Dampening expectations for a complete success was a major theme of today’s NASA press conference following the Flight Readiness Review (FRR) for the SpaceX demonstration flight to the International Space Station (ISS). The launch date for the mission remains targeted for April 30.

NASA Associate Administrator for Human Exploration and Operations Bill Gerstenmaier cautioned, however, that some tests remain to be be completed before a final decision is made and the mission is very challenging.

SpaceX will be conducting those tests, primarily related to software, and reporting back to NASA by April 23. NASA does not expect to perform another FRR at that time, but will consider the results before formally committing to the launch date. If it does not go on April 30, May 3 would be the next opportunity.

SpaceX succeeded in convincing NASA to combine the last two of its ISS demonstration flights, so this mission, dubbed C2+, is especially difficult. The company launched its first demonstration flight, C1, of the Falcon 9 rocket coupled with the Dragon spacecraft in December 2010. It was supposed to launch two more test flights, C2 and C3, but this flight combines the objectives of both, hence the C2+ designation. It will be only the third flight of Falcon 9 and the second flight of Dragon. SpaceX founder Elon Musk sounded optimistic about the performance of those two components of his space transportation system, but stressed that other aspects of the mission are firsts — such as rendezvous and berthing with the ISS, and the solar arrays for Dragon. “We’ve got a pretty good shot,” he said, “but there’s a lot that can go wrong on a mission like this.”

Musk noted that SpaceX hopes to launch two more missions to the ISS this year and if berthing is not successful this time it will try again. Gerstenmaier made clear that these commercial cargo missions are “critical” for ISS operations. Dragon, in particular, is the only cargo spacecraft capable of returning materials to Earth. Russia’s Progress, Europe’s ATV and Japan’s HTV spacecraft are not designed to survive reentry and burn up in the atmosphere. Dragon is designed for a survivable water landing, which was demonstrated during the December 2010 mission.

These cargo flights do not carry anyone aboard, but SpaceX hopes to evolve Dragon into a vehicle capable of carrying crew. When pressed today, Musk ventured that a flight with a crew might be possible in about three years if this demonstration flight succeeds.

Following the termination of the space shuttle program last year, NASA has no way to launch cargo or crew to the ISS. It is completely dependent on Russia to take people to and from the ISS, and Russia, Europe and Japan for cargo transport.

This mission will take 521 kilograms of cargo to the ISS, primarily crew provisions including food, as well as one Nanorack with student experiments, and some replacement parts. It will return 660 kilograms of material to Earth, landing in the ocean off the California coast after 18 days berthed with the ISS. Decisions are not final on what will be returned.

Musk said he did not know how much SpaceX has spent on this commercial cargo vehicle specifically, but said the company has spent a total of about $1 billion over the course of its history. NASA provided $381 million of that, while Musk and other investors provided the rest. NASA said it owes SpaceX another $15 million when it achieves the remaining three milestones under the Space Act Agreement through which NASA is facilitating SpaceX’s development of the transportation system. Alan Lindenmoyer, NASA’s manager of commercial crew and cargo at Johnson Space Center, replied to a question by saying that studies have concluded that it would have cost NASA four to ten times more to develop this capability under conventional contracting methods.

Between now and April 23, the company will continue tests to make certain that its hardware and software interact correctly under various circumstances. Musk stressed that Dragon is autonomous — no one is aboard with a joystick to make decisions. It is all done by software. “Dragon is making a lot of decisions all the time,” he said. In the past month, he continued, the biggest problem is “false aborts” when Dragon gets “worried” about something and aborts the mission when it is not necessary.