Atomic Engine

Drive that uses thermonuclear reactors (atomic reactors that heat up chemicals - usually liquid hydrogen - and generate thrust by exhausting created gases). Functional and popular for a long time, ultimately defeated by fast development of chemical and ion drives. Eventually, thanks to advanced fission technology, it became comparable with the other two and even slightly better when it comes to acceleration and fuel efficiency which is why it slowly regains it's popularity - mostly in Outer Colonies, especially in fighters and missiles.