Interest on the national debt is $400-$600 billion per year depending on what you count (a lot of it is paid to the federal government itself). It's a lot of money, but it's a small fraction of the federal budget. It's also less than the amount of money in existence - M1 is $5.5 trillion and can easily be expanded, as mentioned.
(It also wouldn't necessarily matter if the interest per year were greater than the total amount of money in existence. If the money supply were $20, that wouldn't make it impossible for me to pay you $21/year; I could pay it in 21 $1 payments.)
(It also wouldn't necessarily matter if the interest per year were greater than the total amount of money in existence. If the money supply were $20, that wouldn't make it impossible for me to pay you $21/year; I could pay it in 21 $1 payments.)