[jfengel]

You don't need to go that far. Light has momentum, and you can use that directly. It has the maximum possible specific impulse.

https://en.wikipedia.org/wiki/Photon_rocket

However, it requires a lot of energy and we are nowhere near a practical model. It's also not "propellant-less"; the photons are the propellant.

[BurningFrog]

The practical point of being "propellant-less" is that you don't use up some finite supply of propellant.

If the "propellant" is electricity that our solar panels can generate, that's functionally propellant-less!