Elon Musk, Tesla’s Chief Executive, attacked Jeff Bezos, Amazon founder, on Twitter Friday following Amazon’s request to the FCC to deny a SpaceX satellite plan.

Musk founded SpaceX, a rocket company. Bezos founded Blue Origin, the space company. Musk and Bezos have been arguing over space issues for a while.

Amazon wrote to the Federal Communications Commission asking it to reject SpaceX’s plans for a second generation Starlink satellite. The letter did not constitute a lawsuit.

Musk tweeted, “Turns Out Besos [sic] has retired to pursue a full time job filing lawsuits against SpaceX.” Bezos left Amazon’s CEO job last month.

Amazon claims that SpaceX has violated FCC regulations, asking for two configurations of satellites instead. Kuiper Systems, a subsidiary of Amazon, sent the letter. It is currently working on its own satellites.

SpaceX has recently begun offering Starlink internet services on a limited basis. This system utilizes hundreds of small satellites orbiting low-Earth orbit.

As for the Bezos-Musk history of space conflict, in July, Bezos suffered a setback to his goal of traveling to the moon, as the Government Accountability Office denied a protest by Blue Origin of NASA’s award of a $2.9 billion contract to SpaceX for a human lunar landing system.

Then earlier this month, Blue Origin won a protective order in the U.S. Court of Federal Claims sealing documents in its lawsuit against NASA. The suit claims that SpaceX was not properly evaluated its proposal for the human-landing system program.

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Last month, Bezos flew in a Blue Origin rocket that was first built and piloted by a man. It took him to sub-orbital flights to the boundary. Musk has not yet flown in space.

SpaceX has just completed the assembly of its first Starship rocket, in preparation for a test launch. This rocket is nearly 400 feet tall and stands higher than the Saturn V rockets used for the Apollo moon missions.