You can also have solar thermal propulsion where you reflect large amounts of sunlight onto some hydrogen to heat it up before using it as propellant. The effi

You can also have solar thermal propulsion where you reflect large amounts of sunlight onto some hydrogen to heat it up before using it as propellant. The efficiency is roughly the same since in both cases you're basically limited by the melting point of your engine. Solar thermal designs tend to have lower thrust than nuclear thermal but much more thrust than solar electric.