 Air and Space Conference, that the space agency will fund additional milestones for some of the CCDev2 agreements, to accelerate development. These optional milestones were already negotiated with the firms.

“All four CCDev2 partners are performing very well and meeting their milestones,” Phil McAlister, director of NASA’s Commercial Spaceflight Development, said in a statement. “These additional milestones were selected because they sufficiently accelerated the development of commercial crew transportation systems to justify additional NASA investment.”

Those changes include four optional milestones added to Sierra Nevada Corp.’s agreement, which increase the potential value of its deal by $25.6 million to $105.6 million. NASA also amended Boeing’s agreement to add three optional milestones with a potential value of $20.6 million, which could raise the company’s contract to $112.9 million.