Make the transit ship spin. Surround the habitable parts with water and other materials. It’s so simple but nobody seems to be aware that these are solved problems.
It isnt reasonable to assume napkin solutions are "so simple" in practice or have not been considered. There are plenty of people smarter than either of us that look at these problems.
- you need 1 meter of water shielding to reduce radiation to safe levels[0]
- each cubic meter of water is 1 metric ton
- Starship's circumference is ~28 meters
Assuming you only shield 4 meters of the length of the ship, not counting the top and bottom cross sections of the cylinder, you've already blown past the 100T payload capacity of Starship.
Once those are in cycle orbit you now have giant water cylinders that you can catch every once in a while to ride to earth/Mars. Forever. With a tiny energy budget for corrections. Just wait till one passes by and slip your vessel inside it. Man I am good.
However, water isn't light. According to this post on stack exchange it would take something like 330,000kg of water to shield a crew. Getting that into orbit is a big task.
I'm pretty sure it's lots :) I think that we just haven't figured out how to do this, so the "obvious" solution is not on the table (yet).
It’s not a question of power. You could could get it spinning quickly or slowly with high or low power respectively but the question you are trying to ask it how much energy is required. The amount of energy to get the thing spinning is the same no matter what. And once you get it spinning, you’re done. It will keep spinning. I think there is a certain law that pertains to that. Something to do with fig newtons.
The amount of energy that is required is trivial compared to the other energy budgets. And it would be spun up at low power over the course of hours or days.