NASA has more than $300 million for the program this year, and expects to be able to spend up to $500 million per partner between this summer, when awards will be announced, and May 2014, when this phase of the program ends.
Companies bidding for the work also are being asked to determine how much they need to get to an orbital test flight, as well as what they'd be able to do with a flat budget of $400 million a year.
Ideally, NASA would like a test flight by the middle of the decade. Goals for the demonstration would be to stay in orbit three days, reach an altitude of at least 230 miles and demonstrate maneuverability in space. Also, the test vehicles should be capable of carrying four people.
Interested bidders have 45 days to respond to NASA's solicitation, which was issued Tuesday.
Companies currently holding NASA funds for space taxi design work are Boeing, Sierra Nevada Corp., Space Exploration Technologies (SpaceX) and Blue Origin, which is owned by Amazon.com founder Jeff Bezos.
Image: The Dream Chaser, designed by Sierra Nevada, is one of several firms vying to build space taxis for NASA. Credit: Sierra Nevada