Flush with the success of the Falcon Heavy rocket launch last week, SpaceX CEO Elon Musk began discussing the performance of the booster Monday on Twitter. He was evidently miffed about comparisons between the Delta IV Heavy rocket—manufactured by SpaceX competitor United Launch Alliance—and the Falcon Heavy rocket.

Last week on Twitter, Doug Ellison, a Jet Propulsion Laboratory visualization producer, shared some calculations that demonstrated in some cases that the Delta IV Heavy rocket could match the performance of the Falcon Heavy for certain missions to the outer Solar System.

Musk responded that Ellison's numbers were based on flawed underlying data and that even if they weren't, the Falcon Heavy cost substantially less than the competition. Then Tory Bruno, the chief executive of United Launch Alliance, joined the discussion.

Hey @elonmusk , congrats again your heavy launch. Clarification: Delta IV Heavy goes for about $350M. That’s current and future, after the retirement of both Delta IV Medium and Delta II. She also brings unique capabilities, At least until we bring Vulcan on line. — Tory Bruno (@torybruno) February 12, 2018

Musk was not buying those numbers, and he didn't buy the slightly higher estimate of the Delta IV Heavy's cost ($400 million) either.

That was three years ago, before ULA cancelled all medium versions of Delta IV. Future missions have all Delta fixed costs piled on, so their cost is now $600M+ for missions contracted for launch after 2020. Nutty high. — Elon Musk (@elonmusk) February 12, 2018

Perhaps Musk was hungry because he moved from "nutty" to the eating of hats. For context, below, he's talking about United Launch Alliance's plan to replace its Delta and Atlas rockets with a new, powerful booster called the Vulcan rocket. Originally planned for a launch in 2019, the Vulcan rocket's maiden launch now will probably slip into mid-2020 at least. But Musk clearly believes the test flight and Air Force certification process will delay that quite a bit longer, and he's willing to put his millinery where his mouth is.

Maybe that plan works out, but I will seriously eat my hat with a side of mustard if that rocket flies a national security spacecraft before 2023 — Elon Musk (@elonmusk) February 12, 2018

After all of this, Bruno had just a single-word reply: "Wow."