Physicist: In the post “Q: Have aliens ever visited Earth?” I said that the maximum velocity that can be obtained by a rocket using fusion is 11% of light speed. The assumption I had been working with was that a rocket shouldn’t be able to get going any faster than it’s exhaust velocity. Which translates to a final velocity of only about 47% of light speed. In order to get something to 0.9c (again using fusion) would require a fuel-to-ship ratio of around 700,000. Using normal rocket fuel (w = 4440 m/s) to get to 0.9c, you’d need a fuel to ship ratio of about 1076,985.