[douglas]

Let's see, 30 turns. With 300k research, assuming medium tech cost, it'll take 12 turns to get Stellar Manipulation 5 and Base Construction 3. That gives you 18 turns to build the ringworld.

If you have Space Yards 3 and 100% construction aptitude, minimum build time for the ringworld generator is 23 turns, including the bonus for emergency build on the last 11 turns. 127% construction aptitude after cultural modifiers can bring it down to 18 turns. You'll probably need a turn to spare for activating it, which brings the requirement up to 134% construction aptitude. If you don't have construction rates that high, start stocking up resources and figure out a retroseries build, it's your only hope.