Blue Origin is unlikely to deliver two flight-ready versions of the BE-4 rocket engine to United Launch Alliance (ULA) before at least the second quarter of 2022, two sources say. This increases the possibility that the debut flight of ULA’s much-anticipated new rocket, Vulcan, could slip into 2023.

Vulcan’s first stage is powered by two BE-4 engines, which burn methane and are more powerful than the space shuttle’s main engines. The sources said there recently was a “relatively small” production issue with fabrication of the flight engines at Blue Origin’s factory in Kent, Washington.

As a result of this, the engines will not be completed and shipped to the company’s test stands in West Texas until next year. Once there, each engine must be unpacked, tested, and then re-configured to be moved to ULA’s rocket assembly facility in northern Alabama. A reasonable “no-earlier-than” date for the engines’ arrival at the rocket manufacturer is now April 2022, and this assumes a smooth final production and testing phase.