This gave me an idea. I don't know if it is very good, but it is an idea.
Concentrated sunlight can make things very hot, very fast. Okay, let's concentrate sunlight onto a heat exchanger and use the hot gas as a reaction mass in a rocket. That is, to use the sun as an energy source instead of carrying one on-board a spacecraft.
The heat generated in this video is enough to make a respectable amount of thrust, if I am not mistaken. I understand that the amount of power isn't much in terms of its size. At any rate, it may be an interesting thing to contemplate.
This became an 18 part series, if you can believe that.
Here's the links in order with each number as a link: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18.
Looks like 11 is missing. Oops! So, that makes it a 17 part series instead.