1) The United States cannot be forced to default on the debt. We are the sole issuer of our currency, borrow and spend in that currency. Therefore, we cannot "run out" of money. We cannot go bankrupt because there is no bank. Having a monopoly on our currency is why we're different than countries that DO go bankrupt, like Greece, for example.
2) The national debt is an account of all the money spent in to the private sector but not taken out of it via taxes. We can no more "pay off" the debt than you can go back and get the $ you spent on a candy bar when you were 12.
3) The Fed controls interest rates, not the market. If interest payments became too burdensome, guess what? We finance the whole thing in to rolling 6 month Treasuries instead of 30 years, and drop the interest rate to zero.
Wait, WHAT? So if what I'm saying is true then deficits don't matter?
Well, they do and they don't. What matters is the glue that holds the whole thing together. That glue is the productive capacity of the US, the ability to tax that production, and the FAITH in the currency. The US has a lot going for it in those categories.
To the extent that we spend so much that we have a crisis of faith, that's bad. To the extent that we spend on unproductive or mismanaged programs, that's bad. In other words, it's much better to borrow for investment than borrow to fund current consumption.