NASA formally released a tender Monday for the second round of competition for privately-developed spacecraft to carry U.S. astronauts to orbit by the middle of the decade, supplanting the agency's reliance on Russian contractors.

Artist's concept of Boeing's CST-100 crew capsule on top of a Delta 4 rocket. Credit: Boeing

About $200 million will be available to winning U.S. bidders this round, four times the figure awarded in February to five companies. The $50 million for the first commercial crew development, or CCDev, competition came from the Recovery Act of 2009.

NASA is seeking proposals to "further advance commercial crew space transportation concepts and mature the design and development of system elements, such as launch vehicles and spacecraft," the agency said in a press release.

The CCDev program is aimed at fostering a new commercial space industry to eventually take over human transportation duties to and from low Earth orbit, partially filling the void left after the retirement of the space shuttle. Until such providers are available, NASA astronauts can only get to space on Russian Soyuz spacecraft.

Proposals are due Dec. 13 and the agency expects to announce the winners in March 2011 for terms of up to 14 months. The CCDev 2 awards will be structured as funded space act agreements, agile contracting mechanisms that allow NASA to invest in the work of private companies on designs for commercial spaceships or enabling technologies.

NASA says $200 million is expected to be available to dole out to the next series of winners, but the exact figure depends on the passage of stalled appropriations legislation in Congress. Lawmakers will pass new federal budget bills after reconvening following the upcoming mid-term elections, but the timing of such legislation is uncertain.

Congress passed this year's NASA authorization bill in September and President Obama signed the legislation into law Oct. 11. The authorization act provides guidelines for the agency's new budget, calling for $1.6 billion for commercial crew development efforts through 2013.

There are no limitations on the pending procurement. Companies that did not receive space act agreements in February will be eligible for awards next spring. Firms already working on technologies under the CCDev program are not guaranteed more funding in the second round.

But programs kicked off under the CCDev procurement in February will run out of NASA money by the end of December, likely forcing some companies to either continue development using their own capital or temporarily halt design work and testing.

Industry officials say they do not expect any significant federal funds beyond Jan. 1 until new awards are announced in March.

The payments are structured based on milestones, meaning companies receive fresh funding as they accomplish key objectives and tests.

The winners of the two largest slices of the CCDev funding pie this year were Sierra Nevada Corp. and Boeing Co. with $20 million and $18 million, respectively.

Sierra Nevada is designing a lifting body spaceship named the Dream Chaser for launch by 2014 on an Atlas 5 rocket.

Boeing's CST-100 capsule should be ready to ferry astronauts to and from the International Space Station by 2015, according to project officials.

Between 80 and 100 employees are working on the CST-100 spacecraft, according to Boeing.

SpaceX was not one of the winners in the first round of CCDev, but the firm could stand a better chance at federal funds in round two, which will be targeted at maturing technologies.

According to the announcement, NASA will again consider proposals highlighting on enabling technologies that would help accelerate another spacecraft. The first round of CCDev agreements included funding for a life support system, launch abort system and a rocket emergency detection system.

Those technologies could be included again in CCDev, along with main engines, propellants and rendezvous sensors, according to NASA.