The basic problem common to all rockets is that the more massive your ship, the more fuel you'll need to accelerate. Fuel adds mass, which means more fuel, which adds yet more mass...which quickly spirals out of control when trying to design an interstellar spacecraft. However, in the 1960's, an American physicist named Robert Bussard came up with an innovative solution; harvest your propellant from the environment around you. Inspired by ramjet engines on high-speed aircraft, the Bussard Ramjet would use a massive magnetic scoop to gather up the extremely diffuse hydrogen atoms that make up the interstellar medium, and cram them tight enough in the center to undergo fusion. The products of the reaction were expelled out the back for thrust.
Unfortunately, the Bussard Ramjet would not work as designed. First off, coaxing pure hydrogen atoms to fuse (called proton-proton fusion) is near-impossible, and only occurs naturally inside of stars. Secondly, the field of the scoop itself actually imparts a massive drag effect, which lead to the development of the magnetic sail.
However, the concept was later revisited by Alan Bond in 1974. Instead of using the interstellar hydrogen as fusion fuel, he proposed using it merely as reaction mass, with the ship having a separate propulsion system. The matter stream from the scoop is directed towards a target of lithium or boron to initiate a fusion reaction (Boron-proton and Lithium-proton fusion are much easier and more energetic than proton-proton fusion). This is called the Ram-Augmented Insterstellar Rocket, or RAIR.
A variation on the RAIR, the Catalyzed-RAIR, uses antimatter to kick-start the fusion reaction, providing a significant boost in efficiency and acceleration.
This ship is the first Catalyzed-RAIR ever built, designed to reach Alpha Centauri B and enter orbit around a habitable planet. It carries 8 Orbit-to-Surface drop pods which carry vital machinery such as construction robots, manufacturies, hydroponics/carniculture labs, artificial uteri, and other vital technologies for a human settlement. In addition, it carries 100 settlers in cryogenic suspension. The ring of habitats are hidden by the massive fuel tanks, which provide radiation protection. At 0.25c, the journey takes over 18 years.