Hi Ben,
I share your dream of interstellar travel. I also wish we could travel to Mars in less than two hours. Unfortunately, the laws of physics are somewhat uncooperative. Nevertheless, this is how I think it can be done.
I started with the assumption that gravity propulsion drives (gravity field generators) are allowed by nature. For a gravity field generator to work, there has to be a way, separate from the stress energy tensor, to curve space-time. But then this raises the question: what is space-time? I decided to make a best guess at what space-time is really made of. An empty void was not helpful; I needed a medium that I could assign properties to. However, given all that we hear about virtual particles, quantum electromdynamics, DeBroglie waves, the quantum vacuum, etc., I reasoned that any of these things would ultimately be modeled with quantum mechanics and some kind of wave-function. Wave-functions describe all quantum systems (w/o exception). Wave-functions, unlike all the other mathematics in physics, wave-functions look like something that could physically exist. So I decided to elevate wave-functions to the status of an ontological physically existing medium.
The speed of light seems to be entrenched within the geometry of space-time. So I had an idea. Let's make the speed of light a characteristic of the medium. Therefore, permitivity and permeability are built in characteristics of the medium as well. But what was I going to do about space-time geometry and time dilation? Since the speed of light is intrinsically a part of space-time, I added an additional characteristic to this medium.
[math]c = \lambda f[/math]
If the medium is made of waves (borrowed from plane waves of wavefunctions), then I can use the wavelengths of these waves to ontologically create distance in space and use the frequencies to permit the progression of time.