SpaceX engineers readied a Falcon 9 rocket for launch Friday on the company's second operational space station resupply mission, a commercial flight to deliver more than 2,300 pounds of science gear, station equipment, spare parts and crew supplies to the international lab complex.
If all goes well, the unmanned Dragon cargo ship will return to Earth March 25, loaded with some 3,000 pounds of no-longer-needed hardware, broken components and experiment samples bound for laboratory analysis.
"Quite a bit of work has been done to get to this point, by the SpaceX team, by the ISS team on the ground and the crew on orbit," said Mike Suffredini, the space station program manager at the Johnson Space Center in Houston.
"We've spent quite a bit of time over the last few weeks reconfiguring the station to be ready for the berthing of the Dragon spacecraft. We did a major software upgrade on board ... over a million lines of code were upgraded, including the software for the (robot) arm that's going to capture the Dragon."
The station's robotic work station has been set up for grapple operations and the arm has been positioned to capture the spacecraft.
"So on board, we're ready to go for the launch of the Dragon," Suffredini said.
The two-stage Falcon 9 booster was scheduled for liftoff from launch complex 40 at the Cape Canaveral Air Force Station at 10:10:09 a.m. EDT (GMT-5), roughly the moment Earth's rotation carries the pad into the plane of the space station's orbit.
Forecasters predicted an 80 percent chance of acceptable weather Friday and Saturday.
During the first operational SpaceX resupply launch last October, one of the nine first stage Merlin engines in the Falcon 9 rocket malfunctioned, prompting the flight computer to shut it down. The other engines performed as designed and the Dragon cargo ship was released into the intended orbit.
SpaceX President Gwynne Shotwell told reporters Thursday the failure was caused by "a material flaw that went undetected in the jacket of the Merlin engine, resulting in a breach ... causing depressurization of the combustion chamber."
"This vehicle has been designed to accommodagte an engine out," she said. "Though you never necessarily want to see it happen, it's nice that we've demonstrated the vehicle (can operate) as it was designed."
Citing technology transfer concerns, Shotwell declined to provide much in the way of additional details. For his part, Suffredini said SpaceX engineers shared their results with NASA and carried out rigorous inspections and tests to make sure the engines will work normally during launch Friday.
"We have two options as the customer," he said, referring to NASA's commercial contract with SpaceX. "We can either put our hardware on that vehicle or not. When we were done, we felt the risks we were accepting with this flight were the same as we'd accepted with the previous flights. We put all the hardware we needed to fly on that vehicle."
The flight plan called for the Dragon capsule to be released into an initially elliptical orbit with a high point of 202 miles and a low point of around 124 miles. The solar-powered spacecraft then will carry out a complex computer-orchestrated series of rendezvous rocket firings to catch up with the space station early Saturday.
The Dragon capsule 14.4 feet tall and 12 feet wide at its base, with a trunk section that extends another 9.2 feet below the capsule's heat shield. The trunk is equipped with two solar panels and provides an unpressurized cargo bay for components bound for the station's exterior.
The Dragon will pull up to within about 30 feet of the station and then "park" itself. After sending commands to disable the capsule's thrusters, station commander Kevin Ford will use the lab's robot arm to grapple Dragon capsule around 6:30 a.m. Saturday.
At that point, ground controllers at the Johnson Space Center in Houston will take over, sending commands to the Canadian-built robot arm to maneuver the cargo ship to its docking port on the Earth-facing side of the station's forward Harmony module.
The manifest includes 178 pounds of crew supplies, including clothing and food; 300 pounds of space station hardware, including replacement components for the lab's carbon dioxide removal system; and more than 700 pounds of science gear, including a pair of Glacier freezers and experiment components.
For its second operational resupply flight, a spacewalk equipment handling fixture called a grapple bar is mounted in the Dragon capsule's unpressurized trunk. The station's robot arm, again operated by flight controllers in Houston, will be used to extract the grapple bar assembly and stow it on an external attachment fitting.
As the Dragon's pressurized cargo hold is unloaded, the astronauts will re-pack it with 1.5 tons of no-longer needed gear, failed components and experiment samples that are needed by scientists back on Earth.
The return manifest includes 209 pounds of crew equipment; 1,455 pounds of science gear, including a Glacier freezer and cold bags loaded with experiment samples; and 884 pounds of space station hardware.
"SpaceX 2 is a really important mission for us because the cargo that goes to and from the space station is key to completing our research mission," said space station Program Scientist Julie Robinson. "There'll be about 330 kilograms of supplies for research and experiments that go up on the flight and about 571 kg that come down.
"And it covers all the broad disciplines of research that we do on the space station, including human research, biology and biotechnology, physical sciences and education activities. And all of these supplies go to support the over 200 investigations that are active on the space station today."
The SpaceX Dragon capsule is the only space station cargo craft designed to bring cargo back to Earth, a critical capability that was lost when NASA's space shuttle fleet was retired in 2011.
The manned Russian Soyuz spacecraft that carry three-person crews to and from the space station can only bring back a few hundred pounds of cargo. All other station vehicles -- unmanned Russian Progress supply ships and European and Japanese cargo craft -- burn up during re-entry.
In 2006, anticipating the space shuttle's retirement, NASA announced a new program to develop unmanned cargo craft that would be procured by the government on a commercial basis. NASA eventually awarded two major contracts.
SpaceX, short for Space Exploration Technologies, holds a $1.6 billion contract to provide 12 cargo flights to the station for delivery of more than 44,000 pounds of equipment and supplies. Friday's launch marks the second operational resupply mission following a pair of successful test flights.
Orbital Sciences of MacLean, VA, holds a contract valued at $1.9 billion for eight cargo flights to the station. Another $288 million was budgeted for development and at least one test flight. An initial demonstration mission is expected later this year.