The US national debt is denominated in dollars. The US government decides what a dollar is. There's no way they can default. Just trigger hyperinflation.
Sure, that's the "technically correct, but practically worthless" answer. Trigger hyper-inflation and then what? It surely won't be "business as usual" afterwards.
That's why they do it progressively enough so you dont see prices jumping up too fast. But be sure that they do, from year to year...