A first-stage engine chamber over pressure appears responsible for the last -econd shutdown of the Falcon 9 rocket, preventing a May 19 lift off of the Dragon capsule on the the first U. S. commercial resupply mission to the International Space Station (ISS), according to Space Exploration Technologies (SpaceX).

A second attempt to launch the ground breaking mission from Cape Canaveral Air Force Station, Fla., is possible May 22 or May 23 -- depending on the outcome of an inspection of the suspect first-stage No. 5 engine. A longer delay was likely, if the engine must be replaced. A second Falcon 9 already at Cape Canaveral could supply a ready replacement.

Preparations for a May 19 4:55 a.m. EDT liftoff looked promising after a smooth overnight coundown. The Falcon 9 flight control computer activated at the one minute mark and all nine of the Falcon 9 first stage SpaceX developed Merlin liquid oxygen/liquid kerosene engines ignited on schedule at the three second mark.

However, the flight computer commanded a shutdown at the half-second (T minus 0.5 seconds) point, when it noted a rise in the No. 5 engine chamber pressure, SpaceX President Gwynne Shotwell told reporters at a NASA-hosted news briefing.

“We had nominal ignition for all nine engines, with the exception of engine No. 5. Engine No. 5 started fine, then started to trend high due to chamber pressure,” Shotwell said.

Company technicians will examine the engine compartment. Engineers were especially interested in looking at the fuel pre-valve for the No. 5 engine  since the early data indicated a low-fuel concentration, said Shotwell. A borescope inspection of the fuel pump is likely as well.

In the minutes immediately following the shutdown, SpaceX founder and CEO Elon Musk suggested the pressure tolerance limits might be adjusted for a second launch attempt. Musk was monitoring the countdown from the company’s Hawthorne, Calif., headquarters and commenting through Twitter.

Later, Shotwell said further data analysis pointed to an actual pressure rise, similar to one blamed for a do-over, or “recycle,” in the countdown for the first Falcon 9 launch in July 2010.

“We cannot blame a software bias for this one,” said Shotwell.

The Falcon 9 under went a successful April 30 launch pad hot fire test. Data from the recent test was a factor in diagnosing the chamber pressure rise.

SpaceX stressed the experimental nature of the flight.

“This is not a failure,” said Shotwell. “We aborted with purpose. It would have been a failure if we had lifted off with an engine trending in this direction.”

A desire to preserve as much of Dragon’s fuel for orbital maneuvers as possible restricts future launch opportunities to every third day -- which assures the most accurate trajectory of the Falcon 9/Dragon into the orbital plane of the space station. However, SpaceX said a second launch attempt on May 23 is possible.

A May 22 launch would be timed for 3:44 a.m. and a May 23 attempt for 3:22 a.m., according to NASA.

After May 29, the high temperatures from an increasing solar beta angle on the space station’s orbital plane would restrict additional launch attempts until after mid-June.

“We are ready to support, when SpaceX is ready to go,” said Alan Lindenmoyer, NASA’s Commercial Orbital Transportation Services program manager.

The three week SpaceX test mission is flying under the Commercial Orbital Transportation Services (COTS) banner. The initiative was established six years ago to nurture the development of U.S. commercial ISS cargo providers.

During the test flight, the unpiloted Dragon capsule will attempt to rendezvous with the station on the mission’s second day. If a series of NASA monitored navigation, guidance and communications checks go well, the freighter will retreat, then  return the following day. The capsule will attempt to move within reach of the station’s 58-foot-long Canadarm2.

Two astronauts aboard the station will command the arm to grapple Dragon and berth it to the space station.

NASA’s second COTS participant, Orbital Sciences, plans a similar test mission later this year.