Fusion should lead to unbeatable specific impulses, so the huge size of a reactor may not be a problem at all. (Or maybe it is, I don't think anybody can be sure before there are actually working reactors out there.)
> Fusion should lead to unbeatable specific [impulse]
No, because the effective specific impulse is so low, as a fusion reactor will only be able to fuse a small fraction of its mass before it is too radiation damaged to work. ITER, for example, would take 300,000 years to fuse its own mass in fusion fuel, but no DT reactor could operate more than a few years (if that) before parts need replacing.
If you want high effective specific impulse and high thrust at high specific impulse, beamed power is the way to go.
Since we're talking about things that don't exist: -
We should be able to beat fusion's Isp with direct matter to energy conversion.
Certainly the mass of fuel needed will be 3 OM lower.
Edit to add: Isp is a measure of velocity. The thing about velocity is, energy goes quadratically but momentum imparted to the vessel (the impulse) only goes linearly with velocity.
Since no process can be 100% efficient, you're dealing with quadratically more and more waste heat from the engines. And space is an excellent insulator.
Well, if you create a nice way to make and store anti-matter, you would basically solve rocketry on any place that humans could ever care about going to.
There are people researching that too, but not as many as fusion. And well, determining the capacity of things that don't exist is the first step on the work of making them exist.
