WASHINGTON — Space Exploration Technologies (SpaceX) plans to carry more fuel on the next launch of its Falcon 9 rocket to avoid a repeat of the oxygen-rich engine shutdown that marred an otherwise successful demonstration flight last December.

The Falcon 9 safely delivered its unmanned Dragon cargo capsule to orbit Dec. 8, but the rocket experienced what SpaceX and NASA officials consider a minor anomaly during the stage separation phase of the flight.

Two of the nine liquid-fueled Merlin engines that power the rocket’s first stage ran low on kerosene during the cutoff sequence, resulting in a potentially problematic situation that a senior SpaceX official described as “an oxidizer-rich shutdown.”

“The mixture ratios on the engines got biased with a little more oxidizer,” Ken Bowersox, the former NASA astronaut who is now SpaceX’s vice president of astronaut safety and mission assurance, said in a Sept. 9 interview. “So because of that, when you get that mixture change happening, the temperatures can go up higher than you want inside the gas generator.”

Bowersox acknowledged the oxygen-rich shutdown to Space News after a NASA advisory committee member raised questions about the engine anomaly during a public meeting at the U.S. space agency’s headquarters here earlier the same day.

Elon Musk, SpaceX’s founder and chief executive officer, said in a Sept. 13 interview that subsequent Falcon 9 rockets would be loaded with more kerosene fuel. While that will reduce the amount of payload the Falcon 9 can haul to orbit, he said, it will reduce the chance that any of the rocket’s engines run out of fuel before liquid oxygen and prompt a temperature spike that can damage turbines. SpaceX also will make tweaks to the timing of its engine shutdown sequence.

“The engine shutdown command needed to be about, literally, about 0.1 or 0.2 seconds sooner in order for all engines to deplete on oxygen,” Musk said.

Despite the engine anomaly, Falcon 9 successfully delivered Dragon to orbit during the mission, an orbital demonstration flight conducted under NASA’s Commercial Orbital Transportation Service (COTS) program.

A senior NASA official said Sept. 13 that he is satisfied with SpaceX’s proposed fix, and that the Hawthorne, Calif.-based company still appears to be on track to meet its next COTS flight milestone.

“We’ve certainly seen SpaceX’s plans for corrective action,” Alan Lindenmoyer, manager of NASA’s Commercial Crew and Cargo Program Office, said Sept. 12. “We’ve already been briefed on their corrective actions and we don’t have any issues with it.”

SpaceX’s next COTS demonstration, an international space station (ISS)-bound flight, is still awaiting NASA’s formal signoff.

If that mission goes well, SpaceX would be cleared to begin periodic supply runs to the space station under a $1.6 billion contract NASA awarded to the company in 2008.

SpaceX’s acknowledgement of last December’s oxygen-rich shutdown followed a discussion of the incident during a joint meeting of the ISS Advisory Committee and the NASA Safety Advisory Panel at NASA headquarters here Sept. 9. The public meeting was held to discuss the results of an Aug. 9 meeting with SpaceX and Orbital Sciences Corp., another company under contract to fly cargo to the ISS.

During the August meeting, held at the Johnson Space Center in Houston, SpaceX told members of the two advisory bodies that there had been an engine anomaly during the most recent Falcon 9 launch, said Charles Daniel, a shuttle and space station safety expert at Herndon, Va.-based Valador Inc. and a member of the ISS Advisory Committee.

“There was no explanation or root cause analysis or corrective action for this particular anomaly,” Daniel said during the Sept. 9 meeting, which was open to reporters and the general public. “This is a relatively troublesome statement not to recognize that a premature engine shutdown was a significant event.”

SpaceX denies Daniel’s claim that the Falcon 9’s engines had shutdown prematurely during the Dec. 8 mission.

“We did not have a premature engine shutdown,” said Bowersox. Moreover, he added, an oxygen-rich shutdown is “not a catastrophic event for the Merlin engine. We’ve been through this on the test stand and we know what it looks like for our engines, so we know that this was not a risk to the mission.”

Bowersox also said SpaceX informed NASA officials of the engine anomaly right away. “This information was presented to NASA and to the [Federal Aviation Administration] not too long after the mission,” he said.

SpaceX spokesman Robert Block said that NASA received a briefing about the flight Dec. 15. SpaceX subsequently delivered full-rate telemetry for the flight on Dec. 28. A full flight report to the agency followed on Feb. 24, Block said.

The company Daniel works for, Valador Inc., was sued in June by SpaceX in Virginia’s Fairfax County Circuit Court. SpaceX brought the lawsuit forward after another Valador vice president, Joseph Fragola, made what SpaceX said were defamatory statements about the safety and reliability of the Falcon 9.

The lawsuit was settled out of court in early August.

According to SpaceX’s complaint, Fragola emailed Bryan O’Connor, then NASA’s chief of safety and mission assurance, June 8 saying he was trying to verify a rumor that the Falcon 9’s first stage experienced a significant anomaly during the Dec. 8 mission.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.