Complex sentence: Bussard proposed a ramjet variant of a fusion rocket capable of fast interstellar spaceflight, using enormous electro-magnetic fields (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium.
Split into simpler sentences: Bussard proposed a ramjet variant of a fusion rocket capable of fast interstellar spaceflight. It would use an enormous electro-magnetic field (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium.