WASHINGTON — In awarding study contracts for a next-generation booster system for the heavy-lift Space Launch System (SLS) rocket, NASA winnowed down a field of seven finalists to four winners, according to a source selection document released July 16.

NASA opted to fund proposals from Aerojet, ATK Launch Systems, Dynetics Inc. and Northrop Grumman Aerospace Systems. It rejected proposals from Blue Origin, Boeing Co. and Lockheed Martin Space Systems Co.

None of the seven bidders would comment on the details of its proposal.

The selected proposals represent “a diverse cross-section of technologies that will enable competition by mitigating Advanced Booster risks prior to the design, development and test and evaluation of the Advanced Booster,” Dan Dumbacher, NASA’s deputy associate administrator for exploration systems, said in the source selection document. NASA posted the document online.

The four winners will split $200 million in NASA study money, with individual awards ranging from $25 million to $100 million, according a February solicitation. NASA unveiled the winning proposals July 13.

NASA has announced a total of two SLS missions, one in 2017 and one in 2021. In both missions, the rocket will launch the Lockheed Martin-built Orion deep-space exploration capsule on a circumlunar flight. Only the 2021 mission will be crewed.

The most heavily publicized booster study proposals came from Dynetics, which submitted three winning concepts. The Huntsville, Ala.-based company is working with Pratt & Whitney Rocketdyne of Canoga Park, Calif., to adapt the Apollo-era F1 engine for use as an SLS strap-on.

“The high-cost non-recurring engineering typical of engine development was accomplished during the Apollo-Saturn program, and significant risks (e.g., turbopump design and combustion stability) were eliminated, so our team can focus on booster affordability rather than technical feasibility,” Steve Cook, Dynetics’ director of space technologies, said in an April press release promoting the partnership.

In the source selection document, Dumbacher gave the Dynetics proposals top marks for affordability and praised especially the company’s plans to modernize manufacturing techniques for tank structures, saying the approach could provide “significant life-cycle cost advantages.”

Meanwhile, the companies whose proposals were passed over remained tight-lipped the week of July 16.

Brooke Crawford, a spokeswoman for Kent, Wash.-based Blue Origin, declined to discuss that company’s proposal. Blue Origin, which seldom discusses its various space projects publicly, is bankrolled by Amazon.com founder Jeff Bezos.

Patricia Soloveichik, a Boeing spokeswoman, said Boeing executives requested a post-competition debriefing from NASA, but that no such meeting had taken place as of July 20.

Lockheed Martin spokesman Gary Napier did not immediately reply to a request for comment.

Boeing and Lockheed already have major roles in NASA’s post-shuttle deep-space exploration plans.

Blue Origin does not. The company is working on suborbital and orbital launch and crew vehicles, some of which are based on old NASA designs. In 2011, Blue Origin got $22 million in NASA funding in the second round of the agency’s Commercial Crew Program. The company used the money to test a crew escape system and liquid-fueled engines.

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.