Fission Reactor

Oldest of technologies used to supply energy required for ships, rockets and fortifications to work, that achieves it via the controlled chain reaction of atom splitting. With 'oldest' not meaning 'the worst', as it is - together with fussion reactors - one of the two most oftenly used power sources in the Galaxy.
Their biggest advantage is very high fuel efficiency, as reactors of that type practically do not require fuel tanks of any sorts. Their biggest disadvantage are complicated security procedures required to safely shut down reactor in case of malfunction (which happen, albeit extremely rarely) or enemy fire. Every hit not even into reactor but near it causes a high risk of disastrous explosion.
Outer Colonies prefer atomic reactors in raider and escort ships, which gives them a way bigger combat range, not to mention letting escort ships pack much bigger punch. They are also standard power sources of a variety of space fighters, corvettes and frigates, since they tend to die after being hit once so they is no use care about their survivability too much. Also among Shipkiller and Light Shipkiller class rockets they are predominant.