Sierra Nevada Corp., the losing bidder in NASA’s recent multibillion-dollar “space taxi” competition, has gone to court seeking to block winners Boeing Co. and Space Exploration Technologies Corp. from proceeding with work until a pending contract protest is resolved.

Sierra Nevada on Wednesday asked the U.S. Court of Federal Claims in Washington to prevent Boeing and SpaceX, as the Southern California company is known, from receiving payments under separate contracts awarded last month by the National Aeronautics and Space Administration to develop, test and operate commercial-crew vehicles to take U.S. astronauts into orbit.

The legal move comes as more details emerge about price differences between Sierra Nevada’s unsuccessful bid and Boeing’s more-expensive offering, renewing debate about NASA’s ability to ultimately pay for all the projected trips to the international space station over the next several years.

The two contracts, amounting to a maximum of $6.8 billion, are intended to cover further development and certification of the safety of the vehicles for manned missions, which are supposed to begin no later than 2017. Sierra Nevada’s Dream Chaser vehicle is winged and able to land on a runway, while the other two companies are using capsules.

Sierra Nevada, which previously said its overall proposed price was about $900 million less than the maximum of $4.2 billion awarded to Boeing, filed a protest with the U.S. Government Accountability Office last month challenging NASA’s decision. SpaceX was awarded a maximum of $2.6 billion.