I've been refining the design and inner workings of my spaceships lately and one thing I've been asking myself was what propellant would work best. I needed my ships to travel astronomical distances for exploration purposes and I also needed a way to instantly teleport my characters from one world to another. I really didn't want to copy other sci-fi with warp travel and opted for a completely unique approach. Basically, my ships are giant teleporters. This way, once a planet has been discovered the ship that discovered it functions as a shortcut to and from that planet.
Since I chose teleportation as my fictional element, I decided to make the most of it and use it for propulsion as well. The idea of a never-ending supply of propellant really appealed to me. What ended up happening was: I took a railgun and strapped a teleporter to it. Yup! Very original.
The overall set up resembles an hourglass. Magnetic rods surround the firing chamber and a pair of teleporters are located on both ends. The reaction mass is magnetically accelerated to generate thrust. Once the mass reaches the threshold of a teleporter, it is blipped back to the other end. Teleportation does not affect the objects momentum or orientation in any way, only it's location in space. So while you can't push on it and have it come back at you to push you a second time, you CAN rotate the 'hourglass' to recycle the energy put into the reaction mass. Like a magnetic bottomless hourglass. (Hopefully the edit I made cleared things up.)
FYI The ship is oval shaped, made mostly out of titanium, measures 1 km in length and is powered by a nifty fusion reactor.
And now the question: what propellant would offer the most thrust using this method of propulsion?
Your typical space shuttle has to work with a very limited fuel supply, thus they focus on fuel efficiency. A high thrust-to-weight ratio. But my system is the opposite. Scarcity is not the issue. So that leaves only one thing to improve: thrust! A satellite equipped with an ion engine can accelerate a lot with very little propellant, but it takes waaay too long to do so. So would my engine benefit from a super dense propellant instead?
There is basically no research done on the subject, so I'm relying entirely on your expertise.
Right now, my ships main limitation is that they only operate in space. That means they need to be constructed in space (Star Trek style) and can only move in space. If the system isn't efficient enough for atmospheric flight, that's perfectly fine. There are workarounds. But IF there is a way to make it work, I would be more than happy to lift that limitation.