Atlas 5/Cygnus launch photo
An Atlas 5 lifts off from Cape Canaveral, Florida, on March 22. The rocket's upper stage burned for more than a minute longer than planned, ULA confirmed March 24. Credit: ULA

WASHINGTON — United Launch Alliance engineers investigating a premature engine shutdown on the recent launch of a Cygnus cargo spacecraft have narrowed the cause of the anomaly to the vehicle’s first stage fuel system, the company announced March 31.

In a statement, ULA said the investigation into the March 22 incident, where the first stage shut down six seconds early, was ongoing, but that teams have developed “initial fault trees” to track down the cause of the problem.

“The team has been successful in isolating the anomaly to the first stage fuel system and its associated components,” the company said in the statement, but offered no additional details on what caused the anomaly.

ULA said a “robust review team” is currently working to track down the specific cause of the premature engine shutdown. “The review will thoroughly assess all flight and operational data to determine root cause and identify appropriate corrective actions prior to future flights,” ULA stated.

ULA confirmed March 24 that the RD-180 engine that powers the first stage shut down early after industry sources noted the issue in telemetry of the launch broadcast on NASA TV. The rocket’s Centaur upper stage compensated by firing for more than a minute longer than planned, placing the Cygnus payload into its planned orbit.

That additional burn consumed most of the propellant planned for use in a later firing of its RL10 engine to deorbit the stage and reenter south of Australia. The engine was to fire for 11 seconds on that burn, but shut down 8 seconds early. The stage reentered further east than planned in an uninhabited stretch of ocean.

The March 31 statement is the most detail that ULA has provided about the anomaly to date, as company officials said they prefer to let the investigation play out before discussing it. “We have a pretty good idea about the cause, but won’t share until we are sure,” Tory Bruno, president and chief executive of ULA, wrote in a March 25 post on, commenting on an earlier SpaceNews article about the engine anomaly.

“We avoid jumping to the answer until the investigation is complete,” he added. “That prevents us from narrowing in on a tempting answer too quickly before all the possibilities have been thoroughly investigated.”

Bruno also said that he was “not concerned about impacts to the manifest” caused by the investigation. ULA has delayed the next Atlas 5 launch, of the U.S. Navy’s fifth Mobile User Objective System satellite, by a week since the anomaly. That launch is now scheduled for no earlier than May 12.

In its latest statement, as in earlier comments, ULA stressed that it considered the March 22 launch a successful mission, despite the anomaly, as it placed the Cygnus cargo spacecraft into its planned orbit. That spacecraft arrived at the International Space Station as planned March 26.

That launch’s customer, Orbital ATK, said they had no issues with the launch. “The partnership with ULA has been very good for us,” said Frank Culbertson, president of Orbital ATK’s space systems group, during a March 30 panel session at the National Academies on commercial research in low Earth orbit. The March 22 launch was the second time that Orbital ATK used an Atlas to launch a Cygnus as the company works on an upgraded version of its own Antares launch vehicle.

“They’ve put us right on target both times,” he said. “Even if they’re working a small problem, they still got us on orbit, on target, and we were able to execute the rendezvous as planned.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...