SpaceX CEO Elon Musk voiced his disapproval of acting NASA administrator Sean Duffy Tuesday in a calling him “Sean Dummy.”

The spat between Musk and Duffy began Monday, when Duffy appeared on CNBC News’s Squawk Box to announce that as part of a mission to land on the moon before President Donald Trump’s second term ends, NASA is opening up government contracts that were once solely held by SpaceX.

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