WASHINGTON — Initial analysis of data from the Oct. 28 failure of an Orbital Sciences Corp. Antares rocket indicates that a turbopump in one of the two main engines on the rocket’s first stage malfunctioned seconds after liftoff, company executives said Nov. 5.

“Current evidence strongly suggests that one of the two AJ-26 main engines that powered Antares’ first stage failed about 15 seconds after ignition,” David W. Thompson, chief executive of Orbital Sciences, said in a conference call with financial analysts.

“At this time, we believe the failure likely originated in, or directly affected, the turbopump machinery of this engine,” he said, adding that more analysis was needed before the company could reach a definitive conclusion about the failure.

The AJ-26 engine, a refurbished version of the Soviet-era NK-33 engine provided by Aerojet Rocketdyne, has been the focus of speculation about the failure since the accident. Video of the launch showed the plume from the engines brighten about 10 seconds after liftoff, followed by an explosion at the aft end of the first stage. The rocket then fell back to Earth, triggering a larger explosion that destroyed the rocket and the Cygnus cargo spacecraft it was carrying.

The AJ-26 engine has performed normally on four previous launches of the Antares rocket, dating back to April 2013. However, an AJ-26 failed during a test at NASA’s Stennis Space Center in Mississippi in May, and another failed due to a fuel leak during a test there in 2011.

As part of the “go-forward” plan announced by Orbital Nov. 5, the company is accelerating development of an upgraded version of the Antares rocket that will replace the AJ-26 engines with an alternative engine. That engine has been selected, but the company has not announced that choice, citing competitive reasons.

Thompson said it is highly unlikely Orbital will use the AJ-26 on any future launches. “We will likely discontinue the use of the AJ-26 rocket engines,” he said, “unless and until those engines can be conclusively shown to be flightworthy.”

Asked during the conference call if the decision to discontinue the AJ-26 was linked to a “fundamental reliability issue” with the engine, Thompson responded, “I would say that’s a good assessment.”

Thompson said it was premature to discuss any contractual implications of discontinuing use of the AJ-26 engines earlier than planned. Aerojet Rocketdyne did not respond to a request for comment on its AJ-26 contract with Orbital. The company reported a loss of $17.5 million in its latest fiscal quarter on its AJ-26 contract, according to documents filed with the Securities and Exchange Commission on Oct. 10. The company cited costs incurred in previous test failures of the engine.

The ongoing Antares failure investigation is being performed by a seven-person team that the company announced on Nov. 3. Leading that Antares launch failure Accident Investigation Board is David Steffy, the chief engineer of Orbital’s Advanced Programs Group.

Three other Orbital officials are on the board: David Cooper, a member of the Independent Readiness Review Team in the company’s Launch Systems Group; David Swanson, senior director for safety and mission assurance for Orbital’s Technical Operations organization; and Eric Wood, director of propulsion engineering in the Launch Systems Group.

Two NASA officials are also part of the board: Tom Costello, the launch vehicle assessment manager for the International Space Station Program; and Matt Lacey, senior vehicle systems engineer for the NASA Launch Services Program. Wayne Hale, an independent consultant and former manager of NASA’s space shuttle program during its return to flight after the 2003 Columbia accident, is the board’s final member.

The Federal Aviation Administration’s Office of Commercial Space Transportation (AST), which licensed the Antares launch, is providing oversight of the investigation. AST chief engineer Michael Kelly and mishap response coordinator Marcus Ward are supporting the Orbital investigation.

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...