The US always pays its debts when they are due. I think perhaps the problem is you don't understand how US debt works, and why it is a bit special:
So the most important thing to understand is the US doesn't go and beg people to give it money, rather it auctions debt. People come and purchase the debt. You can do it yourself on their Treasury Direct site. The US sells debt instruments to interested buyers. They are bid on, and whoever bids the lowest interest rate wins. The upshot is the US sets the terms of the debt instruments sold. They have a variety, some are as short as 4 weeks, some as long as 30 years. When you buy something, the terms of repayment are stated up front: What it'll pay, and when. There is no provision to cash out early, and you don't get to dictate any terms, you just choose what note you want to buy (if they are available).
This is how public debt works in a lot of countries, but it isn't how things go when you are getting loans from the IMF.
The other important thing is that all US debt is denominated in US dollars. A US debt instrument specifies how many dollars it'll pay out and that number is NOT inflation adjusted, except in a few very special cases. Well the US government also controls the US mint, which makes US dollars. So the US government can literally print money, and inflate its way in to payments. There are negatives to that, of course, but it is perfectly doable. The US controls its fiscal and monetary policy regarding its debt. Since all its debts are in US dollars, and since US dollars are the world's reserve currency, the US cannot face a crisis where it can't pay, unless such a crisis is internally generated (via the debt limit).
Not the case with Greek debt, it is in Euros and Greece doesn't control the Euro.
Finally, there's the fact that the US has great credit. Doesn't matter if you disagree that it should, fact is it does. Investors are willing to loan the US money for extremely low interest rates because they see it as a very safe investment. 4 week T-Bills have been going for between 0%-0.015%. 30-year bonds have been going for 2.5%-3.75%. Investors bid the interest rates very low because they desire it as a safe investment.