(Bloomberg) -- Northrop Grumman Corp. has purchased three Falcon 9 launches from Elon Musk’s SpaceX as Northrop works to replace Russian-made rocket engines on its own vehicle that ferries cargo to the International Space Station.

The launch pact will allow Northrop to continue cargo resupply missions as it readies an upgraded version of its Antares rocket with startup Firefly Aerospace, a spokesman for the defense giant said Monday. Russia halted sales of rocket engines to the US after Russia’s military invaded Ukraine, pushing some space companies to seek alternatives.

The move underscores the critical role that Space Exploration Technologies Corp. plays in hauling loads to orbit as its biggest US competitors -- United Launch Alliance and Northrop -- transition away from Russian-built engines for their mid-size rockets.

Northrop’s Antares 230 rocket was upgraded to run on Russian-made RD-181 engines to power its cargo missions. An initial version of the rocket, which exploded over a Virginia launch pad in 2014, relied on Soviet-era motors built in the 1960s and refurbished by Aerojet Rocketdyne decades later.

Northrop earlier on Monday said it would partner with closely held Firefly to field a US-built upgrade for the Antares and develop a new rocket in the medium-launch class. The SpaceX launch purchase was reported earlier by Reuters.

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