WASHINGTON — NASA is reassuring commercial space firms that it will be a supportive customer for privately built space taxis even as it cautions that Congress could stymie efforts to foster development of such vehicles.

During an Aug. 19 industry event at NASA headquarters here, agency officials said 35 companies had responded to a May 21 NASA solicitation seeking input on the fledgling commercial crew initiative that U.S. President Barack Obama has proposed for a $5.8 billion investment over the next five years.

“We believe that we can fund up to four providers with that $5.8 billion,” Phil McAlister, NASA’s commercial crew planning lead, told an audience of mostly space industry executives and advocates attending the forum. “This is going to be a challenging program for both NASA and the private sector, and if somebody stumbles along the way we would like to have other providers that hopefully we can rely on.”

Commercial space advocates — including the companies aiming to build the private space taxis and launchers NASA says it needs — remain upbeat despite a series of legislative setbacks in the U.S. House and Senate, which pared back Obama’s commercial crew request in draft legislation moving through Congress and revived elements of the Moon-bound Constellation program Obama seeks to abandon.

“Of course, there is a lot of discussion here in D.C. and throughout the nation on what this program is going to be going forward,” McAlister said. “We are going to adjust if necessary, but this is sort of the baseline program that we are putting together today.”

McAlister said if NASA is directed to invest less money in commercial crew than the nearly $6 billion it has requested, it will have to re-evaluate its plans.

“However, competition is a fundamental aspect of the strategy,” he said. “I don’t want to say exactly what we would do, but we would have to trade schedule for the potential for competition, and I would say competition is a very strong driver.”

McAlister and other NASA planners are pressing ahead with commercial crew despite its uncertain future, laying the groundwork for an unorthodox procurement strategy that bears little resemblance to the way NASA buys spacecraft today. During the event, NASA officials from the Kennedy Space Center in Florida and Johnson Space Center in Texas vowed to work closely with industry to define human-rating requirements for NASA astronauts riding in commercial vehicles and said they would adopt a hands-off approach to overseeing their development, much like the agency has with the Commercial Orbital Transportation Services (COTS) program. Under COTS, Hawthorne Calif.-based Space Exploration Technologies (SpaceX) and Dulles, Va.-based Orbital Sciences Corp. are leveraging roughly $500 million in NASA seed funding to build and demonstrate new rockets and spacecraft designed to resupply the international space station. Once those demos are completed, SpaceX and Orbital are obligated to make regular cargo flights to the station under fixed-price contracts worth a combined $3.5 billion.

NASA officials had little to say about the thorny issue of indemnification, or shielding commercial launch service providers against catastrophic third-party liability claims. They also warned prospective commercial partners that NASA expects complete, end-to-end systems integration for the new space taxis, including ground and launch operations, mission planning, space station operations, crew training and vehicle and crew recovery.

“The commercial partner is going to be responsible for everything, and we expect a fully integrated solution in support of commercial crew,” said Maria Collura, a deputy in the space transportation office at Kennedy Space Center.

In addition, private companies would be expected to reimburse NASA if they choose to use government-owned facilities or infrastructure in executing commercial crew missions.

“There is some uncertainty regarding the availability of NASA facilities,” Collura said, encouraging private firms to alert individual NASA centers regarding anticipated infrastructure needs so they can “plan accordingly, and hopefully reduce the costs overall.”

While funding for commercial space taxis remains in flux on Capitol Hill, three of the four NASA spending bills moving through Congress would direct the agency to immediately embark on development of a government-owned space exploration system leveraging shuttle heritage and Constellation designs that could be ready by around 2015 to serve as a backup to commercial systems, potentially undermining the business case for commercial crew.

Brett Alexander, president of the Commercial Spaceflight Federation, an advocacy group here, said if Congress ultimately directs NASA to build a government-owned crewed system, it must not compete with the private sector.

“It is absolutely important that NASA clarify that it is planning to buy commercial services when those services are available and not rely on the government service to do it,” he told Space News Aug. 19, adding that the Orion crew exploration vehicle Congress appears inclined to continue funding is too costly and over-designed for routine crew and cargo runs to the space station.

The Planetary Society, a Pasadena, Calif.-based space advocacy group, is urging lawmakers in an Aug. 19 open letter to support the Obama administration’s commercial crew plans and to refrain from requiring NASA to begin development of a heavy-lift launch vehicle before a specific deep space destination has been selected.

“[W]e are concerned that the path on which the legislative process is proceeding will lead to an incomplete plan, which would be worse than no plan at all,” the letter states.