WASHINGTON — Despite its proposed $5.8 billion investment in developing private space taxis capable of ferrying astronauts to and from the international space station, NASA is making no promises to procure their services.

Phil McAlister, NASA’s commercial crew planning lead, says the agency’s goal in fostering development of privately owned spaceships and rockets is to ensure a robust commercial enterprise that offers redundant access to the orbiting outpost while allowing NASA to concentrate on more-challenging human spaceflight endeavors. Following retirement of the space shuttle next year, NASA plans to rely exclusively on Russian Soyuz spacecraft for astronaut transport until a domestic commercial alternative comes on line, hopefully by mid-decade. But if commercial capabilities prove too costly, McAlister said, NASA could continue purchasing astronaut rides on Soyuz craft.

“It would be a tough thing,” he told a public meeting of the NASA Advisory Council exploration subcommittee Sept. 21. “We didn’t want to lock ourselves in at this stage because we just weren’t sure how they were going to play out at the end of this development.”

McAlister said NASA wants private space firms to feel confident that the agency will procure their services, “but of course there are legal considerations of what you can guarantee five years in advance.” NASA has no plans to establish a minimum threshold for private investment in crew taxi development, but McAlister said, “We’re going to ask them to invest, and to get sufficient investment they’re going to have to have a very strong business case, and we’ve got to help them with that.”

In the meantime, McAlister said, NASA is continuing to refine stringent human-rating requirements for commercial crew vehicles and determining how the government would certify and license such services. A draft human-rating requirements document was issued in May, and another round is expected “in about a month,” with a final version anticipated by year’s end, he said.

NASA’s commercial crew proposal has been hotly contested on Capitol Hill, where draft bills that would curtail spending on the effort continue to wind their way through the legislative process. This could drag well into the federal budget year that begins Oct. 1 and potentially delay spending on commercial crew initiatives.

That could buy the agency time to refine its human-rating requirements for commercial crew vehicles before accelerating the pace of development already under way at some companies. But while NASA wants to limit future changes to hardware design requirements, which can slow development and increase cost, some future wiggle room is needed, McAlister said.

“We know we’re not going to be able to baseline those requirements today for what it’s going to be five years from now,” he said. “We’re going to have to have a method for incorporating changes that limits the government’s exposure.”

He said NASA had not settled on mission safety requirements with design implications when it signed its Commercial Orbital Transportation Services (COTS) program agreements with Space Exploration Technologies Corp.(SpaceX) and Orbital Sciences Corp. The companies are developing space station cargo delivery systems under COTS agreements with a combined value of $448 million. NASA administrative costs plus $32 million spent on a now-terminated COTS agreement bring the current program’s value to $500 million.

McAlister said the COTS agreements put the burden of paying for hardware changes on the companies.

“Any kind of cost growth was absorbed by the commercial partner,” McAlister said. “The government’s investment was fixed at $500 million, and that has not grown,” he said.

However, in its 2011 budget request, NASA requested an additional $300 million for COTS above the program’s original $500 million price tag. In addition, NASA provided $24 million in 2009 economic recovery funding to pay for improvements to ground infrastructure at test facilities for the two COTS providers.

In 2008, both SpaceX and Orbital signed Commercial Resupply Services (CRS) contracts worth a combined $3.5 billion to begin routine resupply runs to the space station following successful completion of the COTS program. To date, the two firms have received a combined $214 million in milestone payments under their CRS contracts, bringing the government’s combined investment in their vehicles to roughly $750 million. SpaceX and Orbital have yet to conduct their initial COTS demo flights.