Don't mean to be an inner-solar-system-centric, but we all have to respect the economics of deltaVee. I'll grant you that if we're harvesting reaction mass from the Oort, it may become moot, provided we have some way to accelerate it to high velocity. Solar thermal probably isn't practical, there probably aren't practical concentrations of fuel for fusion and certainly not fission. I wonder if Mercury has relatively easily mined fissile material?