Something like 99% of all government debt is fixed (minus inflation protected securities, but they make up less than 1% of the total stock of federal government debt). So, how on earth can the effective debt service rate go to 6%? Currently, the effective interest rate on gross government debt (GGD) is a little north of 2%. If we assume that by the end of fiscal year 2012, we had $16,159,487,013,300.35 in GGD, debt service for fiscal year 2012 was $359,796,008,919.49 (the lowest since 2005) and a projected 2013 deficit of $1 trilion, interest rates would have to increase to 66.975% for the 2013 deficit.
((((3. 5979×10¹¹)+(1000000000000x)))/(1. 7159×10¹³))=0.06 ; x=0.66975
So while you are correct that 66.975% interest rates on the 2013 deficit would be catastrophic, the chance of it actually occurring is zilch!
If borrowing rates in 2013 were to magically go to 6% (another unlikely scenario for reasons i will explain later), the effective cost of debt service goes to 2.4465% with a price tag of $419,806,849,780.39. This is an additional cost of $60 billion.
Let's get something straight; interest rates cannot go higher unless there is sufficient economic growth to facilitate the sheer existence of higher interest rates. Meaning: if interest rates were to go to 6%, U.S. economic output would have to be greater than or equal to 6%. This is simply a matter of fact.