Past public reviews and discussions.
Once per level of technology. Plus, if you can design a new one for every planet, some players will definitely design a different one at least for each empire that borders them based on that empire's tactics. And if you can't design a new one for each planet, well that's still one more redesign to make when your technology levels up; unless it automatically levels up all your weapons to the next one of that type. That would be pretty cool. Could work.
Warning: Antarans in dimensional portal are closer than they appear.