3% interest times $20 trillion = $600 billion per year in interest. (interest rates will not stay this low indefinitely, and even 3% would be crazy low given the actual default risk via inflation that the US poses)
That interest basis alone would effectively wipe out the ability to continue Social Security (or almost the entire US military, take your pick).
The national debt can never be repaid under any circumstances. We can't afford the real interest cost right now, which is why the Fed is paying for that via debt monetization (aka QE). Throw in just a trillion per year in principle, and it becomes a sad joke.
We will never, and have no plans to ever pay for the national debt. There's no scenario under which the math works out, unless our government suddenly becomes hyper disciplined and initiates an uncompromising 50 year payback plan that takes a hatchet to the entire welfare state (corporate, social, military).