Vulcan aces engine test, but upper stage anomaly will delay launch for a while

The Vulcan rocket took a critical step toward its much-anticipated debut launch last week with a successful engine-firing test. However, one critical issue remains unresolved before the large booster can lift off.

This final hurdle involves modifications to the rocket's Centaur V upper stage, which exploded during a test at the end of March. On Tuesday, the chief executive of Vulcan manufacturer United Launch Alliance, Tory Bruno, wrote on Twitter that a root cause of the failure had been identified, and the investigation has been concluded.

As part of their recent updates, neither Bruno nor United Launch Alliance established a new target launch date. Two sources indicated to Ars, however, that this flight likely would not occur before the fourth quarter of this year due to additional needed work.