WASHINGTON — NASA dropped one company from its short list of potential commercial crew providers and relegated another to a backup role because of doubts that the proposed vehicles could be completed within the time and money the agency has available, according to an internal document NASA released Sept. 4.

The 13-page document, known as a source selection statement, details NASA’s reasons for passing over Alliant Techsystems (ATK) and three lesser-known firms in August to pick Boeing, Space Exploration Technologies Corp. (SpaceX) and Sierra Nevada Space Systems for Commercial Crew Integrated Capability (CCiCap) awards totaling $1.1 billion.

While Sierra Nevada stands to receive more than $200 million from NASA to spend the next year and a half maturing its lifting-body Dream Chaser vehicle, Boeing and SpaceX were awarded $460 million and $440 million, respectively, to develop competing commercial crew solutions featuring capsules. The awards announced Aug. 3 call for Boeing and SpaceX to complete a critical design review by May 2014 meant to clear the way for construction of their systems to begin.

NASA also received CCiCap proposals from Space Operations, American Aerospace Inc. and Spacedesign Corp., but rejected them as “unacceptable” during the first of a five-step selection process, according to the source selection document.

CCiCap is the third round of the agency’s Commercial Crew Program to develop privately operated astronaut taxi systems to restore independent U.S. access to and from the international space station by 2017. NASA currently pays Russia for those services.

Boeing and Sierra Nevada were the big winners during the first two rounds of the Commercial Crew Program, receiving $130.9 million and $125.6 million, respectively, since 2010 to develop their designs.

ATK and SpaceX both joined the program in 2011 during the second round. But while SpaceX received $75 million to prepare its flight-proven Dragon capsule to carry humans, ATK’s work on its Liberty rocket-and-capsule design proceeded under a Space Act Agreement that entitled the company to NASA technical advice but no funding.

ATK’s proposal made it through to the end and received high marks for the amount of money the company pledged toward development, but was ultimately rejected because it “lacked enough detail to determine if a safe crew transportation system could be developed in a timely and cost effective manner out of the heritage components ATK selected for this concept,” William Gerstenmaier, NASA’s associate administrator for human exploration and operations, wrote in the July 31 source selection statement.

Justifying the backup role assigned to Sierra Nevada, Gerstenmaier wrote that Dream Chaser’s winged design “presents some unique challenges not found in a capsule design.”

In particular, Gerstenmaier wrote, NASA selection officials were unconvinced, even after a second review of Sierra Nevada’s proposal, that the company had a plan to adequately address risks associated with Dream Chaser’s thermal protection system, which protects the vehicle from the extreme heat generated during atmospheric re-entry.

“I think it’s fair to say there’s additional risk associated in creating the first orbital vehicle, but once those risks are retired you actually have a vehicle that can do many more things in space,” Mark Sirangelo, executive vice president of Sierra Nevada Space Systems, said Sept. 7.

ATK spokesman George Torres said Sept. 5 that Liberty officials learned during a NASA debrief “that although our proposal focused on the strategic goals called out in the CCiCap solicitation — such as cost to the government, schedule and safety — those categories were not given clear weighting in the ratings of the proposal.”

Because NASA used Space Act Agreements instead of traditional government contracts for CCiCap, ATK and the other losers do not have the option of protesting the awards. Torres said it was “too early to tell” whether ATK would continue Liberty’s development absent NASA funding. “We’ll have to reassess our plans,” he said.

Boeing, meanwhile, garnered praise for a “strong technical design” that demonstrated “how heritage subsystem components would be integrated into the design” of the company’s CST-100 space capsule. Gerstenmaier said this offset the proposal’s most notable shortcoming: a “lack of significant corporate financial commitment” on Boeing’s part.

Boeing spokeswoman Paula Korn declined to say how much money the company intends to invest in CST-100, though she did say, “NASA is funding the preponderance of this activity.”

“The investment Boeing has committed to NASA is consistent with balancing NASA/Boeing risk for this phase of the program acquisition process, as well as Boeing responsibility to shareholders,” Korn wrote in a Sept. 6 email.

SpaceX’s proposal, meanwhile, “provides the earliest crewed demonstration flight under a credible schedule at the lowest development cost,” according to the source selection statement.

Gerstenmaier said he was concerned that SpaceX would not be able to adapt its cargo-only system into a crew taxi as quickly as the company proposed. However, he wrote, planned SpaceX cargo runs to the international space station, the first of which is slated for early October, should give SpaceX some of the flight data it needs to reduce risks associated with modifying its systems for crew transport.

SpaceX wants to fly a crewed orbital test flight of its astronaut transportation system in 2015. Boeing plans a similar flight by 2016, according to redacted CCiCap Space Act Agreements released in August.

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.