Hello, I am in the process of writing space based sci fi novella, I made a few posts last year and earlier this year regarding different aspects of the ship and crew. If anyone is interested, I wanted to concentrate on the fusion engine of the ship itself and hopefully come up with a feasible model. The novella takes place several hundred years in the future and details Earth's first trip to Alpha Centauri. The ship will take 40 years to get there and will be traveling at a peek velocity of 0.12c . The ship will be coasting for most of the journy because there was no way I could work out a constant acceleration trip without making the ship the size of texas just to hold the fuel :) The question is regarding how the engine might work, Its a Fusion engine but the problem I have is that most types of fusion reaction require hydrogen isotopes that are only stable for a decade or so. This isnt a problem for shorter journeys but for a trip lasting 40 years I couldn't come up with an engine that would still have viable fuel to decelerate with. That had me thinking about a proton to proton fusion reaction. I know we are no where near that level of design today so this will all be pure speculation. Assuming the civilization in question could develope a proton to proton engine, what might it look like? I know stars get away with it but is this something that is even feasible for us to reproduce and what are some of the ways we might go about it? If the idea of a proton to proton reaction is completely off the table then what are some of the other methods my ship might utilize to facilitate a 40 year journey? Thanks for all your help.