Astra’s new Rocket 4 launch vehicle will use Ursa Major’s Hadley liquid engine to power its upper stage, the two companies announced Monday.

Astra has been tight-lipped about the new upper-stage engine that would power its new Rocket 4, with CEO Chris Kemp only telling investors last year that the rocket’s substantially increased payload capacity was thanks in part to engine upgrades. Outsourcing the engine helps clarify how Astra was able to so quickly pivot its plans for Rocket 4, including doubling the launch vehicle’s payload capacity from 300 kilograms to 600 kilograms.

Rocket 4 marks a major deviation for Astra, which has historically focused on extremely lightweight, high-cadence rockets. After a series of launch failures and in response to customer input, the company said it would go in a decidedly different direction. When Astra announced the change last August, Kemp said the company was especially keen to target mega constellation operators with its higher capacity system.

Astra plans on conducting initial test launches later this year. Whether the company resumes commercial launch operations this year will depend on how those tests go, Kemp said.

Astra is the latest launcher company to use Ursa’s engines. Ursa also counts Phantom Space and Stratolaunch amongst its customers. The Colorado-based engine maker also recently closed a deal with the U.S. Air Force, bringing their total number of engines currently under contract across defense and commercial customers to around 30. In addition to Hadley, Ursa is developing a substantially more powerful Ripley engine, which will be able to generate about 10 times more thrust.

The news confirmed a long-held rumor amongst space aficionados that the two companies had inked a deal. Rumors started swirling when Astra released its first payload users guide for Rocket 4 last November, and space sleuths noticed that the upper stage’s engine was listed as being turbopump-fed and capable of generating approximately 6,500 pounds of thrust. People speculated that it could only be a Hadley variant, given the time and financial constraints facing Astra. Turns out they were right.

The story has been updated to reflect that Ursa Major has orders for around 30 engines across both defense and commercial customers, not just defense customers.