NASA and SpaceX, the agency’s six-year-old Commercial Orbital Transportation Services program partner, stressed the experimental nature of the company’s bid to lift off early May 19 on the first U.S. commercial resupply mission to the International Space Station.

The launch of the Falcon 9/Dragon rocket and spacecraft combination from the company’s launch complex at Cape Canaveral Air Force Station is set for 4:55 a.m. EDT, during a nearly instantaneous launch window.

“We really are on the eve of a very significant mission,” Phil McAlister, NASA’s acting director of commercial space-flight development, told a May 18 news briefing. “This is a test flight, and NASA views test flights primarily as learning opportunities. They don’t fit neatly into the characterization of success or failure. If it gets us in a better posture to fly the next time, that is really a good thing.”

Forecasters from the U.S. Air Force 45th Weather Sqdn. predicted a 70% chance of favorable conditions on the eve of the three-week mission. A desire to preserve as much of Dragon’s fuel for on-orbit maneuvers as possible restricts future launch opportunities to every third day. After May 29, the high temperatures from an increasing solar beta angle on the space station’s orbital plane would preclude additional launch attempts until after mid-June.

“If successful, there is no question this is a historic flight,” says Gwynne Shotwell, SpaceX president. “What’s important from a SpaceX perspective on a test flight is to make sure we learn something. Hopefully, we learn a lot. But we are really here to demonstrate this spacecraft, wring it out to the maximum extent possible. Obviously, the ultimate goal is to berth.”

With a successful liftoff, the unpiloted Dragon will attempt to rendezvous with the six-person orbiting science lab early May 22, following a series of guidance, navigation and communication system checkouts. Under a flexible timeline, Dragon would approach to a point 1.5 mi. below the station for more system checks, including a test of the astronauts’ ability to remotely command the capsule.

If the May 22 checkout goes well, Dragon will maneuver out in front of the station, then above and behind. Early on May 23, Dragon would return to the station, this time moving close enough for station astronauts Don Pettit of NASA and Andre Kuipers of the European Space Agency to grapple the freighter with Canadarm2. The two fliers will be working from a control console for the 58-ft.-long robot arm in the station’s Cupola observation deck.

Once Dragon is captured, the astronauts will berth the freighter to the station’s U.S. segment Harmony module, potentially at the 75-hr. mark in the mission.

“I don’t think there will be a lot of sleep for the folks at SpaceX,” Shotwell says. “It’s all hard.”

If Dragon is successfully berthed, it would remain at the station for about 18 days. The capsule will be unberthed with Canadarm2 and commanded to reenter the Earth’s atmosphere, descending for recovery by SpaceX in the Pacific Ocean, off the coast of Southern California.

Regardless of the mission outcome, SpaceX is preparing for a follow-on cargo delivery mission to the station in the fall. Once the test objectives are met, SpaceX intends to carry out a 12-mission supply delivery agreement under a $1.6 billion contract awarded to the company by NASA in late 2008.