I had an idea for a gravity drive. It's silly and kind of complicated, but think of it like this...
Every gravity well (a planet, star, moon, or even something as small as a satellite/ship) is part of the mass of gravity in the universe. Since matter cannot be created or destroyed, the amount of gravity is a constant.
Now, here's where interstellar travel comes in. Imagine that in the 'gravity' spectrum, distances are based on gravity. Empty spaces (spaces with no gravity) are essentially shorter in the gravity spectrum, since there is no... 'resistance', let's call it.
We all know of the model for gravity where space is a plane and each object makes an indent. Imagine it reversed so each object makes a 'bulge' rather than an intent - making it harder to travel through.
If you've followed this far, and it makes any sense, congrats. I have a hard time articulating it.
Now what a gravity drive would do would envelop the ship in a field (not uncommon in scifi) that allows the ship and its crew to shift into the gravity spectrum. So you would activate it while sitting next to a planet, and you would be pushed *AWAY* from the planet, thrown into 'empty' space (little or no gravity so no resistance) and very quickly arrive at the first gravity well you hit.
This would mean for interstellar travel, you'd need to calculate based on your local gravity wells exactly where to activate your drives from, so you'll be pushed in the *exact* right direction. If you activate it from the wrong location, you could be flung god knows where - even outside of the galaxy. There's no energy required for the drive (besides what is necessary to phase the ship in and out of the gravity spectrum), and you'd still need propulsion to move around with in a star system safely.
Make sense?