The Plasma Fusion Engine

As great as the discovery of faster than light travel is, it would mean nothing without the plasma fusion engine. The technology was born in the middle of the 21st century, when space travel outside Earth orbit became common. Old chemical fueled rockets used far too much fuel to transport a ship on a journey to a location as far always as Mars, much less Jupiter. Chemical fuels in early interplanetary ships made up more than 90 percent of a ship's mass. Even the crude fusion engines developed in the early 21st century were a thousand times more efficient than chemical fuel engines.

By the early 22nd century the plasma fusion engine were 10 times more powerful than their older cousins.  Ships crisscrossed the solar system in mere weeks, with not even 1/1000th the needed mass of chemical rockets. Humanity spread out all over the solar system, starting settlements on Mars, the moon, Europa, in the asteroid field, and even on Venus. Travel was cheap, and materials were in abundance. Over a thousand people left the cradle of the solar system on generation ships, hoping to found new worlds (over a century later one was still burning its plasma fusion drive when it was overtaken by a jump-capable ship heading for the same destination).

By the time ships jumped to other stars, engineers had developed the next great advance in plasma fusion technology: the "scoop." It is an electromagnetic mesh generated from a generated from a ship that funnels hydrogen from the atmospheres of gas giants like Jupiter.  With the refueling option ships could stay out on mission for years, even long enough to build a new colony, and even new ships.

Today's ships have developed even better processing capabilities, better refueling efficiency, and even more power. Modern Ships can create two million kilograms of thrust for ten seconds from less than a thimble full of liquid hydrogen. Despite this, fuel makes up more than half of the mass of today's warships. The fuel is needed for combat maneuvers, which use much more fuel than non-combats maneuvers.

source:  Terran & Avarian Fleet Book