Re: World building: implications of counter gravity

 Don't recall from the actual book, but from the movie First Men on the
Moon, the Cavorite did use a 'directional' gravity attraction, blocking
the Earth's and using the moon's. Did ignore the whole inverse square
thingy, but I can be picky.
Might help avoid using it AS a thrust, though.
As for slingshot, as there's as much gravity on both approach and
withdrawal from a body, so that you can use to increase or brake,
depending on moving with or against the body as you pass.
Cutting the gravity on either leg would be HUGE. I think. Can't do the
math. Can't even do the math as to whether 'as much gravity on both' is
true as you are accelerating as you approach, and decelerating after
you've passed. After half a century, calculus is a bit rusty.
Combining a launch system, say a canon or XL-5's trolley, with gravity
blocking would get something off the ground without lot of reaction
material.
Does that help?
Course, the whole Hyperloop talk has re-energized an idea I had for
reusing a long distance tube system for launches...

Doug