Interest rates explode after 2029

This is a very simple model, probably flawed.  If interest rates ever exceed 5% of GDP then we have a serious problem. At this point, at least 25% of revenues would go just to pay interest.  After this point, the situation deteriorates rapidly.

I got the numbers through 2022 from the official OMB budget FY2013 proposal. After that, I assume that GDP increases 4.5%/year.  For interest, I assume that it increases 10%/year after 2022.

Interest rates really have nowhere to go but up, and on a skyrocketing base. 

Year	GDP (trillions)	Interest (trillions)	Int % of GDP
2011	15.1	0.23	1.52%
2012	15.8	0.22	1.39%
2013	16.5	0.25	1.51%
2014	17.4	0.31	1.78%
2015	18.4	0.39	2.11%
2016	19.5	0.48	2.46%
2017	20.7	0.57	2.76%
2018	21.7	0.65	2.97%
2019	22.7	0.72	3.16%
2020	23.7	0.78	3.31%
2021	24.7	0.85	3.43%
2022	25.8	0.92	3.55%
2023	26.9	1.01	3.74%
2024	28.1	1.11	3.94%
2025	29.4	1.22	4.14%
2026	30.7	1.34	4.36%
2027	32.1	1.47	4.59%
2028	33.5	1.62	4.83%
2029	35.1	1.78	5.09%

This is model L-1.

Update: 6/25/2012.  This is similar to Model G-1, which saw danger in gross interest paid exceeding 40% of revenue.   Net interest seems to be about half of gross interest.