What do you think about gravity as a source of propulsion in space? But you know how a sail uses the wind to navigate. There is no wind and space and currently no known fuel that you could store in sufficient quantity that would enable you to make a very long journey. I was wondering about a psuedotech that would have found an ore capable of strong reactions to gravities energy the same way a magnet reacts to magnetic energy. But shifting the polarity of said ore, a ship using these special sails could control the rate of attraction and repulsion to a gravitational body. The differing levels of gravity from different and moving sources create shifting gravitational currents that require professionals to interpret and navigate. Because of this, a ship may be able to latch on to large planetary bodies and such in leap-frogging fashion. This would be the equivalent of land-hugging for a spaceship where it could only employ useful navigation where the gravitational density was considerable. An alternative to this would be using the Solar Wind as propulsion through the solar system. Of course, this would limit the range and effectiveness of said ships the further from the sun it gets.