WASHINGTON — NASA Administrator Charles Bolden said Sept. 15 the agency is ready to proceed with development of a congressionally mandated Space Launch System (SLS), a heavy-lift rocket expected to cost $18 billion by the time it makes its first unmanned test launch in 2017.

Bolden made the long-anticipated announcement here Sept. 14 from a podium shared with Sens. Kay Bailey Hutchison (R-Texas) and Bill Nelson (D-Fla.). The two lawmakers have been pressuring NASA and the White House for months to commit to building the Space Launch System that Congress prescribed in the NASA Authorization Act of 2010.

As expected, the rocket design Bolden and the lawmakers unveiled during a press conference at the Dirksen Senate Office Building here consists of a cryogenic core stage based on the space shuttle’s external tank and powered by five space shuttle main engines. The upper stage will use the same J-2X engine that NASA had been developing under the defunct Constellation back-to-the-Moon program for use on the since-canceled Ares 1 and Ares 5 rockets.

Both the core and upper-stage engines would be made by Pratt & Whitney Rocketdyne of Canoga Park, Calif.

“Our decision to go with a liquid hydrogen/liquid oxygen launch vehicle system was based on NASA’s analysis to reduce costs, increase flexibility and leverage the U.S. leadership in this technology,” Bolden told reporters.

Bolden also said NASA will hold a competition for the Space Launch System’s side-mounted boosters. However, he said, early test flights will use space shuttle-derived solid-rocket boosters made by ATK Aerospace Systems of Magna, Utah.

“The development flights will take advantage of existing boosters and other hardware while companies compete for advanced boosters to enable greater capability for our new heavy-lift rocket,” Bolden said.

The first SLS demonstration mission, an unmanned flight that would launch the Multi-Purpose Crew Vehicle (MPCV), is slated for 2017, Bolden said. MPCV is essentially the Orion capsule from the Constellation program. Construction on the first space-bound MPCV has already begun.

During the press conference, Nelson  insisted that NASA can afford to build SLS, which he said will cost less to develop than lawmakers predicted when they wrote the 2010 NASA Authorization Act mandating construction of SLS and MPCV.

“This rocket is coming in at the cost of what not only what we estimated in the NASA Authorization act, but less,” Nelson said. “The cost of the rocket over a five- to six-year period in the NASA authorization bill was to be no more than $11.5 billion. This costs $10 billion for the rocket.”

MPCV, Nelson said, will cost $6 billion. Building out ground support infrastructure for the massive SLS will cost about $2 billion. All told, he said NASA would need to spend about $3 billion a year, or $18 billion through the first test flight in 2017.

Hutchison, eager to see SLS development get going, put NASA on the spot to make short work of modifying numerous contracts awarded under the Constellation program.

“I’m told by all of those who can make it happen that we are, in fact, mere weeks away, maybe one or two weeks away, from modifying the contracts and ensuring that we are going to have the designers with the experience to take us to the next level,” Hutchison said.

NASA, in an emailed response to questions from Space News, outlined the agency’s process for modifying SLS-related contracts: “NASA is currently working through a Procurement Strategy meeting in Washington this week, which has been scheduled for some time. The process includes releasing a synopsis of our approach, with a review by next week, and an industry day on September 29th.

“In parallel, and as quickly as our procurement process will allow, we will initiate negotiations for contract modifications as needed on existing contracts,” the statement concludes.

Despite the unified front presented during the press conference,  the agency’s human spaceflight chief warned that NASA will have to scramble to send up a test flight by 2017.

“We’re trying to get to 2017 and we see that as a pretty hard milestone for us,” William Gerstenmaier, associate administrator for NASA’s Human Exploration and Operations Mission Directorate, said.

“The big risk is still on the core stage,” Gerstenmaier said. “Building this tank is not going to be easy.”

Although the SLS core stage will draw heavily on shuttle heritage,  Gerstenmaier said this was no guarantee that NASA would be able to cruise through that part of the development.

“There’s still a lot of design work that’s a risk to us,” he said. “These components are used in a very new and novel way” in the SLS configuration.

Gerstenmaier also said NASA is looking at flying MPCV before 2017 on an expendable rocket, potentially United Launch Alliance’s Delta 4.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.