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.