When I wrote Starships, I avoided talking about the engine technologies. Real-life rocket engines work by throwing mass out of the back end of a ship, and the fuel-to-payload ratios are too high to zip around like we do in Starships. Near term, real life combat is going to be an exchange of laser fire, and combatant survival will be based on the old Bayushi principle of “Strike First, Strike Last.”
Currently, the only alternative is the propellant-less drive. The general idea is that the engine takes some form of light and bounces it off a mirror in such a way as to generate thrust. The concept is the same one that applies to a solar sail. The difference is that you’re carrying along the laser, or microwave generator, as part of the engine.
Hence the reason why I refer to the drive as propellant-less, instead of reaction-less. There’s still a reaction, and therefore possible. It might need a large amount of power, but that’s a different problem. The initial experiments into the concept have produced conflicting results, so I might need a new technology if I revisit Starships in 2025.
The other elephant in the room is a faster-than-light drive. It doesn’t make much of an impact in Starships, only existing to carry the combatants to the area of operations. If I had to pick one (and I will for scenario six), it would be the Alcubierre drive, as refined by Sonny White. Its worst requirement is the existence of matter with negative mass. Such exotic matter is mathematically possible, but might need to be artificially created in a collider. (That it might be a form of anti-matter creates other problems.)
So that makes me happy as a designer. What doesn’t make me happy is that White’s design requires a big, fat ring to surround a ship. I can’t imagine that weapons fire would miss the thing, especially if the target was showing a flank.