Radiation engines?

[Myasishchev]

Problem with those is slowing down.

For in-system travel, slowing down isn't a huge problem. After all, you wouldn't actually be traveling in a straight line with one of these things----it isn't efficient. Instead, you would place yourself into an orbit around the sun. Angle your sail to increase your orbital velocity, you move farther away (head to Mars). Angle it the other way, you move closer (return to Earth).

Oh, I was thinking about intersystem.

 

[Lindley]

Yes, intersystem travel requires a completely different approach.

I do like the mind-bendingness of orbital mechanics, though. Like the fact that in order to overtake someone else in the same orbit, you need to first slow down (drop to a lower orbit), then speed up again after you pass them to return to their level.