Amazon.com Inc. said it plans to launch the first two test satellites for its Project Kuiper constellation this fall on United Launch Alliance’s Atlas V rocket, rather than its next-generation Vulcan, as previously planned.

The satellite swap comes amid repeated delays of the debut launch of Vulcan, which has been in development for much of the last decade. Previously slated to fly as early as May, Vulcan’s first launch is now set for no earlier than the fourth quarter of this year. It’s possible that target could again slip further.

ULA’s Atlas V has been the company’s workhorse rocket for nearly two decades. ULA, a joint venture of Boeing Co. and Lockheed Martin Corp., will phase the Atlas V out of service once Vulcan begins flying.

The change in rocket rides was first detailed in a US Federal Communications Commission filing made by ULA on Friday and was earlier reported by Ars Technica. An Amazon spokesperson later confirmed the update to the launch plan, though declined to provide more details about the Atlas V flight. A ULA spokesperson confirmed the Atlas launch plan and referred further questions to Amazon.

Amazon’s Project Kuiper aims to launch a massive constellation of 3,236 satellites in low orbits around Earth in order to provide broadband internet coverage to the ground below - similar to Space Exploration Technologies Corp.’s Starlink. Last year, Amazon bought as many as 83 rocket launches from United Lunch Alliance, Jeff Bezos’ Blue Origin and Arianespace SA in the largest commercial procurement of launch vehicles in history.

--With assistance from Matt Day.