So you want to build a fusion-powered spacecraft, and assemble it in orbit? Then check out the current design for a fusion reactor at the International Thermonuclear Experimental Reactor (ITER) homepage. This reactor stands seven-stories tall and has a mass in excess of 30,000 tonnes! The follow-on to the ITER experiment, intended to produce electricity continuously is expected to be 5-times larger, with a mass possibly ~150,000 tonnes!!! These things are made of solid metal and superconducting ceramics - they are REALLY heavy.

How can we hope to assemble a fusion-powered interstellar probe?!!! There is a way! With a launch capacity of 360,000 tonnes annually THE CUBE is what we need to get the job done.