Quote Originally Posted by Renton View Post
My point was that you couldn't maintain a constant rate of acceleration of 1g for very long (within interstellar travel time frames). For some reason when I first approached this problem I just assumed it would take 500 years or whatever for 9.8 m/s/s to add up to relativistic speeds. Not so.
It's cool that you had the hypothesis and then tested it using theory.

The energy required to maintain the thrust would be the limiting factor.

No matter your speed, you can always accelerate. It's just that as your speed approaches that of light, your mass approaches infinity. So the amount of acceleration you get from the same force is ever decreasing as you go faster and faster. A constant force does not yield a constant acceleration when relativistic speeds are involved.

Quote Originally Posted by Renton View Post
One thing is that it seems like you could achieve reasonably high speeds (say 0.1c for example) by applying a small thrust over a period of years that would only require a steady stream of a (relatively) small amount of energy. Could solar panels approach the needs of this, or at least make a dent so not so much fuel is needed?
I was going to point out that they are terribly inefficient, but that's only in comparison to other ground-based methods. Wow, double oops, 'cause I was thinking in terms of monetary cost efficiency instead of mass cost efficiency.

The ISS has ~ an acre of solar arrays on it, producing 75 - 90 kW of power. The solar arrays (panels with many cells) do rotate to orient the cells, but the cells may have great longevity for their operation time. I.e. there is probably a bonus in the fact that the solar cells do not have any moving parts.

Ongbonga has a good point about the amount of light diminishing as you move away from the sun. The utility of a solar panel would be pretty limited as you move toward the outer planets, and practically nothing as you move past the gas giants.

I thought about solar sails being a roughly similar concept. I know that we use solar sail concepts to assist in maneuvering satellites and probes, but I don't think there is a probe which uses solar sails as primary thrust. It faces the same problem with diminishing returns as it moves from the sun.