President-elect Donald J. Trump is traveling to Brownsville, Texas, on Tuesday to attend a test launch of SpaceX’s next-generation rocket in another show of solidarity with Elon Musk, the world’s richest man, who helped catapult the former president back to the White House.

Mr. Trump’s appearance at the launch demonstrates the growing closeness between the two billionaires, and Mr. Musk’s increasing influence in Mr. Trump’s realm. Last week, Mr. Trump announced that Mr. Musk would lead a yet-to-be-created government agency, and Mr. Musk has become a staple in Mr. Trump’s transition. Mr. Musk has frequently attended meetings with him at Mar-a-Lago, Mr. Trump’s private residence, and traveled with Mr. Trump to meetings in Washington. In at least one case, Mr. Musk even spoke to world leaders with him.

As has been the case with many of Mr. Trump’s job candidates, Mr. Musk’s influence and presence brings potential conflicts of interest. Mr. Musk’s SpaceX program has already received a $4.4 billion commitment from the federal government to pay for Starship missions to the moon, and SpaceX has secured more than $10 billion worth of federal contracts over the last decade. Several of Mr. Musk’s companies have been targeted by federal agencies, in at least 20 investigations or lawsuits.

Although he and Mr. Musk only recently became close allies, Mr. Trump had long expressed admiration for Mr. Musk’s rocket launches and would frequently reference SpaceX during campaign speeches.