SpaceX, after replacing a faulty first-stage check valve, readied the Falcon 9/Dragon combination for a second attempt to launch the first U.S. commercial resupply mission to the International Space Station (ISS) in the early morning hours of May 22.

U.S. Air Force weather forecasters offered an 80% chance of favorable weather for a 3:44 a.m. EDT liftoff from the Cape Canaveral Air Force Station, Fla. There has been a slight concern about isolated rain showers in the region. SpaceX has a backup launch opportunity on May 25 at 2:33 a.m. EDT.

Company engineers identified a faulty check valve in the No. 5 engine of the Falcon 9 rocket first stage as the source of the high-pressure reading responsible for the May 19 launch abort.

The NASA Commercial Orbital Transportation Services program test mission, now scheduled to last nine days, was 0.5 sec. from a scheduled liftoff at 4:55 a.m. EDT May 19 when the Falcon 9 flight control system computer detected a high-pressure signature in the No. 5 Merlin liquid oxygen/kerosene engine, triggering the shutdown.

“During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine. The failed valve was replaced on May 19 and, after thorough analysis, the vehicle has been cleared for launch,” spokeswoman Kirstin Brost Grantham said in a May 21 SpaceX statement.

The Falcon 9 counts nine SpaceX-developed Merlin engines powering its first stage and one for its second stage.

As part of the recovery operations, SpaceX reverified the function of the other Merlin engine check valves, according to Mike Horkachuck, NASA’s SpaceX COTS project executive, following a May 21 meeting of the NASA-led ISS Mission Management Team.

“It looked like they understand what the launch problem was,” Horkachuck says. “They still have all the safety checks they had on the initial launch attempt to be able to screen for this problem on any other engines, should it somehow manage to manifest itself again.”

With a successful liftoff, the unpiloted Dragon capsule would maneuver toward a rendezvous with the space station early May 24. After a second series of proximity checkouts, Dragon would maneuver within reach of the station’s Canadarm2. Station astronauts Don Pettit and Andre Kuipers, gathered at an internal control console, would reach out with the 58-ft.-long robot arm to grapple and berth Dragon to the station’s U.S. segment shortly after 11 a.m. EDT May 25.

The Dragon freighter would be opened by the station astronauts the following day, May 26.

Under current scheduling, Dragon would depart the station on May 31, descending to Earth under parachute for a recovery by SpaceX in the Pacific Ocean off the coast of Southern California.