There are many steps that will need to occur along the way before the United States goes bankrupt.
The real end point will not happen until the cost of servicing the debt exceeds the revenue collected. The United States has operated under the notion that the amount borrowed is not critical because there is no intention of paying it back, only to service the debt.
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I would think the "end point" would come well before that...like around the time debt service (interest) exceeds 60%-65% of revenue or so...enough federal programs will have been squeezed out at that point to cause panic. Will have to check to see what % of the budget is purely salaries/wages for federal workers/military...when the paychecks stop coming the "stuff" hits the fan.