NASA has released a total of $20 million so far this year to two of the four U.S. companies developing spacecraft under the second round of the Commercial Crew Development program, or CCDev 2, according to a Feb. 16 progress report from the agency.

Sierra Nevada Space Systems of Louisville, Colo., and Boeing Space Exploration of Houston completed milestones in early January worth $12.5 million and $7.5 million, respectively.

NASA’s other two CCDev partners — Space Exploration Technologies (SpaceX) of Hawthorne, Calif. and Blue Origin of Kent, Wash. — are still working toward their first paid milestones of 2012.

Sierra Nevada, which won a $105 million CCDev 2 award last April, checked off its first milestone of the year Jan. 12, completing delivery of the Dream Chaser Engineering Test Article, a full-scale prototype of the company’s lifting-body Dream Chaser space plane. That mock-up will be used for several tests, including a free-flight test this summer. Sierra Nevada’s seven remaining milestones are worth a combined $42.1 million.

Boeing stands to receive up to $49.4 million more by July as it completes the eight milestones remaining under the $112.9 million CCDev 2 award it won last year to advance the design of its CST-100 space capsule.

SpaceX, meanwhile, completed an unfunded milestone Jan. 30 when it brought in NASA astronauts to examine and evaluate a prototype of a crewed version of the Dragon space capsule. A cargo-only version of Dragon is scheduled to make its second trip to orbit in late April as part of a demonstration mission for NASA’s Commercial Orbital Transportation Services program. The three funded milestones remaining under SpaceX’s $75 million CCDev 2 award are worth a combined $20 million. Blue Origin has five milestones remaining, worth about half of its original $22 million award.

NASA has several other CCDev 2 partners that recently completed milestones under their unfunded agreements with the agency. United Launch Alliance of Denver made progress toward human rating its Atlas 5 rocket, and Alliant Techsystems of Magna, Utah, further refined a concept for its proposed Liberty launch vehicle. Excalibur Almaz Inc., a Houston-based company with access to Soviet-era space hardware, further refined a concept of operations for using that hardware for future human spaceflight missions. The U.S. company has ties to the overseas group Excalibur Almaz, which is based in the Isle of Man.