...and of course, deceleration. Any proposals on how we achieve that? I tell you this, it will not be any kind of chemical reaction, for obvious reasons. What other forms might an engine take that could do this? I think we need to understand more about just what space is really made of, because it most definitely is not "nothing". There is a power source there somewhere, whether it's gravitic, particulate, topographical or something completely outside our current math. My vote is gravitic. If we ever learn to harness or even understand this phenomenon in detail, I think the solution for practical space travel will point to itself.

What we really need to know is what to push against. Helicopter blades push against air and defy gravity. A boat propeller pushes against water and achieves forward motion. What else can we push against to lift a ship into orbit, or accelerate it though space? The earth has "gravity" because it's mass is big enough that it creates a depression in space steep enough that we are all stuck (luckily!) at the bottom of it. It bends space! Think of the stresses involved and tell me there isn't enormous potential energy lying around somewhere waiting to be found.