UPDATED Aug. 3, 6:34 p.m. EDT

WASHINGTON — Boeing and Space Exploration Technologies Corp. (SpaceX) are now the clear frontrunners in the race to provide commercial crew taxi services to NASA, with Sierra Nevada Corp. waiting in the wings in case one of the other two falters.

NASA on Aug. 3 announced its selection of Boeing Space Exploration of Houston and SpaceX of Hawthorne, Calif., for final-phase development funding under its Commercial Crew Program, designed to provide domestic astronaut transportation services to and from the international space station. Boeing will receive $460 million during the 21-month performance period of its Commercial Crew Integrated Capability (CCiCap) Space Act Agreement; SpaceX garnered $440 million.

Whereas Boeing and SpaceX are developing wingless capsules, Sierra Nevada Space Systems of Louisville, Colo., is offering a more maneuverable lifting body design borrowed from an old NASA program dubbed HL-20. Sierra Nevada’s Space Act Agreement is valued at $212.5 million.

Passed over for a CCiCap award was ATK Aerospace of Magna, Utah, the longtime supplier of solid rocket motors for NASA’s now-retired space shuttle fleet.

If Boeing and SpaceX meet all of their self-imposed, NASA-approved milestones in the 21-month CCiCap base period, their designs for astronaut taxi systems will undergo a critical design review, the final hurdle to clear before construction can begin. Sierra Nevada’s crew transportation system would not undergo a critical design review at the end of its Space Act Agreement.

William Gerstenmaier, NASA associate administrator for human exploration and operations, said the agency could not afford to bring three commercial crew systems all the way to critical design review. But he said it was in the government’s best interest to have a fallback option in case one of the two systems being fast-tracked encountered problems.

Sierra Nevada’s award will allow the company “to remove, I would say, a large amount of technical risk” in the design of its Dream Chaser vehicle, Gerstenmaier said in an Aug. 3 conference call with reporters. “That lets us see, once the technical risk is removed, if that is something we might want to continue to pursue to critical design review level if one of the others has some problems.”

All of the companies that bid for CCiCap awards had to propose a complete system, including launch and crew vehicles, capable of carrying astronauts to and from the international space station. NASA wants at least one of these systems to be ready by 2017.

Currently NASA relies on Russia for space station crew transport services.

All three CCiCap winners have previously received NASA funds to work on elements of their crew transportation systems. Boeing is developing a capsule called the CST-100, which the company plans to launch aboard a United Launch Alliance Atlas 5 rocket.

During a post-award conference call with reporters, Boeing officials said the CST-100 leverages not only the company’s heritage as major contractor on the U.S. human spaceflight program throughout its history but also Boeing’s experience as a leading provider of commercial aircraft and satellites. They said the combined experience will enable Boeing to realize production efficiencies on the CST-100 without compromising reliability or crew safety.

John Elbon, vice president and general manager of Boeing Space Exploration, said the company was able to leverage some of its NASA-funded design work on the Orion Crew Exploration Vehicle, which was the subject of a two-way competition to build a lunar excursion vehicle that ultimately went to rival Lockheed Martin Space Systems of Denver. But the Crew Exploration Vehicle had different requirements in terms of size and mission duration, he said.

Numerous components on the CST-100, including its propulsion systems, parachutes and flight computers, have been flight proven under other government programs, allowing the company to minimize risk while bringing a high level of maturity to the vehicle’s design, said John Mulholland, vice president and program manager for commercial programs at Boeing Space Exploration.

Many of these systems, including landing airbags, also were tested under previous funded phases of the Commercial Crew Program and will undergo further testing under the initial CCiCap effort, Mulholland said. Boeing laid out 19 test milestones during the 21-month performance period, including additional wind tunnel and propulsion system testing, he said.

Pratt & Whitney Rocketdyne is supplying the launch abort and orbital maneuver and control thrusters for the CST-100’s service module, which will burn up in the atmosphere following each flight. The capsule’s pressurized crew module is reusable, with each certified for up to 10 flights.

The CST-100 module is designed to land on the ground, with the primary landing sites at White Sands Missile Range, N.M., and Edwards Air Force Base, Calif., Mulholland said. Boeing is looking for a third landing site as well, he said.

Sierra Nevada’s Dream Chaser will also launch on an Atlas 5.

“We are now the only lifting body in this competition and you’ve heard multiple times during the course of the discussion that NASA was seeking diversity in vehicles,” Mark Sirangelo, executive vice president of Sierra Nevada’s Space Systems Group, said in an interview with Space News. “There are two capsules and there’s one lifting body, so we feel very strongly that by NASA making this choice, it was sending a very strong signal that they would like to see a shuttle-like lifting body continue in the program.”

Sirangelo would not quantify Sierra Nevada’s financial contribution to Dream Chaser development, nor say what the company planned to charge NASA for rides to the space station. He did say that Dream Chaser would be cheaper than Soyuz seats, which the Russian government sells to NASA for about $60 million each.

SpaceX, meanwhile, is adapting its flight-tested Dragon cargo capsule for crewed missions. The craft will be launched atop the company’s Falcon 9 rocket. SpaceX founder and Chief Executive Elon Musk said in a post-award conference call that the crewed version of Dragon will make propulsive ground landings — on legs — at a yet-to-be-determined site. The current Dragon, a cargo-only vehicle, is limited to water landings. The passenger-carrying Dragon will seat seven, SpaceX said.

Besides upgrading Dragon, SpaceX also plans to field a more powerful Falcon 9 for the commercial crew missions. The company has been working on a new engine, the Merlin 1-D, to replace the Merlin 1-C engines that powered the company’s May cargo demonstration mission to the space station.

Musk said SpaceX will probably be able to fly a demonstration mission to orbit “later in 2015. We would go up, do a few orbits of the Earth and then return,” Musk said. Before any of the three systems are cleared to carry astronauts, NASA will have to certify that they meet the agency’s safety standards. This work will be done under separate contracts to be awarded at or near the end of the base CCiCap performance period. NASA will reveal more details about the certification process in an Aug. 8 Commercial Crew Program Forum, to be hosted at the Kennedy Space Center in Florida.

All three winners have said they can stage their first demonstration flights — which will not carry astronauts — by 2015 or 2016, but construction and flight tests are not funded under the initial CCiCap awards. Boeing officials said the CST-100 will be ready to conduct its first two flights, the second with a crew aboard, in 2016.

NASA’s selections, meanwhile, delivered a major if not fatal blow to ATK’s hopes for its proposed Liberty crew transportation system. ATK, whose Liberty design has not been funded in previous rounds of the Commercial Crew Program, nonetheless had announced in May that it would compete for a CCiCap award.

The Liberty rocket would have used an ATK-built solid-fuel core stage and the first stage of Europe’s Ariane 5 rocket as an upper stage. ATK proposed capping the rocket with a composite crew module.

ATK was working with NASA through unfunded Space Act Agreements to refine Liberty’s design.

Gerstenmaier confirmed that Blue Origin, the secretive space startup bankrolled by Amazon.com founder Jeff Bezos, did not submit a CCiCap proposal. Blue Origin had been involved in NASA’s Commercial Crew Program since the first round of funding was awarded in 2010. The Kent, Wash., company has received a total of $25.7 million in NASA funding, some of which it put toward a crew escape system for its New Shepard vertical-takeoff, vertical-landing suborbital vehicle.


Warren Ferster contributed to this report from Washington.

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.