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ULA confirms engine issue on latest Atlas launch

Atlas Cygnus OA-6 launch

WASHINGTON — The upper stage of the Atlas 5 that launched a Cygnus cargo spacecraft March 22 fired for more than a minute longer than planned, apparently to compensate for the premature shut down of the rocket’s first stage engine.

United Launch Alliance confirmed March 24 that the Centaur upper stage used on the launch of the Cygnus OA-6 mission burned for longer than scheduled, although the company did not provide a reason for the extended engine firing.

“The Centaur burned for longer than planned,” ULA spokeswoman Lyn Chassagne said in a statement provided to SpaceNews. “The team is evaluating the occurrence as part of the standard post-flight data analysis.”

While United Launch Alliance declared the Atlas 5 launch from Cape Canaveral a success, placing the Cygnus in its planned orbit, industry sources and broadcast telemetry suggest that success required an extended burn by the Atlas’ Centaur upper stage.

According to a mission summary released by ULA in advance of the launch, the Centaur was scheduled to shut down its RL10 engine 18 minutes and 9 seconds into the mission. However, telemetry of the launch, shown on NASA Television, showed that the engine continued to fire beyond that time, shutting down about 19 minutes and 20 seconds into the mission.

That extended burn appeared to catch launch commentators off-guard, who had advised on more than one occasion that engine shutdown was expected at the planned time. There was no public commentary about the extended burn during the live broadcast beyond a statement that the Centaur continued to “perform very well.”

At a post-launch press conference at NASA’s Kennedy Space Center early March 23, Vern Thorp, ULA’s program manager for NASA missions, suggested that the additional burn was a routine discrepancy from original projections for the burn.

“The pre-launch predictions of exactly when the events are going to occur are based on a preliminary trajectory. Typically it’s been developed a few weeks before the launch,” he said. “It’s not unusual for things to vary a little bit based on the actual conditions of the launch.”

Atlas telemetry
Telemetry of the March 22 launch, broadcast on NASA TV, shows booster engine cutoff, or BECO, at T+250 seconds, 5 seconds earlier than planned. While the graphic shows the engine firing, the acceleration displayed in the upper-left corner is 0, confirming the engine has shut down. Credit: NASA TV

But in addition to the extended burn of the Centaur, launch telemetry broadcast on NASA TV indicates that the RD-180 engine in the first stage of the Atlas 5 shut down prematurely. That telemetry shows booster engine cutoff taking place about 4 minutes and 10 seconds into the mission, five seconds before the scheduled cutoff time according to the ULA mission summary.

ULA has not confirmed or otherwise commented on the apparent early shutdown of the first stage. Industry sources suggest that the Centaur burn may have extended to make up for any underperformance by the first stage, such as if it shut down prematurely.

ULA noted that, despite the irregularities, it still considered the launch a success. “The Atlas 5’s robust system design, software and vehicle margins enabled the successful outcome for this mission,” Chassagne said.

“Centaur nailed the orbit,” Thorp said at the press conference, noting that the orbit the Centaur placed the Cygnus in was within a “fraction of a kilometer” of the planned orbit. “We got Cygnus where it wants to go.”

“Like every mission, we’re going to go do a very detailed post-flight review,” he added, “to make sure everything performed properly. But from everything we’ve seen so far, the mission was pretty nominal.”

SpaceNews.com

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ULA confirms engine issue on latest Atlas launch

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