With the retirement of NASA’s shuttle fleet in 2011, the space agency has thus far been dependent on Russia’s Roscosmos to ferry NASA astronauts back and forth to the International Space Station. To prevent this being a permanent measure, the space agency divvied out $1.1 billion back in 2012 between Boeing, Sierra Nevada, and Space X for initial design and development of vehicles to replace the Roscosmos role by 2017.

Earlier in September the time came for NASA to narrow its scope, and the continued contracts were issued to Boeing and Space X. According to The Wall Street Journal, Sierra Nevada said on Friday that they had filed a formal protest with the U.S. Government Accountability Office over the decision.

Sierra Nevada argues that its winged space plane, which could land on a runway after returning from space, would cost up to $900 million less to build and operate than the crew capsules offered by Space X and the Boeing CST-100, which would come back to Earth in much the same way as Apollo-era capsules.

NASA has not made the details of their selection process public yet, and a spokesperson said they “wouldn’t have any comment while the protest is pending.” Unnamed sources for the Journal stated that Sierra Nevada was behind both Boeing and Space X in some technical rankings.

Sierra Nevada claims that its proposal was the second lowest of the three bids and that it “achieved mission suitability scores comparable to the other two proposals.”

In 2012, the company was awarded $212.5 million to continue development of their reusable space plane, the Dream Chaser, which could carry seven astronauts into low Earth orbit.

The Journal estimates NASA’s new contract with Space X at up to $2.6 billion, while the Boeing deal could be worth as much as $4.2 billion.