OA-6 launch
An Atlas 5 carrying a Cygnus cargo spacecraft lifts off from Cape Canaveral, Florida, March 22. Credit: NASA TV

WASHINGTON – United Launch Alliance announced April 8 that it has indefinitely delayed the next launch of its Atlas 5 rocket after the most recent launch of a Cygnus cargo spacecraft included a premature shutdown of the first stage engine.

ULA had planned to use its workhorse Atlas 5 rocket to launch a U.S. Navy narrowband communications satellite May 5. But during the March 22 launch of the OA-6 mission, used to deliver cargo to the International Space Station, the Atlas 5’s first stage engine shut down six seconds early, requiring the upper stage to fire more than a minute longer than planned to place the spacecraft into the proper orbit.

ULA officials said March 27 they would push back the launch of their next Atlas 5 mission, known as Mobile User Objective System-5, by at least a week to no earlier than May 12.

Then late April 8, the company said in a three-sentence press release that “the Atlas 5 MUOS-5 launch is delayed and indefinite on the Eastern Range due to ongoing evaluation of the first stage anomaly experienced during the OA-6 mission.”

Tory Bruno, president and chief executive of ULA, wrote in a March 25 post on Reddit.com that the company has a “pretty good idea about the cause, but won’t share until we are sure.”

As in earlier releases, ULA stressed in the April 8 announcement that it considered the March 22 launch a successful mission after placing the Cygnus cargo spacecraft into its planned orbit. That spacecraft arrived at the International Space Station March 26.

The longer-than-expected upper stage burn consumed fuel the rocket’s RL10-powered upper stage needed for an 11-second deorbit burn to drop the spent stage into the ocean south of Australia. Because of the anomaly, the de-orbit burn shut down 8 seconds early, causing the stage to reenter further east than planned in an uninhabited stretch of ocean.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.